Force on a Moving Charge | CAIE 9702 Paper 4 Notes

Paper 4

Physics9702

Force on a Moving Charge

Current is moving charge inside a conductor. A single moving charged particle can also experience a magnetic force.

Calculate the force magnitude

F=B|q|v\sin\theta

Symbols

$F$ = force magnitude (N)
$B$ = magnetic flux density (T)
$|q|$ = magnitude of particle charge (C)
$v$ = particle speed (m s⁻¹)
$\theta$ = angle between velocity and magnetic field (—)

The force is maximum when the velocity is perpendicular to the field.
The force is zero when the velocity is parallel to the field.
The charge magnitude sets the size of the force. The charge sign sets which of two opposite directions the force takes.

Positive and negative charges turn in opposite directions

Swipe left or right to see the whole diagram →

Use Fleming's left-hand rule with conventional current. For a positive charge, current follows the velocity. For a negative charge, current points opposite to the velocity, so the force reverses.

For a positive charge, conventional current points in the same direction as the particle velocity. Use Fleming's left-hand rule directly.
For a negative charge, conventional current points opposite to the particle velocity. The force direction reverses.
In both cases, the force is perpendicular to the velocity and the magnetic field.

Your turnDirection check

An electron moves to the right through a magnetic field into the page. State the force direction.

Show worked answer

A positive charge would be forced upward. The electron is negative, so its force is downward.