Chapter 24 Magnetic Fields

Objectives

• 24.1 Describe the properties of magnets and the origin of magnetism in materials

• 24.1 Compare various magnetic fields

Objectives

• 24.2 Relate magnetic induction to the direction of the force on a current carrying wire in a magnetic field

• 24.2 Solve problems involving magnetic field strength and the forces on current-carrying wire, and on moving, charged particles in magnetic fields

• 24.2 Describe the design and operation of an electric field motor

Magnetic Poles

• Magnets are polarized– One positive end, one negative end– Compasses are just magnets free to spin

• If you slice a magnet in half, you retain two opposite poles– No scientist yet to make a monopole

Opposites Attract

The Earth

• Earth is a big magnet. The actual location of the magnetic poles change every year by about 40 miles

Magnetic Declination

• Your compass points to the magnetic pole, not to the true north pole

• As you approach the poles, you have to add/subtract degrees to go the right direction

• The north arrow on the compass rose (the large N) is pointed towards the place on the horizon directly beneath the North Star! That is, towards true north!

• And the needle (of course) points towards magnetic north! So the magnetic declination for this locality is 45 degrees west

Temporary Magnets

• Most metal objects are NOT magnetic but they can become magnets when in contact with a magnet– A magnet holding a nail polarizes the nail to make

it a temporary magnet

Permanent Magnets

• Of the common metals, only 3 actually produce a magnetic field– Iron, Nickel, Cobalt (The Iron Triad)– Many rare earth elements also do, but we don’t

need to know about those

Magnetic Flux

• The strength of the magnet.

• Strongest where the field lines are closest together

• Flux per unit area is proportional to strength

First Right Hand Rule

• Current moving causes a field (the current is Positive, opposite of the actual electrons)

• Bigger Current = Stronger field

• Thumb points towards North

Solenoid: Coils wrapped around with a current moving through

Thumb points towards the North End

• How computers work is through magnetized domains

Computers

• Surface covered in magnetic particles• Recording: Current goes through disk drive’s

read/write head (em magnet iron core). Current induces a magnetic field onto bits– Magnetic particles line up and orient

• To read/retrieve, no current is sent to the read/write head. The bands (bits) induce a current to the read/write head. 0 or 1

Questions

• A long, straight, current carrying wire runs from North to South. – A compass needle placed above the wire points

with its N-pole toward the east. In what direction is the current flowing?

– What direction would the needle point if the compass were below the wire points?

More on Fields

• Current induces a field and causes a force to act on current carrying wire (or other wires)– Uses below

• Loudspeakers (wire moves)• Galvonometer: Measures the current• TV’s: Lining up the electrons

Magnetic Fields

• Points towards the South Pole– So, South must be Negative end – Since magnetic fields point towards the negative

charge• Earth’s North pole– Is the south magnet

Magnetic Field

• Measured in Teslas

Calculating Strength of Magnetic Field

• F = BIL• Force = (Magnetic Field Strength) (Current)

(Length of wire)

• A wire 0.50 m long carries a current of 8.0 Amps at a right angle to a 0.40 T magnetic field. How strong a force acts on on the wire?

Force on a moving charged particle

• F = Bqv• Force = Field x Charge x Velocity of particle

• How strong a force acts on a moving electron which is traveling a 3,000,000 m/s through a uniform magnetic field of 0.04 T at right angles to the field.