Chapter 22Chapter 22 Magnetism and Its UsesMagnetism and Its Uses

MagnetismMagnetism Discovered over 2000 years ago Discovered over 2000 years ago

by the Greeks. Named after by the Greeks. Named after Magnesia, Turkey.Magnesia, Turkey.

Magnetic ForceMagnetic Force– You can feel the You can feel the

magnetic ability magnetic ability before magnets touchbefore magnets touch

Magnetic FieldMagnetic Field– Surrounds the magnetSurrounds the magnet– Strongest at the North and Strongest at the North and

South polesSouth poles– Round magnets have poles Round magnets have poles

on the top and bottomon the top and bottom

Magnetic FieldsMagnetic Fields Unlike poles that are the Unlike poles that are the

opposite will attract opposite will attract – Produce new magnetic Produce new magnetic

fieldfield Like poles that are the Like poles that are the

same will repel same will repel Magnetic Field DirectionMagnetic Field Direction

– Goes from the north to Goes from the north to the souththe south

The Earth is a Large MagnetThe Earth is a Large MagnetEarth’s Magnetic FieldEarth’s Magnetic FieldSouth Seeking PoleSouth Seeking Pole North Geographic Pole North Geographic Pole

North Seeking PoleNorth Seeking Pole South Geographic Pole South Geographic Pole

Magnetic MaterialsMagnetic Materials Iron, cobalt and nickel are naturally Iron, cobalt and nickel are naturally

magneticmagnetic– Electron’s magnetic properties not cancelled Electron’s magnetic properties not cancelled

in these metalsin these metals– Each atom magneticEach atom magnetic– Not always magneticNot always magnetic– Groups of atoms ( magnetic domains ) must Groups of atoms ( magnetic domains ) must

be alignedbe aligned

• Even though each domain behaves like a magnet, the poles of the domains are arranged randomly and point in different directions.

Lining Up Domains

MagnetismMagnetism

• As a result the magnetic fields from all the domains cancel each other out.

• If you place a magnet against the same nail, the atoms in the domains orient themselves in the direction of the nearby magnetic field.

Lining Up Domains

MagnetismMagnetism

• The like poles of the domains point in the same direction and no longer cancel each other out.

Domain alignment can be temporary Domain alignment can be temporary or permanentor permanent– Magnet held next to metal will align Magnet held next to metal will align

domaindomain– Intense vibrations/heat can knock the Intense vibrations/heat can knock the

domains out of alignmentdomains out of alignment Permanent magnets have strong Permanent magnets have strong

magnetic domain not influenced by magnetic domain not influenced by atomic motionatomic motion

Electricity and MagnetismElectricity and Magnetism Electric currents creates a magnetic Electric currents creates a magnetic

fieldfield Magnetic field reverses when the Magnetic field reverses when the

current reversescurrent reverses An eAn electromagnet is lectromagnet is

a temporary magnet. a temporary magnet.

Moving Charges and Magnetic Fields

• It is now known that moving charges, like those in an electric current, produce magnetic fields.

• Around a current-carrying wire the magnetic field lines form circles.

Electricity and MagnetismElectricity and Magnetism

Increasing CoilsIncreasing CoilsIncreasing CurrentIncreasing Current Larger Magnetic Field Larger Magnetic Field

Faraday’s LawFaraday’s Law Flux in magnetic force = flux in Flux in magnetic force = flux in

electric voltageelectric voltage

Using Galvanometers • In a galvanometer, the electromagnet is

connected to a small spring.

Electricity and MagnetismElectricity and Magnetism

• Then the electromagnet rotates until the force exerted by the spring is balanced by the magnetic forces on the electromagnet.

A Simple Electric Motor • A simple electric motor also includes

components called brushes and a commutator.

Electricity and MagnetismElectricity and Magnetism

• The brushes are conducting pads connected to the battery.

• The brushes make contact with the commutator, which is a conducting metal ring that is split.

• The brushes and the commutator form a closed electric circuit between the battery and the coil.

Making the Motor Spin • Step 1. When a current flows in the coil, the

magnetic forces between the permanent magnet and the coil cause the coil to rotate.

Electricity and MagnetismElectricity and Magnetism

Making the Motor Spin • Step 2. In this position, the brushes are not in

contact with the commutator and no current flows in the coil.

Electricity and MagnetismElectricity and Magnetism

• The inertia of the coil keeps it rotating.

Making the Motor Spin

• Step 3. The commutator reverses the direction of the current in the coil.

Electricity and MagnetismElectricity and Magnetism

• This flips the north and south poles of the magnetic field around the coil.

Making the Motor Spin

• Step 4. The coil rotates until its poles are opposite the poles of the permanent magnet.

Electricity and MagnetismElectricity and Magnetism

• The commutator reverses the current, and the coil keeps rotating.

Producing ElectricityProducing Electricity

A generator produces electricity A generator produces electricity from from mechanical energy to electrical mechanical energy to electrical energy.energy.– A coil rotates between the poles of A coil rotates between the poles of

a permanent magneta permanent magnet– The Rotations change the direction The Rotations change the direction

of the currentof the current

Types of Electrical CurrentsTypes of Electrical Currents Direct Current = D/CDirect Current = D/C

– Battery Battery – Current in one directionCurrent in one direction

Alternating current = A/CAlternating current = A/C– Household currentHousehold current– Current changes direction 2x per secondCurrent changes direction 2x per second– Produce at 60 Hz per secondProduce at 60 Hz per second

Transformers ( Step-up / Step-down )Transformers ( Step-up / Step-down )– 150,000 to 200,000 volts is generated by 150,000 to 200,000 volts is generated by

power plantspower plants– This is too much voltage, so it must be This is too much voltage, so it must be

lowered for use in our homes.lowered for use in our homes.

Superconductors are Superconductors are materials that have materials that have no electrical no electrical resistance, such as resistance, such as mercury, lead and tinmercury, lead and tin

Conductors, good Conductors, good examples are copper examples are copper and aluminumand aluminum

Semiconductors, Semiconductors, such as silicon which such as silicon which is used in computers.is used in computers.

Credit Cards Credit Cards work by work by using ausing a Magnetic Magnetic Strip to store Strip to store information.information.