Uniform Electric Fields (Cambridge (CIE) A Level Physics): Exam Questions

State what is indicated by the direction of the electric field line.

Fig. 1.1 shows a pair of parallel metal plates with a potential difference (p.d.) of 3500 V between them.

Fig. 1.1

The plates are separated by a distance of 4.6 cm. The plates are in a vacuum.  

On Fig. 1.1, draw five lines to represent the electric field in the region between the plates. 

Calculate the strength of the electric field between the plates.

 electric field strength = .................................... N C⁻¹ 

A moving electron enters the region between the plates from the left, as shown in Fig. 1.2.

Fig. 1.2

The electron is deflected by the electric field. 

On Fig. 1.2, draw a line to show the path of the electron as it moves through and out of the region of the electric field.

Fig 1.1 shows an electron that enters an electric field at right angles to the field. 

Fig 1.1

Sketch the path of the electron on Fig 1.1 once it enters the uniform electric field. State the shape of this trajectory.

A positron enters the field at right angles instead, as shown in Fig 1.2.

Fig 1.2

Sketch the path of the positron on Fig 1.2.

Fig 1.3 shows a small polystyrene ball which is suspended between two vertical metal plates, P₁ and P₂, distance d apart that are initially uncharged.

The ball carries a charge of –0.17 μC. 

Fig 1.3

When the switch is closed, a p.d. of 400 V is applied between P₁ and P₂ and the ball experiences an electrostatic force of 34 mN. 

Calculate the value d.

Because of the electrostatic force acting on it, the ball is displaced from its original position. It comes to rest when the suspended thread makes an angle θ  with the vertical, as shown in Fig 1.4. 

Fig 1.4

On Fig 1.4, mark and label the forces that act on the ball when it is in this position.

An electric field exists in the space between two charged metal plates in Fig 1.1.

Fig 1.1

Sketch the graph on Fig 1.2 of the variation of electric field strength E  with distance d  from A along the line AB.

Fig 1.2

The force on a proton placed between the plates is 50 nN and the separation of the plates is 24 mm.

Calculate the potential difference between the plates.

A negatively charged particle of mass 5.0 g remains stationary exactly halfway from each plate, as shown in Fig 1.3.

Fig 1.3

Calculate the charge of the particle.

State and explain what will happen to the particle if the separation of the plates starts to decrease.