Cyberphysics - Electromagnetism

Electromagnetism

Moving charges create a magnetic field. Therefore all electrical wires have a magnetic field around them (but only when the current flows). The field lines are a concentric circle pattern. The direction of the field lines can be found using the right hand grip rule. You must know this. Grip a pencil in your right hand so that your thumb points in the same direction as the pencil tip and your fingers curve around it. The pencil represents the current and its tip the direction of the current arrow. Place the pencil on the page (tip into or away from page as required) and then your fingers point in the direction of the field lines.

Symbol for a current carrying wire; a section through a current carrying wire has either the arrow tip (current coming out of the page at you) or the cross of the tail feathers of the arrow (current going into the page away from you).
NB Don't treat the circle for the edge of the wire as a field line and put an arrow on it!!

A solenoid is a long coil of wire. It's field pattern is like a bar magnet's, the only difference is that you must extend the field lines through the centre of the coil... they therefore form loops instead of starting and ending on a pole. They are virtually parallel through the centre of the coil.

See the animation of this by clicking here.

Don't draw too many or you'll find it difficult to keep the diagram symmetrical and correct.
Draw field lines in a different colour from the wiring.
Ensure the circuit is complete.
Don't forget to put in the current direction and then find out which end of the coil acts like which pole of a magnet. Mark these clearly on your diagram. .
Finally put in the field line direction.

The solenoid acts like an electromagnet

The bigger the current, the stronger the field, the greater the number of turns, the stronger the field- (The stronger the field the closer the field lines). The presence of a soft iron core increases the strength of the field substantially.

The core must be soft otherwise when the current is switched off the core would still be magnetised.

A d.c. power source (e.g.. battery) must be used so that the current only flows in one direction. If an a.c. current (mains supply) is used the direction that current flows changes so many times in a second that the domains in the core do not have time to line up in one direction before they are pulled into the opposite direction. This results in a random arrangement of domains and a net zero magnetic field in the core.

Making an electromagnet

See the electromagnet experiment

Uses of the electromagnet

To pick up ferromagnetic materials in a scrap yard. Attached to a crane it is useful to pick up scrap iron and steel. There is no need for careful positioning of a hook and you simply switch the current off to drop scrap (disentangling a hook can be tedious).
To sort ferromagnetic materials from non-magnetic materials such as aluminium in a scrap yard.
In circuits like the electric bell.

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