School Science Lessons
30. Electromagnetism, induction, electromagnets, motors and generators, d.c. and a.c.
2012-05-04b SP
Please send comments to: J.Elfick@uq.edu.au Table of contents 30.0.0 Electromagnetism, motors, generators 30.6.0 Electromagnetic induction 30.3.0 Electromagnets, electromagnetism 30.1.3 Motors and generators, d.c. and a.c.
30.6.0 Electromagnetic induction
30.6.0 Electromagnetic induction 30.6.1 Electromagnetic induction 30.6.3 Electromagnetic induction with two solenoids 32.5.5.0 Electromagnetic induction,
motor vehicle circuits 38.2.04 Capacitor (formerly "condenser"),
capacitance in an a.c. circuit 3.85 Daniell cell 30.6.01b Direction of induced current 30.6.01 Faraday's law for induced EMF 30.6.01a Faraday's law, Test Faraday's laws 30.6.04 Impedance, phase, resonance 9.2.6 Induced EMF in conductor moving in magnetic
field, open right-hand rule 30.6.02 Lenz's law, force opposing conductor with
induced EMF 30.6.1.1 Magnetic induction inside a coil, intensity
of an induced magnetism 30.6.9 Make a spark 32.5.5 Motor vehicle ignition system 30.7.2 Radio waves, transmitter: amplitude modulation
(a.m.) and frequency modulation (f.m.) 30.6.03 Self-inductance and mutual inductance 30.6.10 Spark in a spark plug 30.7.1 Spectrograph 30.2.0 Transformers 30.3.0 Electromagnets, electromagnetism
30.3.0 Electromagnets, electromagnetism 30.3.1 Electromagnet, Simple electromagnet 30.1.0 Electromagnetism 6.41 Electromagnets (Primary) 30.3.11 Attraction and repulsion of parallel electric
currents 30.3.12 Attraction and repulsion of parallel coils
carrying electric current 30.3.13 Bar magnet in coil carrying current 38.5.4 Crystal microphone 2.175 Cylindrical electromagnet 4.78 Cylindrical electromagnet 30.3.10 Dancing spring, jumping wire, electric
current in parallel coils 30.1.04 Direction of magnetic field using iron
filings 30.1.03 Direction of magnetic field using plotting
compass 32.5.4.4 Electric bell, motor vehicle 30.4.1 Electric bell, door chime 30.4.8 Electric burglar alarm 30.4.2 Electric buzzer 30.4.7 Electric fire alarm 30.4.3 Electric signal using a solenoid 2.180 Electricity from a magnet and a coil 4.83 Electricity from a magnet and a coil 30.1.06 Forces between conductors carrying current
in opposite directions 30.1.05 Forces between conductors carrying current
in same direction 30.1.08 Forces between magnets 30.1.07 Forces between permanent magnet and conductor
carrying current 4.79 Horseshoe electromagnet 30.1.02 Magnetic field of current loop, Earth's
magnetic field 30.3.3 Magnetic field of solenoid 30.3.9 Magnetic field of solenoid and bar magnet 2.178 Magnetic field of electric current in a wire 4.81 Magnetic field of electric current
in a wire 2.179 Magnetic field of open coil, open solenoid 30.3.7 Magnetic field of open coil, open solenoid 4.82 Magnetic field of open coil, open
solenoid 30.3.5 Magnetic field of electric current in a
circular coil 30.1.01 Magnetic field of wire carrying current,
right-hand grip rule, Oersted 30.3.8 Magnetize inside a coil, solenoid carrying
current 2.166 Magnetizing coil 30.4.6 Microphone 32.5.4 Motor vehicle electromagnets 30.3.01 Nail electromagnet 38.5.03 Reed switch, reed relay,
"make-and-break" 30.3.6 Solenoid affects iron nails 2.177 Strength of electromagnets 4.80 Strength of electromagnets 4.2.3 Study an electromagnet 4.2.4 Substances magnetic fields can
pass through 30.3.2 Suspended solenoid behaves like a bar magnet 30.1.3 Motors and generators, d.c. and a.c. 30.1.3.0 Motors and generators, d.c. and a.c. 30.5.0 Alternating current, a.c. circuits 30.5.1 Alternating current, Simple alternator 30.1.3.1 Bicycle dynamo 30.1.3.1.1 Bicycle dynamo, the "missing wire"
in a bicycle generator circuit 30.1.3.02 Electric generator, alternator, a.c.
generator 30.1.3.5 Electric motor 32.5.7.1 Electric motor, Principle
of the electric motor 30.1.3.2 Electric motor spin 30.1.3.3 Electric motor, Simple electric motor 30.1.3.01 Force on current-carrying conductor
in a magnetic field 6.41 Electromagnets See diagram 32.2.1: Simple electromagnet
Be able to make an electromagnet.
Use Pieces of insulated wire, nails about 7 cm long, batteries, pins
or paper clips that can be picked up by a bar magnet.
Winding the wire on the nail. The wire must be wound around the nail
in one direction only. It must not be crossed over and there should be about
thirty turns around the nail.
1. Give each group one piece of insulated wire, one long nail, pins and
paper clips. Put the nail near some pins and see if it is a magnet. Is it
a magnet? [No.]
2. Show the class how to wind a piece of wire around a nail. Test the
nail with the wire on it and see if it is a magnet. Is it a magnet? [No.]
3. Use scissors to cut away one cm of the plastic at each end of the
wire so that the ends are bare. Connect the ends of the wire to the battery
and see if the nail with the wire around it is a magnet now. Is it a magnet?
[Yes.] Pick up some pins with the electromagnet. Now disconnect the wire
from the battery. Is it still a magnet? [No.]
4. The electromagnet is an important part of a petrol engine. When electric
current flows, the magnet is on, when no current flows, the magnet is off.
Extra Activity:
What happens to the "strength" of your magnet when you use:
1. less turns of wire around the nail? [The magnetic strength is less.]
2. two batteries instead of one? [The magnetic strength is greater.] 30.1.0 Electromagnetism
Open right-hand rule, Fleming's left-hand rule, electric motor effect,
induced EMF proportional to rate of change of magnetic flux, EMF = BLV, electric
motor effect, measure magnetic fields, strength of magnetic field, Biot
and Savart, permeability, eddy currents, magnetic fields and forces, fields
and currents, forces on magnets, magnet/electromagnet interactions, force
on moving charges, force on current in wires, torque on coils See diagram: 30.0: Open right hand rule
1. Open right-hand rule, Fleming's left-hand rule: By the open right-hand
rule, the extended thumb points in the direction of the conventional current,
I, the fingers point in the direction of the magnetic field, B, the pushing
palm points in the direction of magnetic force, F.
2. Induced EMF, dynamo, electric generator: A conductor moving to the
right, at a right angle to a uniform magnetic field, will have an induced
EMF, V, across its ends. The thumb points in the direction of the movement
of the conductor because positive charges move in that direction, i.e. the
direction of the conventional current, the fingers point in the direction
of the magnetic field, B, the pushing palm shows the end of the conductor
that becomes positive. The other end becomes negative as electrons move
to that end. The resulting potential difference across the ends of the rod
is called the induced EMF, in volts. If you connect the ends of the moving
conductor through an external circuit, a conventional current will flow
externally from the positive end to the negative end, and inside the conductor
from the negative end to the positive end. The current through the conductor
moving perpendicular to magnetic field will cause a magnetic force to oppose
further movement of the conductor. The conductor can keep moving only if
an applied force is used. The size of the induced EMF depends on the velocity
of the conductor.
3. Electric motor effect, left-hand rule: A current carrying conductor
at a right angle to a uniform magnetic field experiences a force. Use Fleming's
left-hand rule: the first finger points in the direction of the magnetic
field, the middle finger points in the direction of the conventional current,
the thumb points in the direction of thrust, movement of the conductor.
If the current carrying conductor is at less than a right angle to the uniform
magnetic field, the thrust is less. If the current carrying conductor is
parallel to the uniform magnetic field, the thrust is zero.
4. A motor turns a coil in magnetic field, B. The coil with N loops,
each area A, rotates with frequency F for time t, cutting the magnetic
field lines to induce an EMF between its terminals = 2π × NABF × cos 2pft. The
mechanical energy input from the motor that turns the coil = electrical
energy output + (energy lost to heat and friction) 30.1.01 Magnetic field of wire carrying current,
right-hand grip rule, Oersted See diagram 30.01: Right hand grip rule
1. Danish physicist Hans Oersted (1777-1851) in 1819, discovered the
deflection of a compass needle by a wire carrying electric current showing
a connection between electricity and magnetism and that magnetism produced
by a current would generate a force. Perhaps his discovery was by chance
when demonstrating to students or he was looking for a connection between
electricity and magnetism based on philosophical principles, or he had
heard that when lightning strikes the masts of tall sailing ships the ships
compass spins wildly.
2. When a current flows through a straight wire, a magnetic field occurs
in concentric circles around the wire. The right-hand grip rule states
that if you grip a wire carrying current in the right-hand, with the thumb
extended in the direction of the conventional current, positive to negative,
the fingers will be curled around the wire in the direction of the magnetic
field. In the centre of a current loop the magnetic field points in one
direction.
3. An electric current flowing through a wire produces a magnetic field.
The field is cylindrical around the wire and obeys the right-hand grip
rule. That is, by gripping the wire with the right-hand and pointing the
thumb in the direction of the conventional current (+ ve to -ve), I, the
fingers curl around the wire in the direction of the magnetic field, B.
The magnetic field of a solenoid is a uniform magnetic field that follows
a right-hand rule. The strength of the magnetic field of a solenoid, B =
4 piknI, n = numbers of wrapped wire circles, called turns, I = electric
current, k = the constant of magnetic effect.
4. If a magnetic field is at an angle A to the conductor carrying current
then the magnetic force on the conductor will be force × sin A. The
force will be a maximum when A = 90o, and will be zero when
A = 0o. Use the open right-hand rule to find the direction of
the force. If the angle A = 0o, then the current carrying conductor
is now parallel to the magnetic field, and no magnetic force is produced
on the conductor. 30.1.02 Magnetic field of current loop, Earth's
magnetic field
In the centre of a current loop, all contributions to the magnetic field
point in the one direction. The Earth's magnetic field is caused by rotating
loops of charge inside the Earth. 30.1.03 Direction of magnetic field using plotting
compass See diagram 30.2.1.1: Magnetic field from a straight
wire
A magnetic field surrounds a conductor carrying an electric current.
The magnetic field stops when the flow of current stops. Use a plotting
compass to find the magnetic field about a current-carrying conductor.
Push a wire through of a piece of cardboard and connect to an electric
circuit. Move the compass around the wire and record the compass needle
positions with arrows then join the arrows. Reverse the direction of current
flow through the wire so that the compass needle will point in the opposite
direction as you move it around the wire. 30.1.04 Direction of magnetic field using iron
filings
Remove the insulation at both ends of two copper wires. Connect one end
of each wire to the pole of a dry cell. Put the other ends of the 2 wires
in line on a piece of paper. Sprinkle iron filings on the paper between the
2 ends of the wires. Put the iron filings from one end of a wire to the
other. Cover the ends of the wires with iron filings. Close the circuit
and observe the motion of the iron filings. The iron filings are lifted
with the wire because there is a magnetic field around the electric current.
Open the circuit. The iron filings drop immediately from the wire.
30.1.05 Forces between conductor carrying current
in same direction See diagram 30.1.3: Forces between conductor
carrying current in same direction
Use a flat sheet of copper, a 2 volt accumulator, two 15 cm lengths of
copper wire hooked at the top, a switch and connecting wire. The flat sheet
of copper allows the bottom end of the hooked conductors to move if you apply
any forces to them. A and B are pieces of wire supported at the top and hanging
vertically so that they are free to swing in any direction. Current passes
in the same direction through the conductors A and B. Note how A and B move
when current passes. 30.1.06 Forces between conductors carrying current
in opposite directions See diagram 30.1.4: Forces between conductors
carrying current in opposite directions
Rearrange the circuit so that current flows in opposite direction sin
A and B. Observe what force is exerted between A and B when current passes.
30.1.07 Forces between permanent magnet and conductor
carrying current See diagram 30.1.5: Forces between permanent
magnet and conductor carrying current
N and S are the north and south poles of a horseshoe magnet. Close the
switch. Observe how does the conductor moves. Reverse the magnet and later
the direction of the current. Work out the directions of magnetic fields.
30.1.08 Forces between magnets
Bring the north poles of 2 bar magnets near each other. Feel the force
of repulsion between them. Reverse one magnet and feel the force of attraction.
Reverse the other magnet and feel the force of repulsion when the 2 north
poles come together. 30.1.3.0 Motors and generators, d.c. and a.c. Order online: AC/DC Demonstrator Order online: Hand Crank Power Generator
Simple d.c. electric motor, cycle dynamo, simple d.c. generator with commutator,
power station, alternators, rotor and stator, a.c. generator (alternator),
electromagnetic generation principles See diagram 30.1.3: Simple electric motor
1. An electric motor converts electrical energy into mechanical energy.
The coil in an electric motor moves in the same way as the moving coil in
a meter. Wind the coil on the core so that both move round. This is called
an armature. As the coil in the electric motor turns from one side of the
magnetic field to the other, the current through it must be reversed, so
that forces on it will keep it rotating in one direction. Make the electrical
contacts with the coil in the motor through a split ring commutator, so that
the current reverses at the right position.
2. The current flowing through a coil in a magnetic field produces a torque
on the coil = NIAB sin A, where A = angle between the magnetic field lines
and a line at right angles to the plane of the coil. A split ring commutator
reverses the direction of current, I, each time "sin A" changes sign, so
the torque always rotates the coil in the same direction. However, the rotating
coil also acts as a generator to produce a back EMF in the coil that opposes
the source of voltage that drives the motor.
(Potential difference across the terminals of the coil = voltage supplied
to the coil - a back EMF)
(Current through the coil = voltage supplied to the coil - back EMF /
resistance of the coil) 30.1.3.01 Force on a current-carrying conductor
in a magnetic field See 6.3.1.4: Electric current, ampere
If you think of conventional electric current as a flow of positive electric
charges, the electric current will experience a force if it flows through
a magnetic field in the same way that charges moving through a magnetic
field experience a force predicted by the open right hand rule. The force
F (newton) on a straight uniform wire length L in a uniform magnetic field
= I (amperes) × L (metres) × B (tesla) sin A, where A = angle
between the direction of the current, I, and the direction of the magnetic
field. If the wire is parallel to the magnetic field lines, the force is
zero. If the wire is at right angles to the magnetic field lines, the force
is maximum. 30.1.3.02 Electric generator, alternator, a.c.
generator
An electric generator is a machine that converts mechanical energy into
electrical power. Electromagnetic generators work by either 1. rotating a
magnet inside a coil, e.g. bicycle "dynamo" or 2. rotating a coil inside
a magnet, e.g. motor car generator, power station..
An electromagnetic generator that produces alternating current it may
be called an alternator, but if it produces direct current it may be called
a dynamo. A bicycle dynamo, e.g. the popular sidewall-running bottle-shaped
"dynamo", produces alternating current, so it really is a bicycle "generator"
if this distinction is made. See diagram 32.5.6.4:
Voltage of a generator
In a motor vehicle, a generator gets mechanical energy from the fan belt
and, in turn, produces electrical energy that charges the battery by being
converted into chemical energy converts some electrical energy into heat
in the armature and the connecting wires. Some chemical energy from the
battery may be converted back to electrical energy that may be converted
into light, or sound when the horn is pressed. The generator has an inner
set of coils that become an electromagnet that turn inside an outer set
of coils. 30.1.3.1 Bicycle dynamo
A dynamo converts mechanical energy into ac or dc electrical energy usually
by rotating a conductor in a magnetic field. The 4 different ways to get
a dynamo, to give a higher current are as follows: 1. Use stronger magnets.
This generates a higher voltage that forces a higher current in the circuit.
2. Reduce the load, i.e. the resistance of the external circuit of lamps
and appliances. 3. Rewind the dynamo with heavier wires because its own
internal resistance of windings may not carry a bigger current without
overheating. 4. Run it faster.
People who ride bicycles with lights powered by bicycle dynamos are aware
that the faster they ride the brighter the headlight. To demonstrate the
effect of speed in the laboratory, mount the dynamo on a spindle that can
be turned with a crank. Complete the circuit by connecting the electrical
output leads to 1.5 V lamp in a lamp holder to a galvanometer, 2.0 - 2.5
MA d.c. The faster you turn the crank the brighter the light or the greater
the reading on the galvanometer 30.1.3.1.1 Bicycle dynamo,
the "missing wire" in a bicycle generator circuit
In a bicycle dynamo, the rotor is a permanent magnet, usually an 8 pole
circular magnet and the e.m.f. is induced in two coils that are stationary
in reference to the rotor. Only one wire connects a bicycle generator to
the bicycle light. Where is the other wire to complete the circuit? When the
bicycle is in motion, a permanent magnet inside the generator rotates in
the centre of a coil of tightly wound copper wire. The spinning magnet creates
electricity in the copper coil. Electricity flows through the wire to the
light, through the filament of the bulb, through the light casing, the bicycle
fork, and the metal casing of the generator, back to the coil in the generator.
A screw in the generator casing pierces the layer of paint on the fork to
complete the circuit. 30.1.3.2 Electric motor spin See diagram 30.1.3.2: Spinning magnets
Put 3 ALNICO bar magnets in a line, south north | south north | south
north, such that the middle magnet is in a small glass beaker. Turn the beaker
through a small angle. Tap the beaker. Its magnet returns to the original
line with the opposite poles nearest. Reverse one end magnet. Turn the beaker
through a small angle. Tap the beaker and the magnet does not return to
the original line. In an electric motor a coil carrying electric current
replaces the magnet in the beaker. 30.1.3.3 Simple electric motor See diagram 29.181.1: Simple electric motor
1 | See diagram 29.181: Simple electric motor 2 |
32.5.7.1 Principle of the electric motor,
shunt-wound motor, series-wound motor
1. horseshoe magnet, 2. axle, 3. commutator, 4. coil, 5. brass strip,
6. electric motor with 3 coils, A Contact (brush), Aw Wire from contact to
coil, B Contact (brush), Bw wire from contact to coil 1. Fix a simple coil, mounted on an axle, between
the poles of a horseshoe magnet. Two wires from the coil connect to the
commutator. The commutator is a cylindrical insulator revolving on the axle
with two strips of brass attached. The commutator rotates with the coil.
Each brass strip is joined to one wire from the coil. Two carbon contacts,
brushes, touch the side of the commutator and allow electric current to
pass from the battery to the commutator. Electric current goes from the
battery to brass strip A then along wire Aw, through the coil then back through
wire Bw and brass strip B then back to the battery to complete the circuit.
When the commutator and coil make one half turn, the current enters through
brass strip B and returns through brass strip A, reversing the current in
the coil. The electric motor runs more smoothly if more than one coil is
used. This electric motor uses a permanent magnet but most electric motors
use a field coil that forms a more powerful electromagnet.
2. Using Fleming's left hand rule, direction of thumb is thrust, first
finger is magnetic field and second finger is current. In the diagram, side
7 to 8 of the coil has upward force on it and side 9 to 10 has downward force
on it. So the coil turns until it it vertical and the brushes no longer
touch the brass strips because of the gaps between them, and no current
flows. However, due to inertia of the commutator, the coil keeps turning
so side 7 to 8 is now on the right side and side 9 to 10 is on the left side.
The brushes touch the brass strips again and the coil keeps turning clockwise.
3. Use strong bar magnets, bent pieces of tin plate, a thick piece of
cardboard wound with many turns of thick 22 gauge wire. Fray the ends of
the wire so that they brush against 2 tin plate contacts connected to dry
cell batteries. To start the motor, give a little push to get the current
moving. Instead of the cells supplying the brushes with current, insert a
torch globe receiving current from the brushes. Turn the armature shaft with
a pulley and belt.
4. Field magnets and brushes
Push a 15 cm nail up through the centre of a 20 × 25 cm card to
act as a vertical bearing. Wind a coil 100 turns of bell wire on 2 nails to
act as field magnets. Push the nail down into the cardboard 15.5 cm apart,
each side of the spike. Push down 2 small nails on the diagonal and 5 cm
from the centre of the card. Strip insulation from the ends of each coil so
that you can twist the wire twice around the small nails then touch the central
large nail. The touching ends will act as brushes. Check the direction of
windings of the coils from the diagram. Attach the other ends of the coils
in the field magnets to screws in the corners of the card. Make the armature
coil. Push a 15 cm nail crosswise through a 4 cm cork. Wind 40 turns of
bell wire on the nails, with the direction of windings as shown in the diagram.
Strip insulation from the ends of each coil. Cut out part of the centre
of the lower side of the cork so that the end of a test-tube fits tightly.
5. Commutator
Cut out a rectangular piece of thin sheet copper 4 cm × circumference
of the test-tube. Cut off 12 mm from the circumference length then cut
the piece of copper in half. Fit the copper pieces to curve around the
test-tube with a gap of 6 mm between them. Make a small hole in each copperplate
to attach one end of an armature coil. Fix the copper plates in position
around the test-tube as a commutator. Set up the rotor. The armature and
commutator form the rotor. Set the rotor into position on the vertical bearing.
Adjust the brushes to contact the commutator. Turn the test-tube within
the cork until the brushes lie across the gaps in the commutator when the
armature is in line with the field magnets. Run the electric motor. Connect
the screws to the terminals of a dry cell or lead cell accumulator using
a car headlight bulb to limit the current, or low voltage power supply.
Give the rotor a slight push and it should keep turning. If the rotor does
not turn check the contact of the brushes with the commutator. 30.1.3.4 Principle of electric motor
Suspend a piece of fine wire vertically between the ends of a horseshoe
magnet. Connect the wire to a torch cell. Note what happens when you make
the connection. Reverse the connections on the cell and observe the result.
Note whether the magnet and the source of electrical energy both seem necessary
to produce the effect you have observed. The electric motor uses the same
principle. 30.1.3.5 Electric motor See diagram 30.1.3.5: Electric motor
Make an electric motor from a wooden base, tin plate supports, 3 iron
wire paper clips, seven drawing pins, 2 metres covered copper wire and cellulose
tape. Remove insulation from the brushes and the armature contacts (commutator)
held at 90o to the coil by thin strips of adhesive tape. For
the coils, use 100 turns on each side, all in the same direction. Bend the
field electromagnet with the tips near the patch of the armature. Punch
the holes in the tin plate with a nail. Sketch the positions of the magnetic
poles for different positions of the armature. Alternate attraction and repulsion
between the two electromagnets makes the armatures revolve. Many variations,
e.g. use of a permanent magnet for the field magnet, are possible. 30.2.0 Transformers
Transformer, electromagnetic induction and transformers, measurement
of voltage transformation under step-up or step-down modes of connection,
mutual induction, energy loss in a transformer: resistance windings, eddy
currents, leakage of field lines See diagram 30.3.2.01: Transformers
1. Wind 2 coils of insulated wire on the same iron core, the primary
coil and the secondary coil. If you pass a.c. through the primary coil,
you produce an alternating magnetic flux in the iron core that passes through
the secondary coil and an induced EMF in the secondary coil. The output
voltage in the secondary coil and the input voltage in the primary coil
are related by the number of turns on the secondary compared with the number
of turns on the primary. A step-up transformer, higher output voltage, has
the number of turns of the secondary coil greater than the number of turns
of the primary coil. A step-down transformer, lower output voltage, has the
number of turns of the secondary coil less than the number of turns of the
primary coil. In theory, power input is equal to power output. Direct current
cannot be stepped up or stepped down with a transformer because in a transformer
a changing magnetic field cuts a conductor but there is no changing magnetic
field from steady d.c. electric current.
2. Examine a step-up and step-down transformer. Use a step-up transformer
to supply 6 volts a.c. to d.c. and light a 12 volt lamp at AB. Use a step-down
transformer to supply 12 volts a.c. to AB, measuring the current in the
AB turns. Take off 6 volts a.c. from CD to light up a large 6 volt lamp.
Measure current with an ammeter to show that if you drop the voltage by
half you double the current, and if you double the voltage you halve the
current. 30.3.0 Electromagnets
Simple electromagnet, strength of an electromagnet, current in a coil,
turns of the coil, distance between poles
Electromagnets consist of insulated wire wound around a soft iron core.
Electromagnets are designed to have magnetic effects only when electric current
passes through the winding. Electromagnets are used in switches, electric
bells, door chimes, relays, metal-lifting cranes, telephones, microphones,
loudspeakers. You use permanent magnets to produce the magnetic fields in
magnetos and some alternators. However, a permanent magnet has a constant
magnetic field and so is less useful than an electromagnet whose field you
can vary. Electromagnets are used in the generator cut-out, voltage regulator,
horn relay and some brakes and clutches to close or open a set of contacts
(a switch) in an electric circuit and to provide a magnetic field that you
can control automatically or manually. 30.3.01 Nail electromagnet To study the magnetism and polarity of an electromagnet,
wrap 20 turns of wire around a large nail. Use the connecting wires to
connect the nail to a power supply via a touch bulb. Here the role of the
bulb is to show if the circuit is on or off. Set the power supply to 2,
4, 6 volts d.c., and turn it on in turns. Each time, use a pocket compass
to test which is the north pole and south pole of the electromagnet. Reverse
the connections to the power supply under the condition of the same voltages.
Use the head of the nail to attract pins and note the number of the pins
attracted. Record the phenomenon under each voltage. Increase the number
of the wire turns of the nail to 40 turns. 30.3.1 Electromagnet, simple electromagnet
1. Use an electrical screwdriver or a big iron nail or a bolt as a core
for the electromagnet and 1 metre of insulated copper wire, e.g. SWG 26.
Leave 20 cm of the wire for connecting in the circuit then wind one coil of
the wire about the core. The turns of wire must be close together and all
wound in the same direction. Connect the 2 ends of the wire to a dry cell
and insert a switch, an ammeter and a rheostat in the circuit. Fix the electromagnet
vertically using a wooden stand or pegs, not iron. You can test the strength
of this electromagnet by observing how many pins or paper clips it can pick
up. Close the circuit and use the rheostat to control the current, e.g. 0.25
A, 0.5 A, 0.75 A, 1.0 A Record for each strength of current how many pins
or paper clips the electromagnet can pick up for the magnetic strength when
you switch on the current. Observe the magnetic strength when you switch
off the current. The bolt may still attract the pins for a short time if
it is made of steel and enough current has passed through it but a soft iron
nail cannot still attract pins. Check your results when you reverse the current
in the electromagnet. Increasing the number of coils makes the magnetic effect
stronger.
2. Wind more coils of insulated copper wire around the core and repeat
the experiment. Record how the number of coils affects the strength of the
electromagnet.
3. Hold the electromagnet vertically. Hold a plotting compass next to
the electromagnet and observe which end of the electromagnet is the north
pole using the right-hand grip rule.
4. Lay the electromagnet sideways on the bench. Hold a piece of stiff
white paper over it and sprinkle iron filings on the paper. Compare the pattern
of iron filings to the pattern formed when you do this experiment with a
bar magnet.
30.3.2 Suspended solenoid behaves like a bar magnet See diagram 30.3.2: Suspended solenoid | See diagram 30.3.9: Solenoid behaves like a bar magnet
1. A solenoid is a long coil of insulated wire that can be wound on a
broom handle, a pencil or a glass tube. Wind many turns of thin wire on 8
cm of glass tubing. Support the solenoid freely and switch on the current
and observe that it behaves like a bar magnet.
2. Make a solenoid by wrapping a piece of insulated copper wire around
a pencil. Suspend the solenoid horizontally so that it is free to rotate and
pass a current through it. Observe what happens when current passes through
the coil. Observe what happens when the current is reversed. Place an iron
rod inside the coil. Note whether the response more or less than before.
Mark the end of the coil that points north. Bring a bar magnet near the
coil. When current flows, one end of the bar magnet will attract one end
of the coil and repel the other end.
3. Repeat, using the other end of the bar magnet and reversing the current
in the coil. With the iron rod in the coil and the current on, attach a
string of nails to the end of the rod. Switch the current off. Observe what
happens to the nails. 30.3.3 Magnetic field of solenoid See diagram 30.3.3: Magnetic field of solenoid
1. Place a card through a solenoid. The solenoid could consist of many
turns of a wire spring or a thin wire twisted round a suitable former, e.g.
a test-tube. Connect the solenoid to a switch and battery and sprinkle iron
filings on the cardboard. Note if you see a similarity in the patterns produced
by the bar magnet and the solenoid.
2. Use a magnetic field about a conductor wind a coil of wire around
it. The magnetic fields created by the current in the several loops of
the coil combine into one composite field as to create a solenoid. The
strength of the magnetic field of a solenoid is proportional to the number
of turns of wire in the coil and the current flowing through the wire. You
can measure the strength of the magnetic field of a solenoid in ampere
turns, i.e. current in amperes × number of turns. If a soft iron
core is inside a solenoid, the lines of magnetic force will take the easier
path through the iron, and the magnetic field produced at the ends of the
iron core will be stronger. To make a coil, wrap an insulated connecting
wire around a small test-tube turn by turn and leave connecting wires. Peel
off their end insulation layers then connect them to a d.c. low voltage
electrical source. If you use a dry cell battery do not turn it on for too
long a time. Place several large needles into the test-tube. Turn on the
electrical source. Note the needles' directions such as their needle points
pointing to the bottom or mouth of the test-tube. Take the needles out of
the test-tube and let them to attract fine iron filings or pins to observe
every needle's magnetism intensity. 30.3.5 Magnetic field of electric current in a
circular coil
Wind some turns of insulated copper wire around an iron nail. Switch
on power supply and watch the iron flings on the piece of paper. Switch
off the current and watch the iron filings. More filings are attracted to
the nail when the current is on. The nail becomes a magnet when the current
flows around it. Most of the iron filings fall off when you switch the current
off which shows that you have not permanently magnetized the iron. However,
some filings still cling to the nail showing that the nail must retain some
magnetism produced by the electric current.
30.3.6 Solenoid affects iron nails
Wind fifty turns of cotton covered copper wire on an 8 cm length of glass
tubing, 0.6 cm in diameter. Such a coil is called a solenoid. Connect this
to your power supply with a switch. Make a better switch with a piece of
springy brass from an old torch battery and several thumbtacks. Place a
nail halfway into one end of the glass tube and switch the current on for
a second. The electron current in the coil produces a strong magnetic effect
around it and this pulls the nail into the coil.
30.3.7 Magnetic field of open coil, open solenoid 1. To observe the distribution of magnetic
lines of force inside an open solenoid, wrap 5 turns of single electric
connecting wire around a cylinder with a smooth surface and leave connecting
wires. Put the coil off the cylinder and separate theses turns at certain
distance. Use a cardboard cylinder then cut off 2 grooves, making sure each
groove is wider than the diameter of the connecting wire and the distance
between the 2 grooves is narrower than the inner diameter of the coil, so
that the bar of the cardboard between the 2 grooves can support the coil
when you insert the coil into the grooves. Connect the 2 ends of this coil
to a d.c. source. Connect a slip rheostat in series to prevent the electrical
source overloading. Scatter fine iron filings on the cardboard. Gently tap
the cardboard then observe the distribution of the iron filings. Usually
you call such a magnetic field a magnetic field inside an open solenoid.
If there is no space between turns of a coil, you call it a closed solenoid.
This method is also used to observe magnetic field inside a closed solenoid.
B. Another method is to substitute sheets of plastic or Plexiglas for
the cardboard. Cut holes in the sheets corresponding to turns of the coil. 30.3.8 Magnetize inside a coil, solenoid carrying
current
Use an electrical source, which may provide 5A current. Connect it to
a solenoid and a switch. Use a 4 cm × 5 cm piece of sheet iron as
a movable gauge. At one end solder a piece of wire to move it. At the other
end solder a metal ball to make the gauge keep in a vertical plane. Attach
a piece of wire to the solenoid with adhesive tape. Place the removable
gauge into the solenoid then turn on the switch to make the solenoid current.
Here the solenoid carrying current produces magnetic field inside the solenoid.
If you magnetize the gauge and wire at the same time, they repel each other
so that the gauge is moved. The distance moved depends on the size of the
magnetic force between the gauge and the wire. The magnetic force depends
on the magnetization characters of the materials of the gauge and wire and
the size of the current at the solenoid. So the size of the current may
be estimated according to the distance. 30.3.9 Magnetic fields of a solenoid See diagram 30.3.9a: Magnetic fields of a solenoid
To map the magnetic fields of a solenoid and a bar magnet, draw 2 parallel
lines 2 cm apart on a 10 cm2 sheet of cardboard. Punch two rows
of holes through the cardboard, 0.5 cm apart. Using d.c. C22 gauge copper
wire, wind a solenoid onto the cardboard by threading the wire through
the holes. Support the cardboard horizontally, sprinkle iron filings over
it and connect your solenoid to a 6 volt battery. Tap the cardboard and
notice what happens to the iron filings. The little pieces of iron have
themselves become magnetized by induction and, when jostled by tapping,
set themselves in the direction of the fields at the various positions.
30.3.10 Dancing spring, jumping wire, electric current
in parallel coils (Use of open surface mercury is illegal in some school
systems!) See diagram 30.3.10: Dancing spring
This experiment shows the effect adjacent coils have on one another if
the current is running in the same direction in each coil.
Current is passed through a limp copper spring dangling in a pool of
mercury causing it to dance. A helix of fine wire hanging vertically into
a pool of mercury contracts and breaks contact repeatedly. A wire is placed
in a horseshoe magnet and connected to a battery. The wire jumps out of
the magnet. A wire is placed in a horseshoe magnet and connected to a battery.
A large heavy wire clip rests in pools of mercury between the poles of a
strong magnet. An aluminium bar in a magnet has its ends in mercury. Short
the mercury pools to a storage battery and the aluminium bar hits the ceiling.
A wire hangs into a pool of mercury and between the poles of a U shaped
magnet. s current is passed through the wire it deflects out of the mercury
and breaks the circuit. A coil of wire wound around one pole of a horseshoe
magnet jumps off when energized. Run twenty amps through a wire in a horseshoe
magnet. 30.3.11 Attraction and repulsion of parallel electric
currents (Use of open surface mercury is illegal in some school systems!) See diagram 30.3.11.0: Parallel electric
currents | See diagram 30.3.11.1: Electric
compass | See diagram 30.3.11.2: Magnetic effects
of currents in a coil
Andre Ampere showed that parallel electric currents attract each other
if they move in the same direction and repel if they move in opposite direction
because of the magnetic fields they generate. However, the force of attraction
is negligible at low current values. An electric current passing through
a coil acts like a magnet
1. Use two copper wires 50 cm long on copper loops so that their free
ends are 1 cm apart and hang in mercury.
A. Connect one terminal of a 6 volt battery to the upper ends of both
copper wires and connect the other terminal to the mercury. Turn the switch
on and observe any movement of the copper wires. In this experiment the electrons
were flowing in the same direction in the 2 wires.
B. Change the circuit to make the current flow in opposite directions
in the wires. Turn the switch on and observe any movement in the wires,.
Notice what happens.
C. To show that the forces between wires carrying electric current are
not electrostatic, hold a strip of charged Perspex then a permanent magnet
near the wire. The magnet does not attract small pieces of paper or wood shavings.
Charged Perspex does attract small pieces of paper or wood shavings.
2. To avoid the use of mercury on this experiment, informants reported
that they replaced the mercury pool with a saturated solution of sodium chloride.
The informants observed significant hydrolysis and was able to make coils
that interacted with magnetic fields from bar magnets brought into proximity.
However, they were unable to observe anything more than a very tiny movement
of the coil by itself in one instance, certainly no "dancing". They tried
various sources of copper wire of different gauges with a large six volt
battery to no avail.
3. Wind a coil around a perspex rod. Attach each end of the coil to a
battery and use a hook to raise the wires and so suspend the coil around
the perspex rod. Close the circuit and the coil behave like a suspended
bar magnet affected by a magnetic field.
4. Use a bar magnet to determine the magnetic effects of current in a
coil. 30.3.12 Attraction and repulsion of parallel coils
carrying electric current See diagram 30.3.12: Attraction and repulsion
of parallel coils carrying electric current
1. Wind 2 coils each of 20 turns of insulated copper wire around a plastic
drink bottle. Fix the coils so that they are standing upright and 2 cm
apart. Connect both coils to a 6 volt battery so that the currents flow
around the 2 coils in the same direction.
2. Change the connecting wires to make the current flow in the opposite
direction. When the currents flow in the same direction, the coils move together.
When the currents flow in opposite direction, the coils move apart. 30.3.13 Bar magnet in coil carrying current See diagram 30.3.12: Bar magnet in coil carrying
current
1. Hold one end, then the other end, of a bar magnet near one coil carrying
current. Note the attraction or repulsion depending on the end of the magnet
and the direction of current through the coil. The circular coil carrying
current behaves like a bar magnet because of electric currents within the
iron. This is happening in a magnet.
2. Repeat the experiment by substituting a balanced compass needle for
the bar magnet. 30.3.14 Nail in coil carrying current
Use a big nail that will not pick up iron filings. Hold it inside a coil
and turn the current on and off. The nail will now pick up iron filings. 30.4.0 Application of electromagnets 30.4.1 Electric bell See diagram 30.4.1: Electric bell
When the switch is pressed, current flows and the hammer is drawn back
to strike the bell. As it moves, current is cut off and the hammer falls back
into position. While the switch is on, the hammer moves back and forth. The
copper side of the bimetallic strip is up and the iron side is down. Adjust
the bimetallic strip so that it is just clear of the terminal on the dry
cell when it is cold. Heat the strip so that it bends, touches the terminal
of the dry cell, closes the circuit and the electric bell rings. 30.4.2 Electric buzzer See diagram 32.4.2: Electric buzzer
Wind insulated copper wire around the bolt, clamp it in position and
connect one end of the wire to a battery. Clamp a saw blade firmly at one
end so that its other end is near one end of the bolt. Attach the other
end of the wire to the saw blade. Hammer a long nail through a thick piece
of wood. Bend the nail so that its point touches the middle of the saw blade.
When a switch is closed, electric current flows through the circuit: battery,
coil, bolt, saw blade (near the clamp), bent nail, switch, battery. The
electric current makes the bolt become a magnet which attracts the end of
the saw blade the end away from the clamp. This movement breaks the contact
between the middle of the saw blade and the bent iron nail so no electric
current flows through the circuit. Now the bolt is no longer a magnet and
so the saw blade springs back to its original position. The saw blade keeps
moving backwards and forwards hitting the bolt or the nail and causing a
buzzing sound. 30.4.3 Electric signal using a solenoid See diagram 30.4.3: Signal
When the nail is pulled into the solenoid coil the string pulls the signal
arm up. Put the signal post down at one end of the bench and the switch
at the other. Obtain some plastic covered wire for the telegraph line to
join up with. The more nearly vertical you have the solenoid the better. 30.4.6 Microphone See: 26.9.01: Transducer, carbon
microphone in a telephone
Push 2 pencils connecting wires through the short sides of a matchbox,
just above the base. Scrape off some of the surface, and do the same with
a shorter connecting wire, which you lay across the top. Connect the microphone
with a battery and earphone from a transistor radio in the next room. Hold
the box horizontally and speak into it. Your words can be heard clearly in
the earphone. The current flows through the graphite "leads." When you speak
into the box, the base vibrates, causing pressure between the "leads" to
alter and making the current flow unevenly. The current variations cause
vibrations in the earphone. 30.4.7 Electric fire alarm See diagram 30.4.7: Electric fire alarm 30.4.8 Electric burglar alarm See diagram 30.4.8: Electric burglar alarm
Set up a model electric burglar alarm with a fine cotton thread holding
a switch open. Breaking the thread by opening a window or a door closes the
switch and rings the electric bell. If the bell is in the watch house, the
burglars may be caught. 30.5.0 Alternating current, a.c. circuits See diagram 30.10: When side BC of the coil is
in positions 1. and 3., the coil is moving in the same direction as the magnetic
field from North to South and so no e.m.f. is produced. When side BC of
the coil is in positions 2. and 4., the coil is moving perpendicular to
the magnetic field from North to South and maximum e.m.f. is produced. See diagram 32.5.6.2A: Alternating current,
graphs A | See diagram 32.5.6.2B: Alternating
current, graphs B See pdf: Make
alternating current visible
A.C. generators (alternators) operate on the same principles of electromagnetic
induction D.C. generators. Alternating voltage is generated by rotating
a coil in a magnetic field or by rotating a magnetic field within a stationary
coil. The value of the voltage generated depends on the number of turns
in the coil, the strength of the magnetic field, and the speed of rotation
of the coil or magnetic field.
The transmission of high voltage alternating current, a.c., over long
distances is more efficient than the transmission of direct current, d.c.
Currents that vary periodically in their size and direction with the
time are called alternating current.
Alternating current, like the wave, has its frequency, period, amplitude
and phase. Classify the alternating current into low frequency and high
frequency. Lighting circuits use the low frequency of a sine wave.
Frequency of alternating current
The frequency of alternating current is the number of complete alternations
(cycles) in 1 second., measures in hertz (Hz) (cycles per second, c /sec.).
In many countries. the mains supply has frequency 50 Hz, so 1 alternating
current cycle lasts 1 / 50 = 0.02 seconds
Root mean square values
Instantaneous current, I, varies with time: I = Io ×
sin 2πi × f ×
t, where Io = peak value.
As alternating current fluctuates from positive to negative vales it
is measured by its peak value or by its root-mean-square value, RMS (r.m.s.)
RMS = peak value / √2. This is the steady direct voltage or current
that would give the same heating effect.
Alternating current is produced by an alternator, i.e. a synchronous
alternating current generator. In the a.c. generator, alternator, mechanical
energy turns a coil in a magnetic field, B, and a variable EMF is induced
across the ends of the coil. Permanent sliding contacts are made with the
ends of the coil. A given contact will change from positive to negative,
depending on the relative direction in which that side of the coil is moving
through the magnetic field. slip rings and contacts. If the coil moves clockwise
in the magnetic field, an end-on view of the sides of the loop shows how
the EMF is alternating.
At positions 1 and 3, the coil is moving in the same direction as the
magnetic field B so no EMF is produced.
At positions 2 and 4, the movement of the coil is perpendicular to the
magnetic field B so maximum EMF is produced.
An alternator produces an EMF that changes from positive to negative
so the average will be zero.
The effective EMF is found by squaring the maximum positive EMF (peak
value), and the maximum negative EMF (peak value), adding them together,
dividing by two, and taking the square root.
The root mean square value, RMS = peak value / √2 = peak value ×
approx. 0.71. The RMS values for alternating voltage and current (a.c.)
are equivalent to the same values for direct voltage and current (d.c.).
Average power = Vrms × I rms. Australian and British power systems
operate at 240 volts RMS, at 50 cycles per second (Hertz). The peak values
are 340 volt. American and Japanese systems use 110 volts RMS and 60 cycles
per second.
The alternator or a.c. generator has permanent sliding contacts with
the ends of the coil so that a given contact will change from positive
to negative, depending on the relative direction in which that side of
the coil is moving through the magnetic field. If the coil moves clockwise
in the magnetic field, the EMF is alternating to produce a sine wave pattern
of induced EMF characteristic of alternating current, a.c. When the coil
is moving in the same direction as the magnetic field, no EMF is produced.
A coil moving perpendicular to the magnetic field produces maximum EMF.
The waveform is sinusoidal. As an alternator produces an EMF that changes
from positive to negative the average will be zero.
So the effective EMF is measured by squaring the maximum positive EMF
(peak value), and the maximum negative EMF (peak value), adding them together,
dividing by two, and taking the square root.
The root-mean-square value (RMS) =(1 / √2) × peak value =
0.71 × peak value.
The RMS values for alternating voltage and alternating current (a.c.)
are equivalent to the same values for direct voltage and direct current (d.c.).
Average power = Vrms × Irms.
The RMS values for a.c. in the British power system is 250 volts RMS,
at 50 cycles per second (50 Hz) and Australian power systems is 240 volts
RMS, at 50 cycles per second (50 Hz). The peak values are 340 volt. American
and Japanese systems use 110 volts RMS and 60 cycles per second (60 Hz). 30.5.1 Simple alternator See diagram 30.6.7: Simple alternator
Use 2 solenoids, an inner and an outer solenoid. Observe the movement
of the pointer of the centre zero galvanometer when the inner solenoid is
stationary. The galvanometer pointer does not move. Observe the movement
of the pointer when the solenoid supported by a light spiral spring moves
up and down. The galvanometer pointer moves each side of the zero mark in
a regular manner corresponding to the motion of the coil. The current passing
through the galvanometer is a simple form of alternating current. 30.6.0 Electromagnetic induction See diagram 30.6.0: Force on current in magnetic
field, Fleming's left hand rule
Induced EMF in conductor moving in magnetic field, open right-hand rule
1. An electromotive force, EMF, is produced in a circuit by a change
of magnetic flux through the circuit or by relative motion of the circuit
and the magnetic flux. In a closed circuit an induced current will be produced.
This phenomenon is called electromagnetic induction. The electromotive force
is called inducted electromotive force.
2. Electric current is produced in a closed circuit when there is relative
movement of its conductor in a magnetic field. There is no battery or other
source of power in a circuit in which an induced current appears because
the energy supply is provided by the relative motion of the conductor and
the magnetic field. The magnitude of the induced current depends upon the
rate at which the magnetic flux is cut by the conductor. Its direction is
given by Fleming's left hand rule.
3. If current flows through a long straight wire, the strength of the
magnetic field at distance from the wire depends on the property of the substance
in which the magnetic field is measured called magnetic permeability, µ..
For a vacuum, or air ("free space") µ0 = 4π × 10-7.
4. Relative permeability compares the permeability of a substance with
the permeability of free space. Diamagnetic materials, e.g. lead, have relative
permeability less than 1 so they decrease the strength of a magnetic field
in a solenoid. Paramagnetic materials, e.g. aluminium, have relative permeability
slightly more than 1 less so they slightly increase the magnetism in a
solenoid. Ferromagnetic materials, e.g. iron and iron alloys, have relative
permeability more than 50. So the magnetic field of a solenoid coil is
much greater if it has a "soft" iron core. In a long coil solenoid, the
right-hand grip rule on any coil shows the direction of the uniform magnetic
field in the solenoid. 30.6.01 Faraday's law for induced EMF
Faraday's law of electromagnetic induction states that the induced EMF
(volts) of a conductor moving through a magnetic field depends on the rate
of change of magnetic flux (weber).
In a coil with n loops,
(Induced EMF = - n × change of magnetic flux / time)
Use "-" because induced EMF opposes the change that produces it, as in
Lenz's law.
If a conductor wire length L moves with velocity v in a magnetic field,
B, cutting field lines, it contains an induced EMF = BLv (B, v and direction
of conductor wire mutually at right angles). 30.6.01a Test Faraday's
laws See diagram 30.6.01a: Test Faraday's laws
The induced e.m.f. increases with the speed of the magnet or the coil
relative to each other, the number of turns in the coil, and the strength
of the magnet.
Test Faraday's laws by 1. vary the speed of moving the magnet inside
the coil, 2. decrease or expand the number of turns in the coil while moving
the magnet at constant speed, 3. use a stronger magnet and compare the deflection
in the galvanometer. 30.6.01b Direction of induced
current See diagram 30.6.01b: Direction of induced
current
Fix a wire between the poles of an Ushered magnet. Attach each end of
the wire to a galvanometer. Move the magnet in three directions and note
any deflection of the pointer in the galvanometer. The wire does not move.
Direction 1. AB, the magnet moves vertically up and down.
Direction 2. CD, the magnet moves horizontally, so that one of the poles
becomes closer or further away from the wire.
Direction 3. EF, magnet moves horizontally, forwards and backwards in
the direction of the wire.
In each case, when the magnet reverses direction, the pointer of the
centre zero galvanometer changes the direction off deflection. 30.6.02 Lenz's law, force opposing conductor with
induced EMF See diagram 30.6.02: Lenz's law Order online: Lenz's law tube,
slow-falling neodymium magnet
1. The magnet approaches the coil downwards with its north pole nearest
the coil so the induced current flows from right to left so that the coil
behaves as if it is a magnet with its north pole nearest the magnet.
2. The magnet moves away from the coil upwards with its north pole nearest
the coil so the induced current flows from left to right so that the coil
behaves as if it is a magnet with its south pole nearest the magnet.
An induced current will cause a current flow in a direction to oppose
the cause of the induced current. This law is an example of the principle
of conservation of energy
An induced EMF is found in a loop of wire when there is a change in the
magnetic flux around the loop of wire. Lenz's law states that if an EMF is
induced in a loop of wire, it will produce a current whose magnetic flux
will always oppose the original change of magnetic flux. For example, if a
magnet is pushed into a coil, the induced EMF. in the coil will produce a
current whose magnetic field tends to push the magnet out. If a magnet is
pulled out of a coil, the current produced by the induced EMF in the coil
will tend to pull the magnet back in again. If mechanical energy is used to
turn a coil in a magnetic field then a variable EMF will be induced across
the ends of the coil. Lenz's law follows from the law of conservation of energy. 30.6.03 Self-inductance and mutual inductance
Coils are turns of insulated copper wire to increase called inductance
depending on how many turns, distance between turns, diameter of the coil,
and what they are wound on. While magnetic flux is changing inside a coil,
a back EMF is induced which opposes the changing current = L (change in current
/change in time) = L × the rate of change of current, where L = self-inductance
constant of the coil, in henry, H. One 1 henry, H, = EMF of 1 volt induced
in a circuit by electric current change of 1 ampere per second. Self-inductance
occurs when a.c. passes through a coil because, if the current in a coil
changes, the magnetic flux through the coil caused by the current also changes,
so the changing current induces an EMF in the same coil. The current lags
behind the EMF by a quarter of a cycle (90o). When the change
in magnetic flux magnetic flux from one coil, the primary coil, is experienced
by a second coil, the secondary coil, an EMF is induced in the secondary
coil = M × (change in primary coil current / change in time), i.e. M
× time rate of change in the primary current, where M = the mutual inductance
of that 2 coil system. When a.c. passes through a coil, the changing magnetic
flux induces a back EMF in the coil which opposes the changing current.
Self-inductance of the coil is measured in henry, L. The current lags behind
the EMF by a quarter of a cycle, 90o. 30.6.04 Impedance, phase, resonance
1. Total impedance, Z, is the total opposition to current flow from 1.
inductance, L 2. resistance, R, and 3. capacitance, C, in an a.c. circuit.
Impedance Z = √ (R2 + [XL - XC]2).
Only the ohmic resistance, R, dissipates electrical energy as heat.
2. Resonance in series a.c.
Maximum current will flow in a circuit affected by inductance, resistance,
and capacitance when the inductive reactance XL cancels the capacitor reactance
× C. 30.6.1 Electromagnetic induction See diagram 30.6.8
Observe the current in the secondary when:
1. the primary current is started or stopped,
2. the primary current is increased or decreased by means of the variable
resistance,
3. a bar magnet is moved in and out of the secondary coil.
Observe no current in the secondary coil when conditions are steady. 30.6.1.1 Magnetic induction inside a coil, intensity
of an induced magnetism See diagram 30.2.5 | See 9.2.6: Transform electromagnetic energy
to kinetic energy
1. Use 1 A ammeter and 5 A ammeter, two slip rheostats, two switches,
d.c. current source, solenoid, current balancer. Connect a circuit as in
the diagram. Measure the width of the current balancer, L (See diagram 30.2.5c)
and record the reading at Table 30.2.5.1. Check the two arms of the current
balancer to see whether the contacts are well made, if not, polish them
with sand paper. Put the current balancer inside the solenoid then adjust
the turn button at one end of the current balancer to balance it. Close
the switch at the circuit with the solenoid. Adjust the slip rheostat until
the current solenoid Ic = 4 A. Record it at Table 30.2.5.1.
2. Close the switch at the circuit with the current balancer. Separately
adjust the slip rheostat until the current of the current balancer IL
= 0.2 A, 0.4 A, 0.6 A. Record them at Table 30.2.5.1. The current balancer
is inclined when current flows. Make it balance again by hanging a connecting
wire at its end. Measure the length of the balance lead l and record it
in Table 30.2.5.1. Separately calculate the amplitude of balance weight
F needed corresponding to different IL (0.2 A, 0.4 A, 0.6A) according
to the length of the balance lead l. Record F in Table 30.2.5.2. Calculate
magnetic induction B = F / (ILL). Record it at Table 30.2.5.2.
3. Repeat the experiment by adjusting the slip rheostat to change the
current of a solenoid. Analyse and find the factors related to the magnetic
induction B. You may find that B is direct proportion to Ic.
Table 30.2.5.1
Sequence number
Width
L (m)
Current
IL(A)
Current
Ic (A)
The length of the balance lead
l (m)
.
.
.
.
.
.
.
.
.
.
Table 30.2.5.2
Sequence number
Balance weight
F (N)
Magnetic induction
B (T)
.
.
.
.
.
.
30.6.2.1 Electromagnetic induction See diagram 30.3.1
1. Connect batteries, a galvanometer and a resistance. Let the wire connecting
the resistance touch the negative pole of the battery instantly. Observe
the deflection direction of the galvanometer needle. Observe the relationship
between the direction of the current and the deflection direction of the
galvanometer needle. Move the wire frame ABCD down through the 2 poles to
A'B'. Observe the deflection direction of the galvanometer needle and record
the direction of the induced current at the wire frame. Move the wire frame
up from A'B' to the original position. Observe the deflection direction
and size of the galvanometer needle. Record the direction of the induced
current at the wire frame and its change in size.
2. Repeat the experiment with different speed to move the wire frame.
Observe the deflection direction and size of the galvanometer needle. Record
the direction of the induced current at the wire frame and its change in size.
Move the magnet, i.e. magnetic field, up and down as well while keeping the
wire frame ABCD immovable. Observe the deflection direction of the galvanometer
needle. Move a straight connecting wire AB up and down at the magnetic field.
Observe the deflection direction of the galvanometer needle. Fold a piece
of hard connecting wire into a rectangle and connect it to a galvanometer
but leave an end open, i.e. it is not a closed loop. Move its long side
XY up and down at the magnetic field. Observe the deflection direction of
the galvanometer needle.
30.6.2.2 Electromagnetic induction See diagram 30.3.1
1. Connect a large coil to a galvanometer. Insert a permanent magnet's
north pole into the coil and observe the deflection direction of the galvanometer
needle. Take the magnet off the coil and observe the deflection direction
of the galvanometer needle again.
2. Repeat the experiment with different speed to insert or take off the
magnet. Observe the deflection direction of the galvanometer needle.
3. Repeat the experiment but with the magnet's south pole. Observe the
deflection direction of the galvanometer needle. Place a small coil into the
large coil and connect the small coil to a battery and a switch with connecting
wire to form a closed loop. Turn the switch on to make the small coil carry
current. Take it off the large coil and observe the deflection direction
of the galvanometer needle. Insert the small coil into the large coil again
and observe the deflection direction of the galvanometer needle. Change the
direction of the current at the small coil by inverting the lead connecting
the battery. 30.6.3 Electromagnetic induction with two solenoids See diagram 32.2.67
1. When the number of magnetic lines of force from a conducting circuit
changes, an induced current flows through the circuit during the change.
1. Connect the solenoid 1, with many turns of insulated copper wire,
to the centre zero galvanometer G, with full scale deflection 0.002 amps.
Its terminals should allow the direction of deflection to show the direction
of the current. Push a magnet quickly into the solenoid and observe the galvanometer.
Hold the magnet inside the solenoid and observe the galvanometer. Withdraw
the magnet quickly from the solenoid and observe the galvanometer.
2. Connect the solenoid 2, with few turns of insulated thick copper wire,
to the Daniell cell D.
3. Repeat the experiment with solenoid 2 connected to the Daniell cell,
D.
2. Put a switch S in series with solenoid 2 and put it inside solenoid
1
1. Close switch S and observe the galvanometer G.
2. Let the current flow through the solenoid 2, inside solenoid 1, and
observe the galvanometer G.
3. Open switch S and observe the galvanometer G
3. Show that the direction of the induced current causes a magnetic field
opposing the change taking place.
1. Connect solenoid 1 to the galvanometer G. Insert the north pole of
the magnet quickly into end A solenoid 1. Note the direction of the induced
current, and, knowing the direction of winding on solenoid 1 deduce the
polarity of end A of the solenoid. Withdraw the magnet from end A of solenoid
1 and again deduce the polarity of end A of solenoid 1.
2. Repeat the experiment using the south pole of the magnet.
3. Connect solenoid 2, to the Daniell cell D. Knowing the direction of
the current in solenoid 2, and the direction of the windings of solenoid 2,
deduce its polarity. Using solenoid 2 as an electromagnet, instead of the
magnet, repeat the experiment.
4. Show that the size of the induced EMF and the induced current depends
on the rate of change of the magnetic field associated with the circuit.
5. Repeat the experiment by varying the rate at which the magnet or solenoid
2, is pushed into, or withdrawn from, solenoid 1. 30.6.9 Make a spark
Connect the primary coil to a potential difference of four volts and
adjust the contact points to open until you can hear a buzzing sound. Observe
the points of the adjustable rods connected to the ends of the secondary
coil when you move the secondary coil backwards and forwards over the primary
coil. A spark forms. The length of this spark indicates how much potential
difference. It is greatest when the secondary coil completely surrounds
the primary coil. 30.6.10 Spark in a spark plug See 32.5.5.0: Motor vehicle ignition
system
Connect the primary lead of a car ignition coil to a 6 volt or a 12 volt
battery. Join the secondary lead from the coil to the central electrode
of a spark plug. Connect the outer electrode to the other secondary wire.
They joined to the outer metal case. Use a switch to start and stop the
primary current and observe the electrodes of the spark plug. 30.7.0 Electromagnetic induction applications
Applications of electromagnetic induction, magnetometer, galvanometer,
ammeter, voltmeter, multimeter, electrical meter movements and transformers,
audio and video technology, analytical instrumentation and navigational technology,
magnetometer, galvanometer, ammeter, voltmeter, multimeter, electrical
meter movements and transformers, audio and video technology, analytical
instrumentation and navigational technology.
30.7.1 Spectrograph
The force on a moving charged particle in a magnetic field causes the
charged particle to move in a circular path. The mass spectrograph can be
used to measure the mass of ions for identification. 30.7.2 Radio waves, transmitter: amplitude modulation
(a.m.) and frequency modulation (f.m.) Order online: Light Modulation,
Photo Phone Kit, amplitude modulation
Radio waves are a form of electromagnetic radiation that can be controlled
to produce wavelengths between one metre and a million metres. An a.c.
signal of a given frequency is fed onto twin conductors acting as an antenna.
An electric field that reverses each cycle is produced between the conductors.
The changing electric field produces a magnetic field perpendicular to
it, which changes at the same frequency. The changing fields join up to
form loops, which move away at the speed of light. 2.166 Magnetizing coil
Use glass tubing wound with close turns of insulated copper wire to magnetize
steel knitting needles.
2.175 Cylindrical electromagnets See diagram 2.175
1. Use an iron bolt 5 cm long with a nut and 2 washers. Put a washer
at each end and screw the nut on to the bolt. Leave 30 cm of wire before
winding 3 layers of bell wire on the bolt between the washers. Leave another
30 cm of wire then cut the wire. Twist together the 2 ends of the wire.
Wind insulating tape around the ends of the bolt to prevent the wire unwinding.
Remove insulation from the 2 ends of the wire to link the electromagnet
in a circuit with 2 dry cells or lead cell accumulators in series. Use a
headlight bulb in series with the electromagnet. Switch the circuit on then
pick up some pins. Switch the current off and see the iron objects fall.
The magnetic force exists only when you turn on the current. Use a plotting
compass to test the poles at each end of the electromagnet.
2. Reverse the connections to the source of electricity and test the
poles again.
2.176 Horseshoe electromagnets See diagram 2.176
Do not use a lead cell accumulator for this experiment because the resistance
of these coils is low and the current will be large with a significant
fire risk. For horseshoe magnets or C-shape magnets, you must wind the
coil in opposite directions on each arm of the magnet. Use an U-shaped
a piece of iron. Wind a coil of 3 layers of bell wire on each straight
arm of the iron, but not on the curving part. Leave 30 cm of wire before
you start winding the coil from the end of one arm. Cross to the other
arm. Wind a coil of 3 layers and leave 30 cm of wire at the end. Wind 3
layers of wire on this pole. When you have finished, tape the wire to keep
it from unwinding. Remove the insulation from the ends of the coil, connect
the horseshoe magnet in series with a motor car headlight bulb, connect
to 2 dry cells or lead cell accumulators, and test the poles of the electromagnet.
One should be a north pole and the other a south pole. If each has the same
polarity, you have wound the second coil in the wrong direction, so unwind
the coil and rewind it in the opposite direction. Try picking up different
things with the magnet. Compare the strength of this electromagnet with
the straight one you made.
2.177 Strength of electromagnets
1. Do NOT use lead cell accumulators because the resistance of these
coils is low and the current will be large with a significant fire risk.
Wind 25 turns of bell wire on a straight iron bolt and connect one dry
cell or lead cell accumulator to the ends of the wire. Record the number of
pins or paper clips you can pick up with the electromagnet.
2. Repeat the experiment with 2 dry cells or lead cell accumulators connected
in series. Wind on 25 more turns of wire in the same direction. Join them
to the first 25 turns.
3. Repeat the experiment with 2 dry cells or lead cell accumulators connected
in series. Wind on another 50 turns. Join them to the first 50 turns.
4. Repeat the experiment with 2 dry cells or lead cell accumulators connected
in series. Remove 50 turns and rewind them on the bolt in the opposite
direction.
5. With 100 turns so wound, repeat the experiment with 2 dry cells or
lead cell accumulators connected in series.
2.178 Magnetic field of electric current in a wire See 32.5.4.1: Motor vehicle Magnetic
fields and electromagnetism
1. Pull 25 cm of insulated copper wire through a hole in the centre of
a small white piece card. Connect the ends of the wire to a battery through
a car headlight bulb. Fix the card in a horizontal position. Fix the wire
in a vertical position. Sprinkle iron filings evenly on the card. Switch
on the current. Tap the card gently with the end of a pencil. The iron filings
move into a pattern showing the magnetic field. Switch off the current
2. Repeat the experiment using a small plotting compass instead of iron
filings. Compare the directions of the compass needle to the patterns of
iron filings on the card. Repeat the experiment with direction of current
reversed.
2.179 Magnetic field of open coil, open solenoid See diagram 29.179: Magnetic field of open coil
1. Wind five evenly spaced turns of bell wire around a wooden cylinder.
Slide the coil off the cylinder. Fit the cylinder into slots in a piece of
cardboard so that the cardboard appears to cut the coil in half length ways.
Connect the coil to the terminals of a dry cell or lead cell accumulator
or low voltage power supply using a car headlight bulb in series. Sprinkle
iron filings evenly on the card. Switch on the current. Tap the card gently
with the end of a pencil. The iron filings move into a pattern showing the
magnetic field. Note the pattern inside the coil and outside the coil. Switch
off the current.
2. Repeat the experiment using a plotting compass instead of iron filings.
2.180 Electricity from a magnet and a coil See diagram 2.180
Connect a coil of fifty turns of bell wire to a current detector. Use
long connecting wires so that the coil, and the magnet are away from the
compass in the current detector. Hold the horseshoe magnet or bar magnet
in your left-hand and the coil of bell wire in your right-hand. Hold the coil
vertically. Pass one pole of the magnet through the soil while observing the
compass needle in the current detector. When the coil moves through the
magnetic lines of force, an electric current moves through the circuit.
2.181 Simple electric motor See diagram 29.181: Simple electric motor |
See diagram 29.181.1: Simple electric motor |
32.5.7.1 Principle of the electric motor,
shunt-wound motor, series-wound motor
1. Field magnets and brushes
Push a 15 cm nail up through the centre of a 20 × 25 cm card to
act as a vertical bearing. Wind a coil 100 turns of bell wire on 2 nails to
act as field magnets. Push the nail down into the cardboard 15.5 cm apart,
each side of the spike. Push down 2 small nails on the diagonal and 5 cm
from the centre of the card. Strip insulation from the ends of each coil so
that you can twist the wire twice around the small nails then touch the central
large nail. The touching ends will act as brushes. Check the direction of
windings of the coils from the diagram. Attach the other ends of the coils
in the field magnets to screws in the corners of the card. Make the armature
coil. Push a 15 cm nail crosswise through a 4 cm cork. Wind 40 turns of
bell wire on the nails, with the direction of windings as shown in the diagram.
Strip insulation from the ends of each coil. Cut out part of the centre
of the lower side of the cork so that the end of a test-tube fits tightly.
2. Commutator See diagram: 29.181: Simple electric motor
Cut out a rectangular piece of thin sheet copper 4 cm × circumference
of the test-tube. Cut off 12 mm from the circumference length then cut
the piece of copper in half. Fit the copper pieces to curve around the
test-tube with a gap of 6 mm between them. Make a small hole in each copperplate
to attach one end of an armature coil. Fix the copper plates in position
around the test-tube as a commutator. Set up the rotor. The armature and
commutator form the rotor. Set the rotor into position on the vertical bearing.
Adjust the brushes to contact the commutator. Turn the test-tube within
the cork until the brushes lie across the gaps in the commutator when the
armature is in line with the field magnets. Run the electric motor. Connect
the screws to the terminals of a dry cell or lead cell accumulator using
a car headlight bulb to limit the current, or low voltage power supply.
Give the rotor a slight push and it should keep turning. If the rotor does
not turn check the contact of the brushes with the commutator.
9.2.6 Induced EMF in conductor moving in magnetic
field, open right-hand rule See: 9.2.6: Transform electromagnetic
energy to kinetic energy
If current flows perpendicular to a uniform external magnetic field then
a magnetic force will be produced on the current at right angles to the
directions of both the current and the magnetic field.
To observe electromagnetic induction, wrap 50 turns of insulated wire
diameter >5 cm around a cylinder with a smooth surface and leave connecting
wires. Put the coil on the cylinder and fix every turn of the coil with adhesive
plastic. Connect the 2 ends of this coil to a galvanometer to form a closed
circuit. Insert a pole of a horseshoe-shaped electromagnet into the coil
and observe whether the compass of the galvanometer moves. After the compass
settles down, quickly take the coil off the magnet while keeping the magnet
immovable. Observe whether the compass of the galvanometer moves. Cover the
coil into another pole of the magnet and slowly move it around. Observe whether
the compass of the galvanometer moves again. Not included
30.5.0 Alternating current, a.c. circuits
30.5.2 Impedance
30.5.3 LCR Circuits, a.c. (or LRC circuit) forced damped oscillator
30.5.4 Filters and Rectifiers
30.6.0 Electromagnetic induction
30.6.11 LR Circuits (circuit containing an inductor, i.e. coil of wire
+ resistor)
30.6.12 RLC Circuits, d.c. (resistance + capacitor + inductor)
30.7.0 Electromagnetic induction applications
30.7.3 Magnetic resonance imaging (MRI)
30.7.4 Magnetic sound recording, magnetic tape
30.7.5 Electrocardiograph, ECG, electroencephalograph
30.7.6 Working principles of an appliance, e.g. telephone receiver, telephone,
carbon microphone receiver, microphone, the loud speaker, moving coil loudspeaker,
relay, switch
30.7.7 Oscillatory circuit and transistor oscillator, tuning circuit
simple radio receiver, Television: black and white, colour, X-rays, voltage
amplification by transistor