Fleming's left hand rule
If an electric current flows through a magnetic field, the field exerts a force on the current.
This can happen when there is a magnet close to a wire carrying a current. It can also happen when there is a magnet close to a beam of electrons. The force causes the wire or the beam to move.
The direction of this force depends on the directions of the field and the current. The way to find this direction is called Fleming's left hand rule.
Stretch out the thumb and first two fingers of your left hand. Hold them out so that they are all perpendicular to one another. Your thumb points in the direction of the force, your first finger points in the direction of the field, and your middle finger points in the direction of the current.
To find the direction of the force, point your first finger in the direction of the field and your middle finger in the direction of the current. Then your thumb will tell you the direction of the force. (When you do this, your thumb and two fingers MUST stay perpendicular to one another.)
It is not easy. Your first finger should point straight and your middle finger should be bent. I have seen a student do it the other way round!
The pictures below show two experiments. Stretch out your left hand and check that you know how to apply the rule to these examples.
Example 1. In the experiment below, the current is in the direction given by the arrow next to the sliding wire. Check that your left thumb points in the direction of movement of the wire. That would be the direction of the force.
Example 2. In the experiment below, there is an electron beam in a cathode ray oscilloscope. The current direction is opposite to the the electron beam direction. This is because electrons are negatively charged. (Since current direction is the direction of flow when charges are positive, it is opposite to the flow when charges are negative.)
The direction of force is the direction of movement of the bright spot on the screen, since that is where the beam is moved. Apply the left hand rule and check that your left thumb points in the right direction.