Electricity- Electromagnets continued

Electromagnets continued
If the wire is bent into a loop the magnetic field lines become bunched up inside the loop as shown on the right. If another loop is formed, overlapping the first, the density of magnetic field lines increases to double that of a single loop. It follows that the magnetic field increases as the number of loops increases.
An electromagnet is made by placing a piece of iron inside the coil of current conducting wire. The strength of the electromagnet depends on the number of loops, assuming current is kept constant. The two electromagnets shown on the right are formed by wrapping insulated wire around the iron bolt and nail. When using the same current it is the electromagnet on the right, with 80 loops, that will be stronger than the 30 loop electromagnet on the left. Click to see a 120kb movie of a home made electromagnet at work.
	The magnetic field lines inside a looped wire form the pattern shown on the left when a current flows through the wire. The strength of the magnetic field is strongest at the points where the field lines are closest to each other, for example, at the poles.
	Look at the video on the left. How do we know that this is an electromagnet? At what points along the electromagnet is the magnetic field the strongest? How can you tell? Explain why the paper clips are not attracted to the electromagnet at its centre but appear to be attracted at the ends.
Activity with electromagnets

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