Magnetic force and moving charge
There is an important relationship between magnetism and electricity that powers much of today's world - the fact that a magnetic field is produced when there is an electric current in a wire. An electric bell is one common device that uses a magnetic force created by a current in a wire. Others are electromagnets used in junk yards, electrical transformers and electric motors. Early
discoveries Transformers Simple electromagnets Note: Be very careful not to touch the exposed wire ends as they are likely to be hot. This experiment is best done for a few seconds at a time, to stop the wires overheating.
The nail loses most of its magnetism when the electric current is turned off because it is made of "soft" iron, meaning that the magnetic domains don't stay permanently lined up. Permanent magnets are made of "hard" iron and, once magnetised the domains remain aligned. Electric bells
Here is the clever bit - as the spring is drawn towards the electromagnet, it loses contact with the electrical terminal and turns off the current. Because the electromagnet has only temporary magnetism, the spring is released and flies back towards the contact. The circuit is now complete and the process continues! What an innovative combination of electromagnetism, kinetic energy and sound energy as the vibrating bell compresses air which your ear interprets as sound.
The principle of electromagnetism is applied to many devices, including audio speakers, solenoids that connect your car's starter motor to the engine, machines holding steel sheets in place while being cut or bent, and even as massive cranes that lift scrap steel when the electric current is on, and drop it again when the current is switched off. You can make a model crane using a simple electromagnet and paper clips. Be sure that the paper clips are steel, not brass or plastic which are not attracted to magnets.
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