Section 2: Electricity and Magnetism Chapter 8: Magnetism and Its Uses

Learning Goals Describe the magnetic field produced by an electric current. Explain how an electromagnet produces a magnetic field. Describe how electromagnets are used. Explain how an electric motor operates.

Moving Charges Moving charges, like those in an electric current, produce magnetic fields. The magnetic field around a current-carrying wire forms a circular pattern around the wire.

Moving Charges The direction of the field depends on the direction of the current. The strength of the magnetic field depends on the amount of current flowing in the wire.

Electromagnet Electromagnet: a temporary magnet made by placing a piece of iron inside a current- carrying coil of wire. Magnetic field is present only when current is flowing in the wire coil.

Electromagnet Increase strength of the magnetic field by adding more turns to the wire coil or increasing the current passing through the wire.

Electromagnet If the wire is wrapped around an iron core, an electromagnet is formed.

Electromagnet Magnetic properties of electromagnets can be controlled by changing the electric current flowing through the wire coil. When no current is flowing, the electromagnet loses its magnetic properties. Strength can be increased by adding more turns of wire or by increasing the current.

Electromagnet Converts electrical energy into mechanical energy to do work! The forces exerted on an electromagnet by another magnet can be used to make the electromagnet rotate.

Galvanometer Galvanometer: a device that uses an electromagnet to measure electric current.

Electric Motor Electric Motor: a device that changes electrical energy into mechanical energy. Contains an electromagnet that is free to rotate between the poles of a permanent, fixed magnet. The coil is connected to a source of electric current (like a battery).

Electric Motor When current flows through the electromagnet, a magnetic field is produced. Changing the direction of the current causes the coil to keep rotating.

Electric Motor Rotation speed of electric motors can be controlled. Vary the amount of current flowing through the coil. More current = stronger magnetic field = faster turning coil

Electric Motor Example: A CD player produces varying electric current that causes an electromagnet in the speakers to move back and forth. This causes vibrations that produce the sound you hear.

Section Review How can you make an electromagnet? Answer: By wrapping a wire coil around an iron core and then send current through the wire.

Section Review Which device uses an electromagnet to measure current? Answer: Galvanometer.

Section Review Describe two ways you could change the strength of a magnetic field produced by an electromagnet? Answer: 1.) Change the number of wire loops 2.) change the amount of current in the wire

Section Review Why is it necessary to continually reverse the direction of current flow in an electric motor? Answer: Reversing the current causes the end of the coil to be attracted to the other pole of the magnet. This allows the coil to keep rotating.