S-Cool Revision Summary

S-Cool Revision Summary

Inducing EMFs

Method 1:

Pick up a metal rod and swing it about in a magnetic field - for example, the Earth's magnetic field. Although you won't realise it, you have just induced an emf across the ends of the rod.

A simple version would be this:

As you swipe the metal bar to the left (as shown above) you sweep through the area of field shown by the crosses.

It's this movement through a field that induces (produces) an emf across the bar ends.

Factors Affecting the Amount of Induced EMF

Any of the following would mean that you induced more emf:

- A longer bar would 'sweep' out more area of field.

- A stronger field would mean you swept through more field lines when moving the same distance.

- A faster swipe would mean you swept out more area of the field per second.

So the induced emf depends on the length of the conductor, the strength of the magnetic field and the speed at which the conductor cuts the field.

Magnetic Flux

The magnetic field strength, B, multiplied by the area swept out by a conductor, A, is called the magnetic flux, Φ.

Φ = BA

Units of flux: weber, Wb.

Magnetic Flux Linkage, F

This is the magnetic flux for a coil. It is also measured in weber and has the symbol Φ. The difference is that a coil has more wire in the field, so for a coil, the equation becomes

'n' x the Magnetic Flux

where 'n' is the number of turns in the coil.

Faraday's Law

For a conductor in a changing magnetic field, the factors affecting the size of the induced emf are:

- How quickly the magnetic field is changing;

- The number of turns or loops of the conductor in the field.

This leads to Faraday's Law, which is that:

The emf induced is equal to the rate of change of magnetic flux linkage or the rate of flux cutting.

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Do you Induce a Current or do you Induce an EMF?

If you move a conductor through a magnetic field, you always induce an emf!

If there is a circuit available, the emf will push a current through it.

If there is no circuit you will still get an emf, but you won't get a current.


Φ = magnetic flux, Wb

B = magnetic field strength, T

A = area of the field swept out or area of the coil, m2

n = number of turns in a coil

E = induced e.m.f.

t = time, s

Δ = change in...