An electromagnet is something which is able to produce a magnetic field when an electrical current is passed through it. Electro magnets can be turned off just by stopping the flow of current unlike permanent magnets which produce a permanent magnetic field. The strength of an electro magnet can be altered as needed by increasing or decreasing the amount of current whereas permanent magnets have a constant magnetic field.
An electro magnet consists of a wire wrapped around an iron core. This coil of wire is called a solenoid and the coils are the secret to how a magnetic field can be produced which is similar to a bar magnets magnetic field. When a current is passed through a single straight wire a circular magnetic field is produced around it. When the wire is coiled each turn interacts with adjacent turns in such a way that each circular magnetic field interacts with the next and they strengthen each other. This creates a strong magnetic field through the centre of the coil.
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In the centre of an electro magnet is an iron core and this helps to strengthen the magnetic field produced by the electromagnet. This is because each individual iron atom acts as a very small magnet and in a non-magnetic piece of metal they are arranged randomly and the magnetic field of each iron atom cancels out the next. When current flows through the coil wrapped around the iron core the magnet field produced by the coil causes the atoms inside the iron core to align and the magnetic fields of the iron atoms will no longer cancel out. The magnetic field of the coil cause the atoms in the iron core to align and the strength of the magnet fields all add together resulting in a strong overall field.
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Start by wrapping the iron nail with the insulated wire so that it forms a tight coil around it. Then secure the wire around the nail with sellotape. Now all you have to do is to connect the ends of the wire to the battery and the electrical current will cause a magnetic field to be produced.
The factors which influence the strength of an electro magnet are the amount of current flowing through the wire and the number of coils. If you were to change one of these factors you would be able to see a change in the strength of the magnetic field. To measure the change in the strength of a magnetic field you could use a compass and see what the furthest point away from the electro magnet is until the needle no longer points toward the electro magnet. Or you could try testing the strength of the magnet field by testing what the weight of iron it can pick up. To do this put a piece of iron on a scale weighing 100g or anything you know is more weight than the electro magnet can lift. Then turn on the electro magnet and touch it to the piece of iron. Watching the reading on the scale closely start to lift the electromagnet and record the reading on the scale just before the electro magnet loses contact. Subtract this reading from the weight of the piece of iron and this will give you the weight that the electromagnet was able to lift.
Note: the battery may get very warm in this experiment. Do not leave the electromagnet connected to long. The battery will also be drained very quickly so you should have a fresh battery to use in each test to keep the experiment fair.
There are other ways to increase the voltage produced by a single cell. For example if instead of using a copper coin you could use a small coil of copper the size of the coin and this will greatly increase the surface area speeding up the chemical reaction and thus increasing the voltage.
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