Experiment 4e Class: Name: ( ) Date:
4e
To find out how the magnetic force on a current-carrying conductor is affected by the current and the length of current inside the magnetic field.
1 When a current flows through a conductor in a magnetic field across it, a magnetic force acting on it is produced.
2 The graph in Figure 4e-1 is a straight line passing through the origin, which means x and y are directly proportional, i.e.
y ∝ x or y = kx ( k is a constant) y
0 y x
0
0 k origin
Fig 4e-1 x
0 x
❏ 1 current balance
❏ 1 power pack (0–12 V a.c./d.c.)
❏ 3 pairs of slab-shaped magnets on a steel yoke
❏ 1 electronic balance
❏ 1 ammeter
❏ 1 rheostat
❏ several connecting leads
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Class: Name: ( ) Date: Experiment 4e
Note
The reading is in gram for some electronic balances. Since an object of 1 gram has a weight of 0.01 N on earth, the reading can be changed to values of force easily by multiplying it by 0.01.
1 (a) Set up a current balance (Fig 4e-2).
(b) Adjust the position of the wire until it is balanced.
(c) Set the reading of the electronic balance to zero.
power pack ammeter rheostat
✐ In order to obtain directly proportional relationship of F and
I , the arm of the current balance must be horizontal and sit lightly on the plate of the electronic balance.
Some books of suitable thickness may be necessary to achieve this.
slab-shaped magnets on a steel yoke
S
N insulator electronic balance current-carrying arm
Fig 4e-2
Note
The reading of the electronic balance gives the magnetic force acting on the current-carrying arm.
2 (a) Switch on the power supply. Vary the current flowing through the wire by changing the resistance of the rheostat.
(b) Take the readings of the ammeter and electronic balance. Record the results in Table 4e-1 on p. 66.
New Physics at Work (Second Edition) © Oxford University Press 2007 65

Experiment 4e Class: Name: ( ) Date:
3 Repeat several times with other current values. Record the results in
Table 4e-1.
✎
Results:
Current through wire I / A
Magnetic force on wire F / × 10 –3 N
0.5
1.2
1.0
2.5
1.5
3.6
2.0
4.7
2..5
5.9
3.0
7.1
Table 4e-1
4 Plot a graph of the magnetic force F on wire against the current I through wire in Figure 4e-3.
magnetic force on wire F / t 10 –3 N
0
Fig 4e-3
7
6
5
4
3
2
1
1 2 3 current through wire I / A
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Class: Name: ( ) Date: Experiment 4e
5 (a) Pass a fixed current of 3 A through the wire.
(b) Measure the length of the current inside the magnetic field (the length of the slab-shaped magnets) and take the reading of the electronic balance. Record the results in Table 4e-2 on p.67.
6 (a) Place one and two pairs of slab-shaped magnets round the current-carrying arm in turn to double and triple the length of the current inside the magnetic field (Fig 4e-4).
double the length of the current inside the magnetic field triple the length of the current inside the magnetic field slab-shaped magnets on a steel yoke slab-shaped magnets on a steel yoke current-carrying arm current-carrying arm
Fig 4e-4
(b) Take the reading of the electronic balance. Record the results in
Table 4e-2.
✎ Results:
Length of current inside the magnetic field l / cm
Magnetic force on wire
F / × 10 –3 N
5
2.4
10
5.1
15
7.3
Table 4e-2
New Physics at Work (Second Edition) © Oxford University Press 2007 67

Experiment 4e Class: Name: ( ) Date:
7 Plot a graph of the magnetic force F on wire against the length l of the current inside the magnetic field in Figure 4e-5.
magnetic force on wire F / t 10 –3 N
4
3
2
1
7
6
5
0
Fig 4e-5
5 10 15 length of current inside the magnetic field l / cm
✎ How is the magnetic force F on the wire related to the current I through it?
The magnetic force on the wire is directly proportional to the current through it, i.e.
F ∝ I .
✎ How is the magnetic force F on the wire related to the length l of the current inside the magnetic field?
The magnetic force on the wire is directly proportional to the length of the current inside the magnetic field, i.e. F ∝ l .
The magnetic force F on a current-carrying conductor is
________________________ to the current I through it and the length l of the current inside the magnetic field, i.e. F ∝ _________________.
68 New Physics at Work (Second Edition) © Oxford University Press 2007

Class: Name: ( ) Date: Experiment 4e
✎
How will the graph of the magnetic force on the wire against the length inside the magnetic field be different if the experiment is repeated with a larger fixed current?
The graph is still a straight line passing through the origin, but the slope is steeper than before.
✎ How will the reading of the electronic balance change if pairs of slabshaped magnets with like poles facing each other are placed round the current-carrying arm? Explain your answer.
When slab-shaped magnets are fitted with like poles facing each other, a zero magnetic field will result midway between the magnets. Therefore, no magnetic force is experienced by the current-carrying arm and the reading of the electronic balance will become zero.
✎ Suggest how small copper riders instead of the electronic balance can be used to study the relationship between the magnetic force on a current-carrying conductor and the current through it. Write down the necessary procedures. (Hint: Small copper riders can be placed on the arm of the current balance to keep the balance in equilibrium.)
Set up the current balance. Adjust the position of the wire until it is balanced. Place a pair of slab-shaped magnets round the arm of the current balance. Put a rider on the arm and adjust the current passing through the wire until the wire is balanced again. Record the corresponding current from the ammeter. Repeat to find the current needed to balance different numbers of riders placed on the arm. Plot a graph of the magnetic force (proportional to the number of riders placed on the arm) against the current.
New Physics at Work (Second Edition) © Oxford University Press 2007 69