What is meant by a magnetic circuit?

A magnetic circuit is a closed path of high-permeability material around which magnetic flux is guided, in the same way that a conducting circuit guides electric current.

What is meant by the permeance of a magnetic circuit?

Permeance is the measure of how readily a magnetic path carries flux for a given number of current-turns, and it is the magnetic equivalent of electrical conductance.

On what does the permeance of a piece of magnetic material depend?

It increases with the cross-sectional area of the path and with the permeability of the material, and it decreases as the length of the path increases.

State the corresponding expression for the conductance of a conductor, and say why the comparison is useful.

Conductance is proportional to cross-sectional area divided by length and depends on the conductivity of the material, and permeance depends on the dimensions of the magnetic path in exactly the same way.

OCR A-Level Physics B H557

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Electromagnetism

Magnetic circuits, permeance and machine design

3 things to know
Components 01, 02 and 03

Overview

Recall questions on magnetic circuits and their analogy with electric circuits, flux = permeance × current-turns, what permeance depends on, the conductance of a conductor, iron cores and air gaps, low-resistance windings, ampere-turns, why larger machines perform better and scaling by two, constant flux around a series circuit, soft magnetic cores, and why there is no magnetic insulator.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why is there no true magnetic insulator, unlike the electrical case?

Key term

State the analogy between the quantities of an electric circuit and those of a magnetic circuit.

Key term

Write the relationship between flux, current-turns and permeance.

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by a magnetic circuit?
Model answer
A magnetic circuit is a closed path of high-permeability material around which magnetic flux is guided, in the same way that a conducting circuit guides electric current.
2
State the analogy between the quantities of an electric circuit and those of a magnetic circuit.
Model answer
Current-turns in the coil play the part of e.m.f., the flux plays the part of the current, and the permeance of the magnetic path plays the part of the electrical conductance.
3
Write the relationship between flux, current-turns and permeance.
Model answer
The flux is the product of the current-turns and the permeance of the magnetic circuit, Φ = permeance × NI.
4
What is meant by the permeance of a magnetic circuit?
Model answer
Permeance is the measure of how readily a magnetic path carries flux for a given number of current-turns, and it is the magnetic equivalent of electrical conductance.
5
On what does the permeance of a piece of magnetic material depend?
Model answer
It increases with the cross-sectional area of the path and with the permeability of the material, and it decreases as the length of the path increases.
6
State the corresponding expression for the conductance of a conductor, and say why the comparison is useful.
Model answer
Conductance is proportional to cross-sectional area divided by length and depends on the conductivity of the material, and permeance depends on the dimensions of the magnetic path in exactly the same way.

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