Electricity and Magnetism

Magnetism

CH 27: Magnetism

We have previously discussed electric fields from stationary charges, and we have begun to discuss moving charges as current moving through a wire.

We are now going to examine the electric field present from a moving charge.

When a positive charge is stationary it has an electric field directed away from the charge in all directions. When this charge begins moving the electric field gets distorted. The result of this distortion in the electric field is a Magnetic Field.

Magnetic Field – a field effect that is observed for moving charges.

We typically discuss magnetic fields as the effects from magnets. There are two different categories of magnets:

1. Permanent magnets2. Electromagnets

Permanent Magnets – Materials that have a permanent magnetic field.Where does this magnetic field come from, if magnetic fields require moving charges?

The motion of electrons around a nucleus.

Each atom has a certain magnetic orientation, and groups of atoms with the same magnetic orientation are called magnetic domains.

When a majority of the magnetic domains in a material are aligned, the material becomes a permanent magnet.

Most materials cannot maintain a permanent alignment of magnetic domains.

Electromagnets – magnets created through the use of an electric current.

All current carrying wires generate a magnetic field.

Magnets

All magnets have a North and South pole.

Similar to charges, where the north pole would be the positive and the south pole would be the negative.

The magnetic field of a magnet is always directed from the north pole to the south pole.

There are no magnetic monopoles!

Similar poles repel, opposite poles attract.

The magnetic field of the bar magnet in the picture below is rendered visible by sprinkling iron filings on a plastic sheet resting on the magnet.

Suppose that the magnet is now broken into two parts, with the section on the left including about one-third of the original magnet and the section on the right including about two-thirds. What will happen to the magnet? It might break so that one pole is in the left segment and the other pole is in the right segment. Or. Perhaps it will break up such that each of the parts becomes a bar magnet on its own, containing both North and South poles. Or maybe breaking the magnet into two parts will cause the magnet to demagnetize, so no magnetic fields are present after the break. Exactly what is the result of breaking the magnet into two parts? When the magnet is broken into two parts: (1) each part will contain one magnet pole. (2) each part will be a new bar magnet including both poles. (3) the magnet will become demagnetized, resulting on no magnets.

Magnetic fields, like electric fields interact over distances. Any moving charge creates a magnetic field and hence will interact with any other magnetic field. Therefore if we place a moving charge in an external magnetic field there will be an interaction between the magnetic field created by the moving charge and the external magnetic field.

What we would observe is a deflection in the motion of the moving charge (it changes direction).

What do we do to change the direction that something is moving?Exert a force on the object.

When a moving charge is placed in an external magnetic field it experiences a force.

What do you think this force will depend on?

Magnetic field strength, magnitude of the charge and the velocity of the charge.

BF qv B FB – Magnetic force [N]

q – Charge [C]v – velocity of charge [m/s]B – Magnetic field strength [T]

T - Tesla

N NT

m AmCs

You can also measure the magnetic field strength in Gauss [G]. 41 10T G

Determination of the magnitude of the magnetic force is usually straight forward, but the direction can be more difficult.

The magnetic force is determined from the cross-product. What do we know about the cross-product in general?

sinBF qvB Scalar form of cross product, which will allow us to determine the magnitude.

FB is perpendicular to both v and B

1)

2)

To determine the direction we will use a simple tool called the Right-Hand Rule.

Right-Hand Rule:

The purpose of the right-hand rule is to determine the direction of the resultant of a cross-product using a right-handed coordinate system.

BF qv B

Thumb points in direction of first vector.

Fingers point in direction of second vector

Palm points in direction of resultant vector.(mimics pushing the charge in that direction)

This can be used for any cross-product! It can also be used to determine coordinate directions for a right handed coordinate system. The right-hand rule is for positive charges. Reverse the resultant direction for negative charges.