Electric Charges and Fields — Practice

Question 1

Two point charges of 4 x 10^-7 C and 6 x 10^-7 C are placed 20 cm apart in air. Find the force between them.

Accepted Answer

Substitute directly into Coulomb's law, converting 20 cm to 0.20 m first. Answer: F = 5.4 x 10^-3 N, repulsive.

Question 2

How many electrons must be removed from a body to give it a charge of +1.6 x 10^-7 C?

Accepted Answer

Since charge is quantised, divide the total charge by the elementary charge e. Answer: n = 10^12 electrons.

Question 3

Find the electric field 30 cm from a point charge of 5 x 10^-6 C, and the force on a -2 x 10^-9 C charge placed there.

Accepted Answer

Use E = kq/r^2, then F = qE with the direction reversed because the charge is negative. Answer: E = 5 x 10^5 N/C radially outward; F = 10^-3 N directed towards the source charge.

Question 4

A uniform field of 2 x 10^3 N/C passes through a square of side 20 cm whose normal makes 30 degrees with the field. Find the flux.

Accepted Answer

Flux is E A cos(theta) with theta measured from the normal, and the area is 0.04 square metres. Answer: phi = 69.3 N m^2/C.

Question 5

A dipole of moment 6 x 10^-9 C m is placed at 45 degrees to a uniform field of 4 x 10^4 N/C. Find the torque on it.

Accepted Answer

Torque is p E sin(theta), and the net force on a dipole in a uniform field is zero. Answer: tau = 1.7 x 10^-4 N m.

Question 6

A charge of 8.85 x 10^-6 C is placed at the centre of a cube of side 15 cm. Find the flux through one face.

Accepted Answer

The total flux is q/epsilon_0, and by symmetry each of the six faces carries one sixth of it, with the side length irrelevant. Answer: 1.67 x 10^5 N m^2/C.

Question 7

An infinite line charge produces a field of 4.5 x 10^4 N/C at 4 cm from it. Find the linear charge density.

Accepted Answer

Use E = lambda/(2 pi epsilon_0 r), remembering the field falls off as 1/r rather than 1/r^2. Answer: lambda = 1.0 x 10^-7 C/m.

Question 8

Three charges of +2 microcoulomb each sit at the vertices of an equilateral triangle of side 10 cm. Find the net force on any one of them.

Accepted Answer

Each of the two forces is 3.6 N, and they are separated by 60 degrees, so the resultant is found from the parallelogram law giving 2 F cos(30). Answer: 6.23 N, directed away from the centroid.

Practice — Electric Charges and Fields

Sign in to save attempts to your dashboard and track your accuracy over time

Find a verified tutor

Join a live course

Browse coaching centres

About

Categories

Support

Documentation

Get Study Tips & Updates

Download Our App

Download for iOS

Download for Android

Popular Cities