A charged particle is moving in a uniform electric field. For the motion of the particle due to the field, which quantity has a constant non-zero value?

Question 15

A charged particle is moving in a uniform electric field.

For the motion of the particle due to the field, which quantity has a constant non-zero value?

A acceleration

B displacement

C rate of change of acceleration

D velocity

Reference: Past Exam Paper – June 2016 Paper 11 Q28

Solution:

Answer: A.

The charged particle experiences a force when in the electric field.

Let electric field strength = E             (E is constant as the electric field is uniform)

Charge of particle = q

The force on a particle of charge q in a uniform electric field of field strength E is given by F = qE.

From Newton’s 2^nd law: F = ma

A resultant force causes an acceleration a.

ma = qE

Acceleration a = qE / m                      for mass m

Therefore, the acceleration of the particle is constant (since q, E and m are all constants) and non-zero. [A is correct] If the acceleration was zero, there would be no force on the particle (as F = ma).

The acceleration being constant means that it is NOT changing. So,

Rate of change of acceleration = 0 [C is incorrect]

Also, an acceleration implies that the velocity is changing (and not constant). [D is incorrect] A changing velocity implies that the displacement is also changing. [B is incorrect]