Electric fields

Sample questions

Original practice questions written in the style of the exam.

1. Question 12 marks

   In a simple model of the hydrogen atom, the electron is $5.3 \times 10^{-11}\,\text{m}$ from the proton.

   Calculate the magnitude of the electrostatic force between them.

   Mark scheme

   • Uses $F = \dfrac{kQ_1Q_2}{r^2} = \dfrac{8.99 \times 10^{9} \times (1.60 \times 10^{-19})^2}{(5.3 \times 10^{-11})^2}$ (1)
   • $F = 8.2 \times 10^{-8}\,\text{N}$ (1)
2. Question 23 marks

   An electron enters the uniform electric field between two horizontal parallel plates, travelling horizontally midway between them. The upper plate is positive.

   Describe and explain the path of the electron while it is between the plates.

   Mark scheme

   • The electron experiences a constant force towards the positive (upper) plate, perpendicular to its initial velocity (1)
   • Its horizontal velocity component is unchanged, while it accelerates uniformly upwards (1)
   • So the path is a parabola curving towards the positive plate (1)
3. Question 33 marks

   Two positive point charges, $+4.0\,\mu\text{C}$ at point X and $+1.0\,\mu\text{C}$ at point Y, are $0.30\,\text{m}$ apart.

   Calculate the distance from X of the point on the line XY where the resultant electric field strength is zero.

   Mark scheme

   • At the null point the field strengths are equal: (1)