Exam Style Answers 24 Asal Physics CB

 

 

CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS: COURSEBOOK 

Exam-style questions and sample answers have been written by the authors. In examinations, the way marks are awarded may be different.

 Coursebook answers Chapter 24 Clockwise magnetic field lines around and close to each strip  [1]

Exam-style questions 1

B 

[1]

2

C 

[1]

3

a

(force, F  is  is given by F  = BIl  sin  sin θ)

 

The force is a maximum when the angle θ  between the wire and the magnetic field is 90° (i.e. when sin θ = 1).  [1]

b

5

Elliptical lines further aw away ay from the strips, eventually becoming elliptical around both strips (even farther away, the shape becomes circular, not shown in the diagram)   [1]

b

The force on strip A is towards strip B and the force on strip B is towards strip A, i.e. the strips attract each other.  [1]

 

This is because strip A, on its own, produces a magnetic field vertically down the paper at strip B by the right-hand rule.  [1]

 

The left-hand rule can then be applied to strip B, which has a current into the plane of the paper and a field down the paper.  [1]

 

So, the force is to the left, towards strip A. 

The force is zero when the angle θ  between the wire and the magnetic field is 0°. (The wire is parallel to the magnetic field.)  [1]

4

 

a

F  = BIl  ∝ I  (force  (force ∝ current) 

[1]

 

hence, the force increases by a factor of 3.0 to a value of 1.41 × 10−2 N  [1]

b

F  = BIl  ∝ ∆B  (force  (force ∝ change in magnetic flux density)  [1]

 

hence, the force is halved to a value of 2.35 × 10−3 N  [1]

c

F  = BIl  ∝ ∆l  (force  (force ∝ change in length of wire in the field)  [1]

 

hence, the force is reduced to 40% of its initial value to 1.88 × 10−3 N  [1]

a

F  = BIl  sin  sin θ  F  3. 8 × 10 3 = B  =    Il sinθ  1.2 × 0.03 × sin 50

 

B  = 0.138 T ≈ 0.14 T 

[1]

b

The direction is given by Fleming’s Fleming’s lefthand rule. The wire experiences a force into the plane of the paper.   [1]

7

[1]

a

The current is from Y to X. This is because Q shows that the magnetic field above the wire, produced by the current, is from west to east. The right-hand rule then shows the current is upwards.  [1]

[1]

b

P points towards the north-west. 

[1]

c

Q then points towards the north-west.   [1]

a

F  = BIl   [1]

 

= 4.5 × 10−3 × 2.5 × 0.07 

[1]

−

 

6

1

ο

8

−4

−4

 ≈ 7.9 × 10  N 

[1] [1]

 

= 7.88 × 10

b

The magnetic field is in the same direction as the current (or the wire).   [1]

c

From Fleming’s left-hand rule, PQ experiences a force out of the plane of the paper and RS experiences a force into the plane of the paper.  [1]

a

Cambridge International AS & A Level Physics – Sang, Jones, Chadha & Woodside © Cambridge University Press 2020

   

CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS: COURSEBOOK 

 

Hence, the frame starts to rotate in a clockwise direction (when viewed from the end PS).  [1]

d

torque = F  × d  = 7.88 × 10−4 × 0.040 

 

−5

= 3.15 × 10

a

Left to right 

b

force = weight of paper tape

 

F = mg  = 60 × 10−3 × 10−3 × 9.81 

 

= 5.89 × 10

c

B  = 

 

= 

 

= 1.33 × 10

d

Moves up and down slowly 

10 a

B  = 

d

Axes labelled and balance reading when current is zero marked  [1]

 

Balance reading decreases linearly with current  [1]    g     /    g    n     i     d    a    e    r    e    c    n    a     l    a     B

[1]

4

−

 

[1]

−3

 T ≈ 1.3 mT 

Il 

 = 

0.0136 0136 4.0 × 5.0 ×10  

2

−

= 0.068

T 

[1]

[1]

[1]

8.5 × 0.05 052

F 

102.45 101.06

[1]

0

4

Current/A

[1]

Diagram showing wire and magnetic field and a method of measuring the force (e.g. diagrams similar to either Figure 24.14 or

12 a

Magnetic flux density is the force acting per unit current per unit length on a wire carrying a current  [1]

24.15 in the coursebook)  [1] Measure I , the current, and F , the force [1]

 

when placed at right angles to the magnetic field. 

 

Method of measuring the force force,, e.g. difference in readings on top-pan balance (in kg) × 9.81  [1]

 

 

Measure length of wire at right angles to magnetic field  [1] F  B  =    [1] Il 

Both electric field strength and magnetic field strength are defined in terms of the force on an object. For the electric field strength, the object is a charge of one coulomb (1 C); for the magnetic field strength, the object is a wire of length 1 m carrying a current of 1 A.  [1]

b

i

The two wires attract one another or  there is a force upwards on the wire on the bench.  [1]

 

The wire abo above ve the bench produces a horizontal magnetic field on the wire lying on the bench.  [1]

 

This magnetic field interacts with the current in the wire lying on the bench to produce a force upwards.   [1]

 

 

The currents flo flow w in the same direction causing the wires to attract each other.   [1] 2.0 ×10 7 × 4.0 B  =   = 2.67 × 10−5 T  [1] 0.03 F  = BIl  = 2.67 × 10−5 × 4.0 × 1  [1]

 

= 1.07 × 10−4 ≈ 1.1 × 10−4 N 

 

  b

i

 ii ii

11 a

Using the left-hand rule, the field is horizontally towards the north, the force is upwards and thus the current is from west to east.   [1] F  I  =    [1] Bl  0.02

 

= 

 

= 417 ≈ 420

1.6 × 10

5

−

×

3.0

A 

 

[1] [1]

The wire is carrying a current in a magnetic field and experiences a force.  [1]

 

There is an upwards force on the top-pan balance, reducing the reading.  [1]

b

By Newton’s third law, the force is downwards downwar ds on the wire to produce an upwards force on the top-pan balance.  [1]

 

2

 N ≈ 5.9 × 10−4 N 

 

Il  5.8 89 9 × 10

 

[1]

−4

F 

force on wire =  (102.45 − 101.06) × 10−3 × 9.81 = 0.0136 N   [1]

−5

 N m ≈ 3.1 or 3.2 × 10  N m [1]

  9

[1]

c

[1]

−

 ii ii

[1]

By the left-hand rule, the current in the wire is from left to right.   [1]

Cambridge International AS & A Level Physics – Sang, Jones, Chadha & Woodside © Cambridge University Press 2020