Magnetic Fields Due to Currents (Cambridge (CIE) A Level Physics): Exam Questions

A section of a straight current-carrying wire is placed at right angles to a uniform magnetic field of flux density . When the current in the wire is , the magnetic force that acts on a length  of the wire is . 

A second wire of the same length is placed at right angles to a uniform magnetic field of flux density when the current is . 

Determine the force, in terms of , that acts on the second wire.

Wire 1 is bent into a square coil. 

The coil is mounted on an axle O and is placed with its plane parallel to the flux lines of its uniform magnetic field B, as shown in Fig. 1.2. 

Fig. 1.2

The current, I, is switched on and flows through sides PQRS. 

State the magnitude of the force on sides SP and QR in Fig. 1.2 and explain your answer. 

Wire 2 is also bent into a square coil and mounted on an axle with its plane parallel to the flux lines of its uniform magnetic field 4B. It has the same arrangement as wire 1 in Fig. 1.1. 

Explain the difference in motion between wire 1 and wire 2 in this arrangement. 

The two wires are placed parallel with their currents flowing in the same direction as shown in Fig. 1.1.  

Fig. 1.1

Sketch the correct magnetic field lines on Fig. 1.1. 

State and describe the conditions that must be satisfied for a copper wire, placed in a magnetic field, to experience a magnetic force.

Two long parallel current-carrying wires are placed near to each other in a vacuum. 

Explain why these wires exert a magnetic force on each other. You may draw a labelled diagram if you wish.

A long air-cored is connected to a power supply so that the solenoid creates a magnetic field. Fig. 1.1 shows a cross-section through the middle of the solenoid.  

Fig. 1.1

The direction of the magnetic field at point A is indicated by the arrow. The other points are labelled B, C and D.  

On Fig. 1.1, draw arrows to indicate the direction of the magnetic field at each of the points B, C and D. 

Compare the magnitude of the flux density of the magnetic field: 

(i) At A and C

[1] 

(ii) At A and D.

[1]

A circular coil P carrying an alternating current produces a changing magnetic field. When a second similar coil Q is placed with its centre a distance x from the centre of coil P, as shown in Fig. 1.1, an electromotive force (e.m.f.) E is induced in coil Q.

Fig. 1.1

It is suggested that E is related to x by the relationship

where I is the current in coil P, and k and Z are constants.

Plan a laboratory experiment to test the relationship between E and x.

Draw a diagram showing the arrangement of your equipment.

Explain how the results could be used to determine values for k and Z.

In your plan you should include:

•    the procedure to be followed

•    the measurements to be taken

•    the control of variables

•    the analysis of the data

•    any safety precautions to be taken.