Syllabus Edition

First teaching 2023

First exams 2025

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Electric Field of a Point Charge (CIE A Level Physics)

Exam Questions

43 mins5 questions
1a
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2 marks

Define electric field strength.

1b
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4 marks

Fig. 1.1 shows the electric field lines between two charged spheres, A and B.

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Fig. 1.1

Using Fig. 1.1, state and explain which of the spheres, A or B, has

(i)
a negative charge
[2]
(ii)
a greater magnitude of charge.
[2]
1c
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2 marks

State the formula for the electric field strength E at the surface of the sphere and state the meaning of any other symbols used.

1d
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4 marks

The field strength at the surface of sphere A is 2.6 × 103 V cm–1, and the field strength at the surface of sphere B is 1.3 × 103 V cm–1.

The magnitude of the charge stored on the surface of sphere B is 7.5 µC.

(i)
Show that the radius of sphere B is about 7 cm.
[2]
(ii)
Calculate the magnitude of the charge stored on the surface of sphere A.
Assume both spheres have an equal radius.
[2]

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2a
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1 mark

State the relationship between electric field strength E and the distance r from a point charge.

2b
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3 marks

Sphere X is a small conducting sphere that carries a charge of +2.0 × 10−9 C.

This charge may be assumed to act as a point charge situated at the centre of sphere X.

Calculate the electric field strength at a distance of

(i)
2 cm from the centre of sphere X
[2]
(ii)
10 cm from the centre of sphere X.
[1]
2c
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3 marks

Sphere X is placed at a distance of 20 cm from sphere Y, which is identical to sphere X but carries a negative charge.

At the midpoint between the spheres X and Y:

(i)
Calculate the magnitude of the electric field strength
[2]
(ii)
State the direction of the electric field.
[1]
2d
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2 marks

Sphere Y is replaced with a sphere which is positively charged and identical to sphere X.

Explain why the value of electric field strength at the midpoint between the spheres would be zero.

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1a
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2 marks

Fig 1.1 shows an isolated conducting sphere of radius r is placed in air. It is given a charge –Q.

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Fig 1.1

State assumptions about the charge of the sphere.

1b
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3 marks

The maximum field strength at the surface of the sphere before electrical breakdown (sparking) occurs is –6.2 MV m–1. The sphere has a radius of 40 cm.

Calculate the maximum charge that can be stored on the sphere.

1c
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3 marks

On Fig 1.2, sketch a graph to show the variation with distance d from the centre of the sphere of the electric field strength E due to the charge –Q.

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Fig 1.2

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2a
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3 marks

Calculate the distance d from a point charge of 6.0 µC where the electric field strength is 96 kV m–1.

2b
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3 marks

At some distance x, the electric field strength decreases to 10.8 kV m–1.

Determine the value of x.

2c
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2 marks

Fig 1.1 shows the different distances from the 6.0 µC charge in terms of d.

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Fig 1.1

Identify the distance x by placing an X on Fig 1.1.

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3a
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1 mark

An α-particle is moving directly towards a stationary gold nucleus (A presubscript 79 presuperscript 197 u).

The α-particle and the gold nucleus may be considered to be solid spheres with the charge and mass concentrated at the centre of each sphere.

The α-particle approaches to 0.75 × 10–14 m from the gold nucleus where it is momentarily stationary.

Calculate the electric field strength the α-particle experiences at the point.

3b
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3 marks

Show that the gold nucleus experiences an electric field strength that is roughly 40 times less than the α–particle.

3c
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2 marks

State the direction that the α–particle will travel in after it is momentarily stationary. Explain why this happens.

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