Magnetism and Electromagnetism

MAGNETISM DAY 1

What causes magnetism?How is magnetism linked to electric charges?

• Magnetic fields come from moving charges

• a moving charge around an atom produces a magnetic field

• currents in wires will produce magnetic fields

• The opposite is also true: moving magnetic fields will cause charges to move (electromagnetic induction)

What causes magnetism?How is magnetism linked to electric charges?

• Magnetic forces are NOT the same as electric forces.

• How are they the same?• Like poles repel. Unlike poles

attract.

• How are they different?Magnetic poles always come in pairs. You can’t separate a North from South pole.

Domains and magnetism

• magnets—an object that has an alignment of domains due to alignment of atoms and their magnetic dipoles

• ferromagnetic materials (like iron) can become magnetic near a magnet because the domains line up

Magnetic Poles

• magnetic pole—where the magnetic field is strongest

• like poles repel (North repels North and South repels South)

• unlike poles attract (North attracts South)

Properties of Magnets

• cut a magnet in half and you get two magnets that still have a north and south pole

• heat it up or bang it around and you weaken the magnet by jumbling domains

• Make a magnet by passing current through iron (line up domains)

Earth’s magnetic field• Earth has a

magnetic field because of a rotating iron core

• So why does a North pole of a compass point North?

• Because the Earth’s magnetic south pole is located in geographic North

Moving charged particles

Right Hand Rule #1

*Thumb in direction of current

*Fingers show how the magnetic field wraps around the wire

What does the magnetic field look like around this wire?

Right hand rule #2Force on a moving charge

BvF qBUnits: Tesla (T)

Thumb = v = velocity of + charge (conventional current)Fingers = B = direction of magnetic fieldPalm = F = force on charge

X = into the page= out of the page

A positive particle is fired to the right through a magnetic field directed into the page. What is the resulting force on the particle?

Magnetic field into board

+

v v

FB

+

+

++

+

+

Note speed never changes; this causes the particle to curl.

Force is always to v

What will be the direction of the magnetic force on this proton?

What will be the direction of the magnetic force on this proton?

2nd Right hand rule

A loop of wire is held inside a magnetic field directed into the page as shown. The movable bar is dragged to the right with a speed v. What is the direction of the current in the loop?

MAGNETISM DAY 2

Electromagnet – electric current creates a magnet

• making an electromagnet—wrap a current carrying wire around iron core and it will set up a north and south pole

Electromagnetic inductionReview…so far we’ve

seen that a moving charge (a changing electric field) produces a magnetic field…the opposite is also true….

Faraday’s Law

A changing magnetic field will induce a voltage in a wire (and a resulting current)

Three every day Items that use electromagnetic induction

Electric Motors

Electric Generators

Electric Transformers

Electric motors

• Electromagnet in a magnetic field will rotate

• electrical energy used to produce mechanical energy

Electromagnet

• making an electromagnet—wrap a current carrying wire around iron core and it will set up a north and south pole

Electric motors

Electromagnet in a magnetic field will rotate (kinetic energy)

electrical energy is used to produce mechanical energy

Electric motor

Commutator ring

Commutator allows the current in the electromagnet to switch directions so the motor shaft spins continuously

Electric GeneratorUses mechanical energy to rotate wires in a magnetic field to produce electrical energy

Coil flip-flops so this produces an Alternating Current (AC) in the wire

Burn coal or use nuclear power to create steam from water to turn coils

Wind and water just turn the turbines directly

Something has to turn the coils

Electric Transformers

Alternating electrical current on one side produces a magnetic field which induces a voltage in the other side of the transformer

Steps up voltage with more windings on other side

Steps down voltage with fewer windings on other side

Power and energy are conserved! Remember, you can’t make more energy.

Putting it all together…

Spinning turbines and generators are inside this power plant

Electromagnet

• making an electromagnet—wrap a current carrying wire around iron core and it will set up a north and south pole