# CIE AS & A Level Physics : 20.1 Concept of a magnetic field – Exam style question – Paper 4

### Question

(a) Define the tesla.                                                                                                                                   

(b) A stiff metal wire is used to form a rectangular frame measuring $$8.0cm × 6.0cm$$. The frame is open at the top, and is suspended from a sensitive newton meter, as shown in Fig. 8.1. The open ends of the frame are connected to a power supply so that there is a current of 5.0A in the frame in the direction indicated in Fig. 8.1. The frame is slowly lowered into a uniform magnetic field of flux density B so that all of side PQ is in the field. The magnetic field lines are horizontal and at an angle of 50° to PQ, as shown in Fig. 8.2. When side PQ of the frame first enters the magnetic field, the reading on the newton meter changes by 1.0mN.

(i) Determine the magnetic flux density B, in mT.
B = …………………………………………… mT 

(ii) State, with a reason, whether the change in the reading on the newton meter is an  increase or a decrease.                                                                                                                             

(iii) The frame is lowered further so that the vertical sides start to enter the magnetic field.
Suggest what effect this will have on the frame.                                                                              
[Total: 6]

Ans

(a) newton per ampere per metre
where current/wire is perpendicular to magnetic field

(b) (i) F = BILsin θ
B = 1.0 / (5.0 × 0.060 × sin 50°)
= 4.4 mT

(b) (ii) (from Fleming’s left-hand rule) force on wire is upwards, so reading decreases

### Question

(a) State what is meant by a magnetic field.
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(b) A rectangular piece of aluminium foil is situated in a uniform magnetic field of flux density B, as shown in Fig. 9.1. The magnetic field is normal to the face PQRS of the foil. Electrons, each of charge −q, enter the foil at right angles to the face PQTV.

(i) On Fig. 9.1, shade the face of the foil on which electrons initially accumulate. 
(ii) Explain why electrons do not continuously accumulate on the face you have shaded.
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(c) The Hall voltage $$V_H$$ developed across the foil in (b) is given by the expression

$$V_H =\frac{ BI}{ ntq}$$

where I is the current in the foil.
(i) State the meaning of the quantity n.

………………………………………………………………………………………………………………………….
……………………………………………………………………………………………………………………. 

(ii) Using the letters on Fig. 9.1, identify the distance t.

……………………………………………………………………………………………………………………. 
(d) Suggest why, in practice, Hall probes are usually made using a semiconductor material rather than a metal.
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[Total: 9]

Ans:

(a) region where there is a force exerted on  a current-carrying conductor
or
a moving charge
or
a magnetic material/magnetic pole