Edexcel A-Level Physics 9PH0
Working as a Physicist
Mechanics
Electric Circuits
Materials
Waves and Particle Nature of Light
Further Mechanics
Electric and Magnetic Fields
Nuclear and Particle Physics
Thermodynamics
Space
Nuclear Radiation
Gravitational Fields
Oscillations
Electric and Magnetic Fields
Forces on currents and moving charges
- 2 things to know
- Papers 1 and 3
Overview
Recall questions on F = BIl sinθ and F = BQv, Fleming's left-hand rule, why a magnetic field does no work, and why a charge entering at right angles follows a circle.
What you need to know
Answer each one before you reveal it.
What happens to the direction of the force on a current-carrying wire in a magnetic field if the current is reversed?
Questions from this subtopic
Each one with the model answer underneath.
- 1
State the equation for the force on a charged particle moving through a magnetic field:
Model answer
F = Bqv sin θ, where q is the charge, v its speed and θ the angle between the velocity and the field.
- 2
What do the thumb and first two fingers represent in Fleming's left-hand rule?
Model answer
The first finger represents the field, the second finger the conventional current and the thumb the direction of the force or motion.
- 3
How is Fleming's left-hand rule applied to an electron moving through a magnetic field?
Model answer
The second finger must point opposite to the electron's velocity, because conventional current is in the opposite direction to the flow of negative charge.
- 4
Why does a charged particle entering a uniform magnetic field at right angles follow a circular path?
Model answer
The magnetic force is always perpendicular to the velocity and constant in magnitude, so it acts as a centripetal force and continually changes the direction of motion without changing the speed.
- 5
Why does a magnetic field do no work on a moving charged particle?
Model answer
Because the force is always at right angles to the velocity, so there is no component of force along the direction of motion and the kinetic energy is unchanged.
- 6
What happens to a charged particle that moves parallel to a magnetic field?
Model answer
It experiences no magnetic force, because sin θ is zero, so it continues in a straight line at constant speed.
Other ways to work this
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