Current in Magnetic Field
When a current goes through a magnetic field, the magnet field exerts a force on the current. This effect was first discovered in 1820: A current was passed through a wire on top of a compass, and the compass needle rotated.
We shall look at a different setup here. It will help us determine the direction of the force more easily. We have a magnet above a wire. The magnet points downwards, and the wire is horizontal. This wire is a thin copper rod that is not insulated. It sits on top of two long thin copper rods that are at right angles to it, and is free to move around. The two long copper rods are also not insulated, and are connected to a battery.
When the current is switched on, the rod moves.
If the magnet is removed, the rod does not move. This shows that the rod moves because of the magnetic field (and the current). If we reverse the magnet direction, the rod moves in the opposite direction. If we reverse the battery, the rod also moves in the opposite direction.
The current in the wire is really a flow of electrons. It is possible to produce a beam of pure electrons flowing through space, without a wire. A magnet close by will also cause this beam to move.
Such an electron flow through space happens in a cathode ray tube (CRT). Cathode ray tubes are used in televisions and oscilloscopes. A cathode ray tube is made up of a glass tube that is air tight, with a vacuum inside. There is an electron gun at one end of the tube, and a fluorescent screen at the other end. A fluorescent screen is just one side of the glass that has a layer of special chemical on the inner wall. When electron beam falls on this chemical, it produces light.
The electron gun produces a narrow beam of electrons at goes to the screen. When the electron beam falls on the screen, it produces a bright spot on the screen. We cannot see the electron beam with our eyes, but we can see the bright spot. The bright spot tells us that there is a beam going from the electron gun to the screen.
If we now bring a magnet close to the cathode ray tube, you will see the spot move. This means that the electron beam has moved.
The magnet itself is not in contact with the electron beam, but the magnetic field fills the space all around the beam. Since the beam moves when the field is there, we conclude that the magnetic field must have exerted a force on the beam.
As with the current in the wire, if we reverse the direction of the magnet, the spot moves in the opposite direction.