How do moving charges give rise to a magnetic field?

When electric charges move (as a current or as individual particles), they create a magnetic field in the surrounding space.

Why does a charged particle moving perpendicular to a uniform magnetic field follow a circular path?

The magnetic force is always perpendicular to the velocity, so it changes the direction of motion without changing speed, providing the centripetal acceleration needed for circular motion.

How is the expression r = mv/(BQ) derived?

By equating the magnetic force BQv to the centripetal force mv²/r and rearranging for r.

OCR A-Level Physics A H556

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Electromagnetism

Charged particles in magnetic fields

4 things to know
Papers 1, 2 and 3

Overview

Recall questions on the force on a charged particle in a magnetic field, circular paths and r = mv/BQ, velocity selectors, and calculations of force, orbit radius and selected velocity.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

State the formula for the force on a charged particle moving perpendicular to a magnetic field and define each term.

Key term

State the equation for the radius of the circular path of a charged particle in a magnetic field.

Numbers

Calculate the selected velocity when electric field strength = 9.1 × 10⁴ V m⁻¹, magnetic flux density = 0.02 T.

Key term

In a velocity selector, how are the electric and magnetic fields oriented relative to each other and to the particle's velocity?

Questions from this subtopic

Each one with the model answer underneath.

1
How do moving charges give rise to a magnetic field?
Model answer
When electric charges move (as a current or as individual particles), they create a magnetic field in the surrounding space.
2
State the formula for the force on a charged particle moving perpendicular to a magnetic field and define each term.
Model answer
F = BQv, where B is magnetic flux density, Q is the charge of the particle, and v is its velocity.
3
Why does a charged particle moving perpendicular to a uniform magnetic field follow a circular path?
Model answer
The magnetic force is always perpendicular to the velocity, so it changes the direction of motion without changing speed, providing the centripetal acceleration needed for circular motion.
4
State the equation for the radius of the circular path of a charged particle in a magnetic field.
Model answer
r = mv/(BQ), where m is mass, v is velocity, B is magnetic flux density, and Q is charge.
5
How is the expression r = mv/(BQ) derived?
Model answer
By equating the magnetic force BQv to the centripetal force mv²/r and rearranging for r.
6
In a velocity selector, how are the electric and magnetic fields oriented relative to each other and to the particle's velocity?
Model answer
The electric and magnetic fields are perpendicular to each other and both are perpendicular to the particle's velocity.

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