Magnetic Fields

Properties of Magnets

• Polarized- has two ends (north-seeking and south-seeking)

• Can cause other materials to become temporarily polarized

• Permanent magnets remain polarized due to atomic structure

• Magnetic fields exist around magnets and describe the magnetic force

• Magnetic flux- number of magnetic field lines passing through a surface. They are proportional to strength.

• Field lines start at the north pole and go toward the south pole. They do not stop, but go through the magnet.

• They are 3- dimensional.

The Earth as a Magnet

Magnetic Domains

• Each electron acts as an electromagnet• Neighboring electrons’ magnetic fields combine and

align• Non-magnetic material have domains, but the magnetic

fields cancel each other out making it non-magnetic overall

• Hans Oersted experimented with a compass and a circuit

• He connected the compass to the wire. The wire was connected to a battery.

• He expected to see the compass point in the same direction as the current.

• He was amazed to see that the needle rotated until it pointed perpendicular to the wire.

• When there was no current in the wire, there was no magnetic field that existed.

Electromagnetism

• **If a compass needle turns when placed near a current-carrying wire, it must be the result of a magnetic field.

• A current-carrying wire forms a magnetic field in closed loops around the wire.

• https://www.youtube.com/watch?v=c3WWAghWbjo

• To determine the direction of the magnetic field, take your right hand and point your thumb to the direction of the current. Your fingers will wrap around the wire in the direction of the magnetic field.

• Solenoid- a long coil of wire consisting of many loops.

• The field from each loop adds to the fields of other loops.

• When there is a current in the wire, the coil has a field similar to a permanent magnet. It has a north and south pole. It becomes an electromagnet.

• Increasing the number of loops, increases the strength. Adding an iron rod to the center will also increase strength.

http://phet.colorado.edu/en/simulation/faraday

• Faraday discovered that the force on a wire is at right angles to both the direction of the magnetic field and the direction of the current.

• F= BIL

• B- Magnetic field (Teslas)

• I - Current

• L- Length of wire exposed to B-field

Practice• A straight wire carrying a 5.0 A current is in a

uniform magnetic field. When 0.10 m of the wire is in the field, the force on the wire is 0.20 N. What is the magnetic field’s strength?

• F= BIL

• 0.20 = B (5.0)(0.10)

• B= 0.40 T

• A wire carrying a current of 3.0 A is in a uniform magnetic field. The wire makes an angle of 15° with the field. The wire is 35 cm in length and the force on the wire is 0.80 N. What is the strength of the magnetic field?