Chapter 7 (3-5) Magnetism from Electricity from Magnetism

Electromagnetism- Using electricity to produce magnetism Hans Christian Oersted (1820) –discovered the relationship between electricity an magnetism –Experiments with compass and electric current in a wire –Results: Electric current produces a magnetic field and the direction of the magnetic field depends on the direction of the current

Oersted

Pg 207 All of the compass needles point north. A current is produced in the wire. Page 207

Electromagnetism The interaction between electricity and magnetism

Figure 1 – pg 208 When there is no current in the wire, the compass needles point north. When current is present, the compass needles align with the magnetic field produced by the current in the wire.

Changing the Magnetic Fields It can be turned on by turning on or off the current. The direction of the field can be changed by reversing the current. Changing the strength: –Increasing the amount of current. –Increasing the amount of loops.

Figure 2 – pg 209 Turning current off Reversing direction Changing strength.

Solenoid A coil of wire that produces a magnetic field when carrying an electric current –The strength of magnetic field increases if you add more loops or increase the current

Solenoid

Electromagnets A magnet that consists of a solenoid wrapped around an iron core The magnetic field produced by the solenoid causes the domains within the iron to become aligned, making it magnetized

Electromagnets

To increase the strength of an electromagnet: –Increase the number of loops in the solenoid –Increase the size of the iron core –Increase the electric current in the wire –Wind the coils closer together

Uses of Electromagnetism APPLY IT –pg 211 The steel bar will be attracted to the magnet. The clapper will be pulled up with the steel bar and will hit the bell. The rising steel bar breaks the circuit.

Energy and Motion and Energy Transformation Magnets can move a wire that has an electric current – The direction of the movement depends on the direction of the current.

Types of Energy Energy: ability to move and object over a distance Electrical Energy: energy associated with electrical current Mechanical Energy: energy an object has due to it’s movement or position

When a wire with a current is places in a magnetic field, electrical energy is transformed into mechanical energy. Mechanical Energy Electrical Energy

Measuring Electromagnetism Galvanometer –A device used to measure current through the interaction of an electromagnet and a permanent magnet –Pg 214

Galvanometer

APPLY IT – pg 215 Person’s skin would have more sweat because it was conducting more electricity. So the person might be nervous.

Electric motors –A device that changes electrical energy into kinetic energy –Diagram of parts of a motor (p216) Commutator Brushes Permanent magnets Armature

Electromagnetic Induction -Using magnets to produce electricity. Faraday and Henry (1831) –Discovered a magnetic field can produce an electric current –Experiments with iron ring, 2 wires, and galvanometer –Results: Electric current was produced in the second wire only when the magnetic field was changing (when the battery was connected and disconnected)

Faraday

Electromagnetic Induction The process by which an electric current is produced by a changing magnetic field

Electromagnetic Induction Faraday’s experiments also showed: –The magnetic field around a wire can be changed by moving either the magnet or the wire –So…an electric current can be induced by moving a magnet in a coil of wire or by moving a wire between the poles of a magnet

clockwisecounterclockwise clockwisecounterclockwise When part of the circuit moves up, the current flows clockwise. When parts move down, the current is counterclockwise.

Uses For EI Transformers –A device that increases or decreases the voltage of an alternating current Generators –A device that uses electromagnetic induction to change kinetic energy into electrical energy

Generators The slip ring turns with the armature and transfer current to the brushes. The motion of the armature in the magnetic field induces a current When the brushes are connected to a circuit, the generator can be used as an energy source. The crank rotates the armature.

Magnetic field current current turn

Transformers Two separate coils of wire wrapped around an iron core. –Primary coil attached to a circuit with a voltage and current –Secondary coil is attached to a non voltage current.

How does it work? The changing current in the first coil produces a magnetic field The magnetic field induces a current in the second coil. –The change of voltage from the primary coil to the secondary coil depends on the amount of loops in each coil

Types of Transformers Step-up: –Primary coils have smaller amount of loops than the secondary coil Increases Voltage Step-down: –Primary coils have more amount of loops than the secondary coil Decreases Voltage

Transformer

How much increase or decrease? VOLTAGE (primary)Coils (pri) VOLTAGE (secondary) Coils (sec) The greater the difference between the number of loops in the primary and secondary coils in a transformer, the more the voltage will change.