A wire of length 0.15 m carrying 3.0 A lies at right angles to a field of flux density 0.20 T. Calculate the force on it.

F = ILB = 3.0 × 0.15 × 0.20 = 0.090 N.

How is the direction of the force on a current-carrying wire in a magnetic field found?

Fleming's left-hand rule is used, with the first finger along the field, the second finger along the conventional current and the thumb giving the force.

Explain the force between a magnet and a current-carrying coil in terms of induced poles.

The current in the coil makes it behave as a small magnet with its own north and south faces, and the attraction or repulsion between those poles and the poles of the magnet provides the force.

OCR A-Level Physics B H557

Planning

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Implementing

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Analysis

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Probing Deep Into Matter

Topic overview Particle accelerators and high-energy beams Scattering and the nuclear atom Paths of scattered particles and closest approach Discrete energy levels in atoms Electron standing waves and the confined electron Quarks, hadrons and leptons Conservation laws and nuclear equations Charged particles in detectors and magnetic fields Relativistic energy and the relativistic factor

Ionising Radiation and Risk

Topic overview The nature of ionising radiations Effects on living tissue and radiation safety Absorbed dose, effective dose and risk Binding energy and nuclear stability The Nuclear Valley and modes of decay Fission, fusion and nuclear power Activity, half-life and the decay constant Investigating absorption and half-life

Electromagnetism

Forces on current-carrying conductors

3 things to know
Components 01, 02 and 03

Overview

Recall questions on F = ILB and when it applies, force, flux density and force per unit length calculations, Fleming's left-hand rule, currents parallel to a field, the force explained by flux lines and induced poles, parallel wires attracting and repelling, electric motors and moving-coil loudspeakers, the turning effect on a coil, reversed currents, wires at an angle, and power lines.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State the equation for the force on a straight current-carrying wire in a uniform magnetic field, and the condition under which it applies.

Key term

What is the force on a wire that carries a current parallel to the magnetic field?

Key term

Explain qualitatively, in terms of flux lines, why a current-carrying wire in a field experiences a force.

Questions from this subtopic

Each one with the model answer underneath.

1
State the equation for the force on a straight current-carrying wire in a uniform magnetic field, and the condition under which it applies.
Model answer
The force is F = ILB, and it applies when the current is at right angles to the magnetic field.
2
A wire of length 0.15 m carrying 3.0 A lies at right angles to a field of flux density 0.20 T. Calculate the force on it.
Model answer
F = ILB = 3.0 × 0.15 × 0.20 = 0.090 N.
3
How is the direction of the force on a current-carrying wire in a magnetic field found?
Model answer
Fleming's left-hand rule is used, with the first finger along the field, the second finger along the conventional current and the thumb giving the force.
4
What is the force on a wire that carries a current parallel to the magnetic field?
Model answer
The force is zero, because there is no component of the current perpendicular to the field.
5
Explain qualitatively, in terms of flux lines, why a current-carrying wire in a field experiences a force.
Model answer
The field of the current adds to the applied field on one side and cancels it on the other, so the flux lines bunch on one side, and because flux lines behave as though they tend to contract the wire is pushed towards the sparser side.
6
Explain the force between a magnet and a current-carrying coil in terms of induced poles.
Model answer
The current in the coil makes it behave as a small magnet with its own north and south faces, and the attraction or repulsion between those poles and the poles of the magnet provides the force.

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