Electric Field of a Point Charge (Cambridge (CIE) A Level Physics): Exam Questions

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

Fig 1.1

State assumptions about the charge of the sphere.

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.

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.

Fig 1.2

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

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

Determine the value of .

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

Fig 1.1

Identify the distance by placing an X on Fig 1.1.

An α-particle is moving directly towards a stationary gold nucleus ().

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.

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

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