Moving charges in a magnetic field (A-level only) Flashcards – AQA A-level Physics (7408)

What is the magnitude of the magnetic force on a charge moving perpendicular to a uniform magnetic field?

$F=B|Q|v$, where $B$ is magnetic flux density, $Q$ is charge and $v$ is speed.

How is the magnetic force on a moving charge oriented relative to its velocity and the magnetic field?

It is perpendicular to both the velocity and the magnetic field.

How does the direction of magnetic force depend on the sign of a moving charge?

For a positive charge, it points in the direction of $\mathbf{v}\times\mathbf{B}$; for a negative charge, it points in the opposite direction.

Which three individual reversals would each reverse the magnetic force on a moving charge?

Reversing the charge sign, the velocity direction or the field direction.

How does magnetic force affect a charged particle’s speed and direction, and why?

It changes the particle’s direction, not its speed: the force is perpendicular to the velocity, so it does no work.

Under what conditions does a charged particle follow a circular path in a uniform magnetic field?

It moves perpendicular to the field, and the magnetic force is the only significant force acting on it.

What role does the magnetic force play in a charged particle’s circular motion?

It remains perpendicular to the velocity and provides the centripetal force.

How does a charged particle’s velocity change during circular motion in a uniform magnetic field?

Its speed stays constant, while its direction changes.

What is the radius of a charged particle’s circular path in a uniform magnetic field?

$r=\frac{mv}{B|Q|}$

How does increasing speed affect the radius of a charged particle’s circular path, with mass, charge magnitude and magnetic field fixed?

The radius increases.

How does increasing magnetic field strength affect the radius of a charged particle’s circular path, with mass, charge magnitude and speed fixed?

The radius decreases.

How do opposite charge signs affect the curvature of particles with the same initial velocity in the same magnetic field?

They curve in opposite directions.

What is the non-relativistic orbital period of a charged particle moving in a circular path in a uniform magnetic field?

$T=\frac{2\pi m}{B|Q|}$

How does speed affect the non-relativistic orbital period of a charged particle in a uniform magnetic field?

The period is independent of speed.