Lecture 10Magnetism

Chapter 5.12 5.20

Outline

• Magnets• Magnetic Field• The Electromagnetic Field• Electromagnetic Induction

Magnets

The simplest magnet is a somehow magnetized bar of iron.It attracts and holds other pieces of iron.Most of the force a magnet exerts from its ends.

A magnet’s ends point north and south.The north-pointing end is called the north pole.The south-pointing end is called the south pole.

Like magnet poles repel one another,unlike poles attract one another.

Magnetic Poles

Positive and negative charges in neutral matter can be separated from each other.The north and south magnetic poles cannot be separated.

Every time we saw a magnet in half, it gets the 2 poles.

Magnetic properties have all substances, but to a different extent. Iron is one of the best magnets.

Magnetic Field

The presence of a magnet alters properties of space near it.

The altered space is called a force field.We cannot see a force field, but can detect its presence by its effects.The form of the magnetic field can be traced by a pattern of iron filings.They line up in the direction in which a piece of iron would move if put there (field lines).They gather most thickly where the force on the iron would be the greatest (larger field line density).

Origin of the Magnetic Field

Every electric current has a magnetic field around it.This was first shown by Oersted in 1820.

The current and the field are perpendicular to each other.The direction of the magnetic field can be found by encircling the wire with the fingers of the right hand.The direction of the current along the wire is then shown by the thumb.

All magnetic fields originate from moving electric charges.

The Electromagnetic Field

An electric charge at rest is surrounded by only an electric field.When it is moving, a magnetic field around it appears.

The relative motion between the charge and observer is needed to produce a magnetic field.

Both an electric and magnetic field are aspects of a single electromagnetic field that surrounds every electric charge.

Electromagnetic Induction

If an electric current produces a magnetic field, then a magnet should be able to generate an electric current.

A current is produced in a wire when there is relative motion between the wire and a magnetic field.

Such a current is called an induced current.The effect is called electromagnetic induction (discovered by Michael Faraday in the XIX century).The strength of the current depends on the magnetic field strength and the wire’s speed.

Summary

• Gravity, electric, and magnetic forces alter properties of the surrounding space.

These properties are called force fields.

• A single electromagnetic field surrounds every moving electric charge.

• Electromagnetic forces is one of the 4 fundamental forces which exist in the Universe.