Electricity and Magnetism

AP Physics

History

2000 years ago Greeks

Chinese Use for Navigation

1296 Pierre Maricourt – needle orientation

1600 William Gilbert – Earth is a magnet

1820 Hans Christian Oersted – Connection between electricity & magnetism

Today – Maglev Trains (No Friction!)

Magnets

Have the property of magnetism.

Ferromagnetic materials (ie iron): Spin of electrons line up in small regions called domains.

Magnetic domains can align in a given direction to allow a magnet to induce magnetism.

Lines of magnetic flux: the field lines of a magnet (similar to electric field lines)

Two types of Magnets

1.Temporary or Soft – iron can easily lose its magnetic property. Iron is magnetic while placed inside a current carrying coil. This is the basis for electromagnets.

2.Permanent or Hard - steel

Magnetic Poles

Hang a bar magnet by a thread.

It will line-up in a north-south direction.

The end pointing north is the “north pole”

The other end is the “south pole”

Magnetic Poles

Similar to electric charges

Always occur in pairs

Opposite (unlike) poles attract

Like poles of a magnet repel

Earth as a Magnet

Earth is one big magnet.

If the north pole of a bar magnet is pointing to the north on Earth, what pole of the Earth is the bar magnet pointing to?

A bar magnet’s north pole is attracted to the Earth’s south pole.

Magnetic Fields

Used to describe magnetism.

Describe the condition in space – moving charges experience a force perpendicular to their velocity.

a.k.a. Defined by measuring the force the field exerts on a moving charged particle, such as an electron. 

Magnetic Field is a Vector

A magnet produces a vector field, the magnetic field, at all points in the space around it. 

Magnetic Field Lines

Run out of North and into south.

Used to picture the magnetic field

Similar to an electric field but…

Not the same. Electric charges

can be isolated, magnetic poles

cannot be isolated.

Magnetic Field

Strength of magnetic field measured in Teslas

SI unit T Symbol is a B

Magnetic Field

B = (µo /2π) (I/r) r = distance from wire (m) µo = permeability of free space = 4πx10-7 T m/A

So µo /2π = 2x10-7T m/A

I = current

Magnetic Force

One of the four fundamental forces

Part of Electromagnetic Force

Acts within a magnetic field.

Strongest at the poles of a magnet

Force of Magnetic Field on Current Carrying Wire

F = ILB a.k.a. F = ILB Sin Ө

I is current, L is the length of the wire, B is the strength of the magnetic Field, Ө is the angle the wire makes with the

magnetic field (angle the direction of the magnetic field is going to the direction the current is going in the wire.)

Force on a current carrying wire

Force is greatest when magnetic field is ┴ to the magnetic field (Ө = 90º or 270º)

Into the page symbolized by an x or a circled x

No force where Ө = 0º or 180º

Force of a Magnetic Field on a Moving Charge

Hans Christian Oersted figured out that there is a relationship between electricity and magnetism.

He aligned a compass and a wire along the Earth’s magnetic field. When put current through the wire the compass swung East-West.

Force of a Magnetic Field on a Moving Charge

The force (F) is equal to the charge (q) times the speed of the particle (v) times the magnitude of the field (B), or

F = q*v x B, where the direction of F is at right angles to both v and B as a result of the cross product.  This defines the magnetic field's strength and direction at any point. 

F = qvBsinӨ

Right Hand Rules - Purpose

Relate directions when dealing with magnetism

Right Hand Rule for Magnetic Field of a Current Carrying Wire

a)Use right hand to grip wire

b)Point thumb in the direction of I

c)Fingers curl in the direction of B

RHR to tell the Pole of a Magnet

a)Wrap fingers of right hand around the electromagnet in the direction of I.

b)Thumb points to North

RHR for Direction of Force produced by a Current Carrying Wire in a Magnetic Field

a)Point fingers of the right hand in the direction of B.

b)Point thumb in the direction of I

c)Palm points in the direction of F.

Sources

http://www.windows.ucar.edu/tour/link=/earth/Magnetosphere/earth_magnetic_poles.html

http://www.coolmagnetman.com/magfield.htm

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html#c1

http://library.thinkquest.org/16600/advanced/magneticfields.shtml