Attraction and Repulsion Magnetism and Its Uses. Magnetism Discovered over 2000 years ago in Magnesia, TurkeyDiscovered over 2000 years ago in Magnesia,

Attraction and RepulsionAttraction and Repulsion Magnetism and Its UsesMagnetism and Its Uses

MagnetismMagnetism

• Discovered over 2000 years ago in Discovered over 2000 years ago in Magnesia, TurkeyMagnesia, Turkey

• Magnetic ForceMagnetic Force– You can feel the magnetic ability before You can feel the magnetic ability before

magnets touchmagnets touch

• Magnetic FieldMagnetic Field– Surrounds the magnetSurrounds the magnet– North and South pole is strongest pointNorth and South pole is strongest point

• Round magnet has poles on the top and bottomRound magnet has poles on the top and bottom

– Field has direction north to south Field has direction north to south

• Unlike poles attract Unlike poles attract – Produce new magnetic fieldProduce new magnetic field

• Like poles that are the sameLike poles that are the same– Repel Repel

• Magnetic Field DirectionMagnetic Field Direction– Goes from north to southGoes from north to south

Earth’s Magnetic FieldEarth’s Magnetic FieldSouth Seeking PoleSouth Seeking Pole North Geographic North Geographic

PolePole

North Seeking PoleNorth Seeking Pole South Geographic South Geographic PolePole

Reason for earth’s Reason for earth’s

magnetism is unknown magnetism is unknown

Magnetic MaterialsMagnetic Materials

• Iron, cobalt and nickel are naturally Iron, cobalt and nickel are naturally magneticmagnetic– Electron’s magnetic properties not Electron’s magnetic properties not

cancelled in these metalscancelled in these metals– Each atom magneticEach atom magnetic– Not always magneticNot always magnetic– Groups of atoms must be alignedGroups of atoms must be aligned

• Called Magnetic DomainCalled Magnetic Domain

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

Lining Up Domains

8.18.1MagnetismMagnetism

• 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

8.18.1MagnetismMagnetism

• 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– Vibrations/Heat will knock domain out Vibrations/Heat will knock domain out

of alignmentof alignment• Permanent magnets have strong Permanent magnets have strong

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

Electricity and Electricity and MagnetismMagnetism

Electricity and Electricity and MagnetismMagnetism

• Electric currents creates a magnetic fieldElectric currents creates a magnetic field• Magnetic field reverses when the current Magnetic field reverses when the current

reversesreverses• Electromagnet Electromagnet

– TemporaryTemporary– Solenoid-cylindrical wire coilSolenoid-cylindrical wire coil– Strength determined by coilStrength determined by coil– Speakers and motors (Electric to Speakers and motors (Electric to

Mechanical)Mechanical)

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.

8.28.2Electricity and MagnetismElectricity and Magnetism

Increasing CoilsIncreasing CoilsIncreasing CurrentIncreasing Current Larger Larger

Magnetic FieldMagnetic Field

Faraday’s LawFaraday’s Law

• Flux in magnetic force=flux in Flux in magnetic force=flux in electric voltageelectric voltage

Using Electromagnets to Make Sound

• The electromagnet changes electrical energy to mechanical energy that vibrates the speaker cone to produce sound.

8.28.2Electricity and MagnetismElectricity and Magnetism

Using Galvanometers • In a galvanometer, the electromagnet is

connected to a small spring.

8.28.2Electricity 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.

8.28.2Electricity 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.

8.28.2Electricity 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.

8.28.2Electricity 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.

8.28.2Electricity 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.

8.28.2Electricity and MagnetismElectricity and Magnetism

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

• Superconductors-mercury, lead and Superconductors-mercury, lead and tintin

• Conductors-copper and aluminumConductors-copper and aluminum• Semiconductors-siliconSemiconductors-silicon

• Credit Card Magnetic StripCredit Card Magnetic Strip

Producing ElectricityProducing Electricity

• From Mechanical to ElectricalFrom Mechanical to Electrical• Electromagnetic inductionElectromagnetic induction• GeneratorGenerator

– Coil rotates between the poles of a Coil rotates between the poles of a permanent magnetpermanent magnet

– Rotations change the direction of the Rotations change the direction of the currentcurrent

– Rotate 60 times a second-How many Rotate 60 times a second-How many times per minute?times per minute?

Types of Electrical Types of Electrical CurrentsCurrents

• Direct CurrentDirect Current– Battery Battery – Current in one directionCurrent in one direction

• Alternating currentAlternating current– 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– How many times does direction change How many times does direction change

per minute?per minute?

• TransformersTransformers– 150,000 volts generated by power 150,000 volts generated by power

plantsplants– Too much for houseToo much for house– Voltage needs to be loweredVoltage needs to be lowered