School Science Lessons
30. Electromagnetism, induction, electromagnets, motors and generators,
d.c. and a.c.
2012-01-28 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 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 = 2 pi X NABF X 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 X 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 Motors and generators, d.c. and a.c.
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)
X L (metres) X 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 X 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 X 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 X 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 X 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 X sin 2pi
X f X 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 X 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 X 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) X peak value = 0.71 X 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 X 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 = 4pi X 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 X 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 X 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 X (change in primary coil current / change in time), i.e. M X 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
X 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 X 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 X 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