P3bL1 Electromagnetic Force

Key Words

Conductor - material that allows electricity to flow.

Current - a flow of charge.

Electricity - energy transported as charge.

Electric motor - a device that turns electrical energy into kinetic energy

Electromagnet - a device that produces a magnetic field when a current flow through it.

Magnetic field - a force field which affects magnetic materials.

Motor effect - the interaction between a magnetic field and an electric current which results in movement

Grade E

Electricity is always associated with magnetic fields.  If there is a current, there is a magnetic field, even though the wire itself might be made of copper, which is non-magnetic.

The field around a single wire looks like this:

In this picture, the current is going down into the board.  By sprinkling iron filings, we can show that there is a magnetic field that is circular.  If we use a compass to plot the direction, we see that it will be clockwise.  The direction of the field is determined by the screwdriver (or corkscrew) rule.  When you screw into the wood, you turn the screwdriver clockwise.

If we turned the current up, we would find that the magnetic field lines around the conductor would get closer together, that is, more concentrated.  The magnetic field will be stronger.

Grade C

The Motor Effect

This experiment tells us that if a magnetic field interacts with an electric current, there is a force, which results in the carbon rod moving.  It is called the motor effect.

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When the current is turned on in the way shown in the picture, the carbon rod rolls along the rails from left to right as shown.  We can then:

• Reverse the current and the rod will roll the other way (right to left);

• Reverse the magnetic field and the rod will roll the other way;

• Reverse both the magnetic field and the current, and the rod will roll in the original direction (left to right);

• Take away the magnet.  The rod will stay where it is (and get hot).

The reason behind this is that whenever an electric current flows, it ALWAYS makes a magnetic field.  It doesn't matter that the material the current flows through is not magnetic, a magnetic field is formed.  Carbon is certainly non-magnetic, but a field is formed around the carbon rod when the current flows.  This interacts with the field from the magnet.

If the current were larger the force would be larger.

Suppose we now turned the magnetic field so that it was horizontal rather than vertical:

The magnetic field is now parallel to the current.  We find that the rod does NOT move at all, however strong the current.

In order for there to be a force on a wire:

• there has to be a current;

• there has to be a magnetic field;

The force is maximum when the angle between the magnetic field and the current is 90 degrees.  It is zero when the current and the magnetic field are parallel.

The force can be increased by using a coil of wire instead of a single wire.

Grade A

We can determine the direction of the movement in the motor effect using Fleming's Left Hand Rule: