Electromagnets…

An electromagnet is a magnet which is activated by the flow of electricity. This magnetism is temporary and can be switched off by removing the current. In fact, any wire with an electric current passing through it has a magnetic field surrounding it. This field is intensified by wrapping the coils around an iron core. The strength of the magnet can be increased by…

1. Increasing the current (up to a saturation point)
2. Increasing the numbers of coils
3. Using a softer ferromagnetic core

In a piece of soft iron, areas of potential magnetism are randomly spread about so that they cancel each other out, meaning that the iron is not magnetic overall. When a current is passed though a coil around the iron, the magnetic regions line up greatly increasing the strength of the magnetic field. When the current is switched off the magnetic areas go back to their original random arrangement.

Since the only resistance in an electromagnet is the resistance of the wire (which tends to be low) high currents are often produced and so the energy applied is released as heat. A little care is needed when building them so that they do not overheat and cause a fire. High currents will also drain the batteries very quickly.

Electromagnets are important as key components in speakers and microphones; motors and dynamos; relays (electric switches) and magnetic locks; electric bells. I have included some diagrams below to show these uses.

Here is a simple electric motor we could build in the lab – a seven minute video if you are short of time. Very like things you may have built with Mr Yeung.

In the video below, the liquid nitrogen cools the metal ring to make it behave like a superconductor. DC current is ‘direct current’ as you would get from a battery, AC current is ‘alternating current’ as you would get from the mains.

You may have seen me build similar/simpler homopolar motors but nothing as clever as this.

2B (from 2011) built some fabulous homopolar motors – click here to see them.

Questions…

1. As well as being magnetic, iron does something else that other metals don’t do, but it needs both water and oxygen to be present. What is iron’s other unique property?
2. How could an electromagnet be used to separate aluminium and iron in a recycling plant?
3. What equation links voltage, current and resistance?
4. Iron and steel both contain iron atoms but what is the difference between iron and steel?
5. Name a transducer that (a) converts electrical energy to kinetic energy (b) converts kinetic energy to electrical energy (c) converts electrical energy to sound energy (d) changes sound energy to electrical energy