Chapter 22 Magnetism and its Uses

Objects

• 22.1 Describe the properties of magnets • 22.1 Define the region of force around a

magnets • 22.1 Model magnetic behavior using domains

Objects

• 22.2 Explain the magnetic effects of a current in a wire

• 22.2 Compare and contrast ammeters and voltmeters

• 22.2 Describe the function of an electric motor.

Objects

• 22.3 Describe how a generator produces an electric current using electromagnetic induction

• 22.3 Distinguish between alternating and direct current

• 22.3 Explain how a transformer can step up or step down the voltage of an alternate current

Objectives

• 22.4 Describe the characteristics of superconductors

• 22.4 Consider various applications of superconductivity

Key Words• Magnetism Magnetic Pole Magnetic Field • Magnetic Domain Electromagnet Ammeter

• Voltmeter Electric Motor

• Electromagnetic Induction

• Generator Direct Current Alternating current

• Transformer Solenoid

Magnetism

• Magnetism is a property of matter in which there is a force of attraction or repulsion between unlike or like poles– One positive end, one negative end

• Most metals area actually NOT magnetic– Iron, Cobalt and Nickel metals which are actually

magnetic– Other metals can be temporary magnets

Opposites Attract

The Earth

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

• Earth’s magnetic poles change yearly

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

• Compass is wrong close to poles

• 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

Magnetic Domains

• Groups of atoms with aligned magnetic poles are called

Magnetic Fields• Points towards the South Pole

– So, South must be Negative end – Since magnetic fields point towards the negative charge

• Magnetic Pole: End of magnet, strongest magnetic field• Earth’s North pole

– Is the south magnet

Electromagnet

• A type of magnet created by current passing through a coil

• Electromagnetic Induction: The process by which moving a wire through a magnetic field produces a current

Solenoid: Coils wrapped around with a current moving through. Used in MRI’s

Thumb points towards the North End

Magnetic Field Force– The current causes the magnetic field– The magnetic field in turns pushes on the wire

• Electric Motor: Turns electrical energy into mechanical energy

Reading Current and Voltage

• Voltmeter: Determines the voltage drop across a resistor

• Ammeters: Determine the current through a wire– Determined by deflection caused by the magnetic

field

Generator

• Produces electric current by rotating wires in a magnetic field– See simulator

Direct Current

• DC = Direct Current. The flow (current) of electrons is in one direction– Like a river runs only down hill, direct current only

flows one way.– Batteries provide this type of current

Alternating Current

• AC = Alternating Current Electrons flow back and forth. The electrons are pushed one way, then pushed the other way. – The electrons stay in roughly the same area, just

going back and forth. Our outlets provide this, alternating current.

Alternating Current

– In the USA, there are three wires that transmit the flow of energy. • Two are at + or – 120 Volts (180 degrees out of phase)• One is at 0 Volts

– The US has adopted a frequency of 60 Hz• The polarity (direction of electrons) changes 120 times

per second• Actual voltage of peaks are + and – 170 volts

Transformers

– A transformer is just a piece of iron with a pair of wires coiled around it - one with many more turns in the coil than the other.

– The coils of wire are not physically connected. The iron core is immersed in an insulating oil bath which does not conduct electricity well.

• Step Up or Step Down the Voltage

Transformers1

2

1

2

NN

VV