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Electromagnetism: 10.1 Introduction to electromagnetism

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INDEX for physics notes on electromagnetism and its uses

10.1 Introduction to electromagnetism

This is all about the connection between electricity and magnetism.

How do you make a magnet using electricity?

When a current flows through a wire (or any conductor) a magnetic field is created around the wire.

The field (of the magnetic flux) can be imagined as a series of concentric circles at right-angles (perpendicular) to the wire - which is at the centre of the magnetic field (see the diagrams below).

You can show the magnetic field patterns by sprinkling fine iron filings on a sheet of paper at 90o to the copper wire carrying a d.c. electric current.

You can demonstrate the presence of the magnetic field with iron filings and thickish wire carrying a relatively high current.

For the above diagram - imagine the current flowing through a straight wire and the magnetic field can be envisaged as a series of concentric rings about the axis of the conducting wire.

The direction of the magnetic field can be predicted from the 'right-hand thumb' rule.

If the current is flowing 'up' through the wire, the magnetic field runs anticlockwise and perpendicular to the wire.

You can show the direction of the field with a small plotting compass - two shown on the diagram above - and you can trace out the circular pattern of the magnetic field.

A few simple rules (apart from the right-hand thumb rule)

(i) If you reverse the direction of current flow, you also reverse the direction of the magnetic field.

(ii) The strength of the magnetic field is increased overall by increasing the current.

(Don't say by 'increasing the p.d.' without saying to increase the current!)

 (iii) For any current carrying wire, the closer you are to the wire, the greater the strength of the magnetic field.

The magnetic field flux lines get closer and closer together the nearer you are to the wire - meaning the magnetic field strength increases the nearer you are to the wire.

The strength of the magnetic field falls away quite rapidly at first as you get further from the wire, then the reduction rate slows down with increasing distance.

Its a non-linear graph.

INDEX for physics notes on electromagnetism and its uses

Keywords, phrases and learning objectives on electromagnetism

Know that electromagnetism is when a magnetic field is created around a wire carrying an electric current.

Know that from the direction of convention current, you can predict the magnetic field direction from north to south with the right-hand thumb rule.

Be able to describe how to show or plot the magnetic field around an electromagnet using iron filings or a plotting compass.


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