Copper - electromagnets. Solenoids.

page 3

Solenoids

Picture 5. A coil of copper. We call this a solenoid. It is made from very thin strands of wire which have been covered in enamel to insulate them from each other.


		An electromagnet is a coil of wire with an electric current flowing through it. When the wire is coiled around in a cylinder, we call this a solenoid. The solenoid becomes an electromagnet when a current flows through it.

	Why use copper?

		Copper is used because it has a low electrical resistance (see conducting properties). This means that it is easy for the current to flow through it. Also, copper wire can be easily shaped to make a coil.

Interactive graphic of solenoid and poles

Picture 6. Closing the switch lets a current flow. This makes the coil into an electromagnet.
You can remember the poles because the letter N will point anticlockwise and an S will point clockwise.

	What's the field like?

		When the current flows through the wire, it makes the coil into a magnet. We call this an electromagnet. The field of the electromagnet is similar to the field of a bar magnet. The coil has a north pole at one end and a south pole at the other. Remember, that we show the lines of force coming out of the north pole and going into the south pole.

	How to remember the field

		When a current passes through the solenoid in picture 6, it becomes an electromagnet. So one end is a north pole and the other end is a south pole. There is a little trick for remembering which end becomes which pole. It's a question of seeing whether the letter S or N will point in the same direction as the current. When we look at one end of the coil, the current is going anticlockwise. In this case, we can put in the letter N and it will point in the same direction as the current. An S will not. So this is the north pole. If we look at the other end of the coil, the current is going clockwise. In this case we can fit in an S. So this is the south pole.

Picture 7. An entryphone system has a special latch holder in the doorframe. When the occupier presses the button, the iron armature is pulled back allowing door to be opened.

	Using electromagnets

		One use of an electromagnet is a simple door entry system for a block of flats. Someone on the fourth floor doesn't want to run downstairs to let people in. So they have a switch which operates an electromagnet in the lock. When they press the switch, the coil of copper wire becomes an electromagnet. So it attracts the armature and pulls back the lock. The door can now be opened.