School Science Lessons
Electromagnetism, electromagnets, motors and
generators, d.c. and a.c.
2009-09-15
Please send comments to: J.Elfick@uq.edu.au See 7.0: Tools for electrical
experiments See 8.0: Electrical experiments
equipment See
32.2.00: Electrical symbols See:
Electrical hazards See 2.12: Radiation
hazards See: Interesting
websites
Table of contents 30.1.0
Electromagnetism 30.1.3 Motors and generators, d.c. and a.c. 30.2.0 Transformers 30.3.0 Electromagnets 30.4.0 Electromagnet applications 30.5.0 Alternating current, a.c.
circuits 30.6.0 Electromagnetic induction 30.7.0 Electromagnetic induction applications 32.7.0 Electric current detectors 32.5.4.0 Electromagnets in motor vehicles 6.41 Make electromagnets (Primary) 30.1.0
Electromagnetism 4.78 Cylindrical electromagnet 4.79
Horseshoe electromagnet 4.80
Test the strength of
electromagnets 4.81
Magnetic field from electric
current in a wire 4.82
Magnetic field inside an open
coil, open solenoid 4.83
Electricity from a magnet
and a coil 6.41 Make electromagnets (Primary) 30.1.01 Oersted, magnetic field of a
current carrying wire, right-hand grip rule 30.1.02 Magnetic field of a current
loop, Earth's magnetic field 30.1.03 Direction of magnetic field
with plotting compass 30.1.04 Direction of magnetic field
with iron filings 30.1.05 Force between conductors
carrying current in the same direction 30.1.06 Force between conductors
carrying current in opposite directions 30.1.07 Force between a permanent
magnet and a conductor carrying current 30.1.08 Forces between magnets 30.1.3 Motors and
generators, d.c. and a.c. 30.1.3.01
Force on current-carrying
conductor in a magnetic field 30.1.3.02 Electric generator,
alternator, a.c. generator 30.1.3.1 Electric motor, electricity
from magnet and coil, d.c. generator, d.c. bicycle dynamo, rotor coil,
rotor coil 30.1.3.2 What makes an electric
motor spin 30.1.3.3 Simple electric motor 4.84
Make a simple electric motor 30.1.3.4 Principle of electric motor
30.1.3.5 Make an electric motor 30.1.3.6 Bicycle dynamo, "missing
wire" in a bicycle generator circuit 30.3.0
Electromagnets 30.3.01 Nail electromagnet 30.3.1 Make and test a simple
electromagnet 30.3.2 Make an electric compass with
suspended solenoid 30.3.3 Magnetizing coil, inner
magnetic field of a solenoid 30.3.4 Magnetic field of a solenoid 30.3.5 Magnetism from electric current
in a coil, magnetic field from a circular coil 30.3.6 Solenoid affects iron nails 30.3.7 Magnetic field inside an open
coil, open solenoid 30.3.8 Magnetize inside a coil,
solenoid carrying current 30.3.9 Magnetic fields of a solenoid
and bar magnet 30.3.10 Dancing spring, jumping wire, electric
current in parallel coils 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 30.4.0 Electromagnet applications 30.4.1 Electric bell, door chime 30.4.2 Electric buzzer 30.4.3 Make an electric signal with a
solenoid 30.4.6 Microphone 38.5.4
Crystal microphone 30.4.7 Electric fire alarm 30.4.8 Electric burglar alarm 38.5.03 Reed switch, reed relay,
"make-and-break"
32.5.2.7
Motor vehicle
electromagnetic circuit breaker 30.5.0 Alternating
current, a.c.
circuits 30.5.1 Simple alternator
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 30.6.01 Faraday's law for induced EMF. Test
Faraday's laws.
30.6.01a Test the direction of induced current 30.6.02 Lenz's law, force opposing
conductor with induced EMF 30.6.03 Self-inductance and mutual
inductance 30.6.04 Impedance, phase, resonance 30.6.1 Electromagnetic induction 30.6.1.1 Measure magnetic induction
inside a coil, measure the intensity of an induced magnetism 30.6.3 Electromagnetic induction with
2 solenoids 3.85
Daniell cell 30.6.9
Make a spark 30.6.10 Spark in a spark plug 38.2.04 Capacitors 32.5.5 Motor vehicle ignition
system 9.2.6 Induced EMF in conductor moving
in magnetic field, open right-hand rule
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 30.7.1 Spectrograph 30.7.2 Radio waves, transmitter:
amplitude modulation (a.m.) and frequency modulation (f.m.)
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 32.5.4.0
Motor vehicle electromagnets 32.5.4.1 Magnetic field sand
electromagnetism 32.5.4.2 Simple relay, horn relay 32.5.4.3 Cut-out 32.5.4.4 Electric Bell 32.5.4.5 Voltage Regulator 32.5.2.7 Motor vehicle
electromagnetic circuit breaker 30.8.00 Electromagnetic Radiation
Radiation hazards 27.2.0 Electromagnetic Spectrum 32.7.0
Electric current detectors, instruments to detect electric current,
meters, moving iron meter,
repulsion type and attraction type 32.7.1 Simple instrument to show
electric current, current detector 32.7.2 Galvanometer 32.7.2.1 Sensitivity and resistance
of a galvanometer 32.7.2.2 Convert a galvanometer to a
voltmeter 32.7.2.3 Convert a galvanometer to
an ammeter 32.7.2.4 Convert a galvanometer to
an ammeter, hot wire ammeter, heat a wire red-hot with electricity, hot
wire current meter + 32.7.2.5 Measure reduction factor k
of a tangent galvanometer 32.7.3 Ammeter 32.7.4 Voltmeter 32.7.4.1 Connect a voltmeter 32.7.4.2 Voltmeter as cell counter 32.7.4.3 Calibrate a voltmeter 32.7.4.4 Potential difference and
electromotive force 32.7.4.5Loading
by a voltmeter
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,
torques on coils See diagram: 30.0
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 Oersted, magnetic field of a current,
right-hand grip rule See diagram 30.01
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 a 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 with a
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 with 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 Force between conductor carrying
current in the same direction See diagram 30.1.3
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 Force between conductors carrying current
in opposite directions See diagram 30.1.4
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 Force between a permanent magnet and
conductor carrying current See diagram 30.1.5
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
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 (newtons) 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 converts mechanical energy into electrical
energy. 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. 30.1.3.1 Electric motor, electricity from magnet
and coil, d.c. generator, d.c. bicycle dynamo, rotor coil, rotor coil
The
4 different ways to get a dynamo, to give a higher current are as
follows: 1. Run it faster. 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. Use stronger magnets. 30.1.3.2 What makes an electric motor spin See diagram 30.1.3.2
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 30.1.3
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. 30.1.3.4 Principle of electric motor See diagram 31.1.0d
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 Make an electric motor See diagram 30.1.3.5
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. 30.1.3.6 Bicycle dynamo, the "missing wire" in a
bicycle generator circuit
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. 30.2.0 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
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 See diagram 4.2.3 | | See
2.5.5.6.1 Magneto | See diagram 30.3.1 |
See
diagram 30.3.2
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 See
diagram 30.03: 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 Make and test a 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 Make an electric compass with suspended
solenoid See diagram 9.2.2 | See
diagram 36.10 | See diagram 30.2.1.1:
Magnetic field from a solenoid | See diagram
30.3.13:
Suspended solenoid
1. A solenoid is a long coil of insulated wire that can be wound on a
broom handle, a pencil or a glass tube. Make yours by winding 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 as if it were a
bar magnet. Support the solenoid freely and switch on the current and
observe that it behaves as if it were 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 Magnetizing coil, inner magnetic field of a
solenoid See diagram 30.2.2
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. 30.3.4 Magnetic field of a solenoid See diagram 9.22 | See
diagram 29.2.2: Magnetic field of a bar magnet
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. 30.3.5 Magnetism from electric current in a coil,
magnetic field from 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 inside an 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 and bar
magnet See diagram 36.11 | See
diagram 29.2.2
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
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 Dancing spring, attraction and
repulsion of parallel
electric currents See diagram 36.7A and 36.7B
Be careful! They may not allow the
use of free surface mercury in your school system!
1. Use two copper wires 50 cm long on copper loops so that their free
ends
are 1 cm apart and hang in mercury. Connect a 6 volt
battery and switch on. Observe what happens. In this experiment the
electrons were flowing in the same direction in the 2 wires. Make the
current flow in opposite directions. Notice what happens. 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. 30.3.12 Attraction and repulsion of parallel
coils carrying electric current See diagram 36.8
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 36.8 | See
diagram 30.3.13: Solenoid behaves like a bar magnet
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 9.10 | See 32.5.4.4: 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 9.10a
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 Make an electric signal with a solenoid See diagram 9.12
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 32.5.6.2: Alternating current, graphs
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 / sqrt 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 / sqrt 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 / sqrt 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, mu (Greek). For a vacuum, or air ("free space") mu0
= 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). Test Faraday's laws
The induced e.m.f. increases with the speed of the magenet 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 nuber of turns in the coil nwhile
moving the magnet at constant speed, 3. use a stronger magnet and
compare the deflection in the galvanometer. 30.6.01a Test the
direction of induced current
See diagram 30.6.01a: Direction of induced current
Fix a wire between the poles of an U-shaped 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
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 = sqrt (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.
You observe no current in the secondary when
conditions were steady. 30.6.1.1 Measure magnetic induction inside a
coil, measure the intensity of an induced magnetism See diagram 30.2.5 | See 9.2.6
1. Use 2 ammeters (1 A and 5 A), 2 slip rheostats, 2 switches, a d.c.
source, a solenoid, a current balancer. Connect a circuit as in the
diagram. Carefully 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 2
arms of the current balancer to see whether the contact is 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 2 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.)
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 Make a 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-shaped 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 Test the 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 from 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 inside an open coil, open
solenoid See diagram 4.82
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 Make a simple electric motor See diagram 2.181 | See 32.5.7.1 Motor Vehicle
Principle of electric motor
2.181.1 Make 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.
2.181.2 Make the commutator See diagram: 2.181c
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
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.