State the condition for a charged particle to experience a force in a magnetic field.

It must be moving, and its velocity must have a component at right angles to the field.

What force acts on a charge moving parallel to a magnetic field?

None, since sinθ is zero when the velocity is along the field.

How is the direction of the magnetic force on a positive charge found?

With Fleming's left-hand rule, taking the second finger along the direction the positive charge is moving.

How is the rule applied to a negative charge?

The conventional current is in the opposite direction to the motion of the negative charge, so the force comes out opposite to that for a positive charge moving the same way.

CCEA A-Level Physics 1210

Physical Quantities, Scalars and Vectors

Topic overview Physical quantities and SI base units Prefixes and standard form Scalars and vectors Resolving vectors and finding resultants Coplanar forces in equilibrium

Moments and Equilibrium

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Linear Motion and Dynamics

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Newton's Laws of Motion

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Linear Momentum and Impulse

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Work, Energy and Power

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Current, Charge and Potential Difference

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Resistance and Resistivity

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Internal Resistance and Potential Dividers

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Refraction

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Lenses

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Superposition, Interference and Diffraction

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Wave-Particle Duality

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Thermal Physics

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Simple Harmonic Motion

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Fundamental Particles

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Charged Particles and Accelerators

Charged particles in a magnetic field

3 things to know
Unit A2 2

Overview

Recall questions on why the charge must be moving, F = Bqv and its sinθ form, the zero force for motion along the field, applying Fleming's left-hand rule to a positive and to a negative charge, the derivation of F = Bqv from F = BIl, why the magnetic force does no work, and the circular and helical paths that result.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Show that F = Bqv follows from F = BIl.

Key term

Why does a stationary charge feel no magnetic force?

Key term

Give the equation for the force on a moving charge in a magnetic field and define each symbol.

Questions from this subtopic

Each one with the model answer underneath.

1
State the condition for a charged particle to experience a force in a magnetic field.
Model answer
It must be moving, and its velocity must have a component at right angles to the field.
2
Why does a stationary charge feel no magnetic force?
Model answer
The magnetic force depends on the velocity of the charge, so a charge at rest experiences none however strong the field.
3
Give the equation for the force on a moving charge in a magnetic field and define each symbol.
Model answer
F = Bqv, where F is the force in newtons, B the magnetic flux density in tesla, q the charge in coulombs and v the component of velocity perpendicular to the field in m s⁻¹.
4
What force acts on a charge moving parallel to a magnetic field?
Model answer
None, since sinθ is zero when the velocity is along the field.
5
How is the direction of the magnetic force on a positive charge found?
Model answer
With Fleming's left-hand rule, taking the second finger along the direction the positive charge is moving.
6
How is the rule applied to a negative charge?
Model answer
The conventional current is in the opposite direction to the motion of the negative charge, so the force comes out opposite to that for a positive charge moving the same way.

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