Circles in a Field

Part of the Electricity and Magnetism section of Coddy's Physics journey. Lesson 43 of 51.

At right angles to a uniform field, a charge moves on a circle: the magnetic force is the centripetal force.

q v B = m v² / r, r = m v / (q B)

A positive charge moving on a circle in a field into the page, the force toward the centre

The magnetic force always points to the centre: the charge goes round.

One turn takes T = 2π m / (q B), whatever the speed.

Challenge

A proton moving at 10⁶ m/s enters a 0.5 T field at right angles to it. What is the radius r of the circle it moves on?

Try it yourself

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Electricity and Magnetism

1Electric Charge and Force

4Charged Conductors

7Real Batteries

10Magnetic Fields

2The Electric Field

5Capacitors

8Kirchhoff's Laws

11Magnetic Forces

3Energy and Potential

6Current and Resistance

9Capacitors in Circuits

12Induction