Experiment 5c Class: Name: ( ) Date:
5c
To find out how the induced e.m.f. in a coil is affected by the number of turns of the coil and the speed of the magnet moving towards and away from the coil using a voltage sensor.
1 An e.m.f. can be induced in a conductor when the magnetic field through it is varying, or it is cutting through magnetic field lines.
2 Faraday’s law of electromagnetic induction states that the e.m.f. induced in a conductor is directly proportional to the rate at which the conductor cuts through the magnetic field lines, or at which the magnetic field changes.
3 Lenz’s law states that the direction of the induced e.m.f. tends to oppose the change causing it.
4 An object falls freely under gravity with a uniform downward acceleration.
❏ 1 voltage sensor
❏ 1 data-logger interface
❏ 1 computer with data-logging program installed
❏ 1 plastic tube
❏ 1 PVC-coated copper wire
❏ 1 bar magnet
❏ 1 metre rule
❏ 1 retort stand and 2 clamps
❏ some cotton wool or soft materials
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Class: Name: ( ) Date: Experiment 5c
Precaution
The turns of the coil should be evenly and closely packed.
Precaution
This protects the bench from being damaged by the falling magnet and prevents the bouncing of the magnet.
1 Set up the apparatus as shown in Figure 5c-1:
(a) Connect a voltage sensor to a data-logger interface and then connect the data-logger interface to a computer.
(b) Clamp a metre rule vertically on a stand.
(c) Wind a coil of 20 turns around a plastic tube and clamp it vertically on the stand (or fix it to the metre rule with rubber bands).
(d) Connect the voltage sensor to the coil.
(e) Place some cotton wool or soft materials underneath the coil.
metre rule bar magnet
✐ As the duration of the magnet through the coil is relatively short, a fairly high sample rate should be chosen.
Also, the data-logger should be programmed at automatic triggering mode (if applicable).
coil of 20 turns voltage sensor
N plastic tube
✐ Possible errors in the experiment:
1 The turns on the coil are not evenly distributed.
2 As the time of falling is very short, the e.m.f. induced momentarily in the coil could not be measured with a high accuracy.
3 Orientation of magnet might not be exactly vertical when dropped.
to computer
Fig 5c-1
1
2
3
4 data-logger interface
A
B
C cotton wool
2 (a) Run the data-logging program on the computer and display the voltage–time graph.
(b) Set up the properties of the voltage sensor as follows:
Sensitivity: high (e.g. 100×)
Sample Rate: high (e.g. 2500 Hz)
New Physics at Work (Second Edition) © Oxford University Press 2007 97

Experiment 5c Class: Name: ( ) Date:
Precaution
The magnet should be dropped to fall vertically.
✎ Explain why a high sample rate should be set.
This is because the duration of the magnet through the coil is relatively short.
3 (a) Start data recording using the data-logging program.
(b) Drop a bar magnet from a height of 20 cm above the coil with the north pole facing downwards.
(c) Stop data recording and save the voltage–time graph plotted in real-time.
(d) Paste the voltage–time graph in Figure 5c-2(i).
✎
Describe the induced e.m.f. in the coil when the magnet enters and leaves it.
When the magnet enters the coil, e.m.f. is induced in the coil momentarily and then drops back to zero (positive pulse).
When the magnet leaves the coil, e.m.f. is induced momentarily in the opposite direction and then drops back to zero (negative pulse).
(i) magnet dropped from 20 cm above the coil of 20 turns
✐ To check the polarity of the magnetic field set up by the induced current:
1 From the graphs and the voltage sensor, determine which terminal of the coil is at a high potential.
2 Apply right-hand grip rule to check the polarity. Induced current is flowing from the higher-potential terminal to the lowerpotential terminal through the external circuit .
98 New Physics at Work (Second Edition) © Oxford University Press 2007

Class: Name: ( ) Date: Experiment 5c
(ii) magnet dropped from 20 cm above the coil of 40 turns
(iii) magnet dropped from 30 cm above the coil of 40 turns
Fig 5c-2
New Physics at Work (Second Edition) © Oxford University Press 2007 99

Experiment 5c Class: Name: ( ) Date:
Precaution
The orientation of the magnet should be the same when dropped.
4 Repeat step 3 with a coil of 40 turns. Paste the voltage–time graph obtained in Figure 5c-2(ii).
✎
How does the induced e.m.f. in the coil change with its number of turns?
The magnitude of induced e.m.f. in the coil increases with its number of turns.
Precaution
The orientation of the magnet should be the same when dropped.
✐ Increasing the dropping height can give a more noticeable effect
(i.e. closer peak and trough).
5 Repeat step 3 with a height of 30 cm of the magnet above the coil of
40 turns. Paste the voltage–time graph obtained in Figure 5c-2(iii).
✎
How does the induced e.m.f. in the coil change with the speed of the magnet moving towards and away from the coil?
The magnitude of induced e.m.f. in the coil increases with the speed of the magnet moving towards and away from the coil.
✎
Why are the two peaks of the voltage–time graphs opposite in sign and of different magnitude?
Current is induced momentarily in one direction to oppose the increase in magnetic field due to the incoming magnet and in the opposite direction to oppose the decrease in magnetic field due to the outgoing magnet, so the two peaks are opposite in sign.
The magnet is moving faster as it leaves the coil than as it enters the coil, so the two peaks are of different magnitude.
The induced e.m.f. in a coil can be increased by towards and away from the coil.
100 New Physics at Work (Second Edition) © Oxford University Press 2007