T7 - Magnets

explain a permanent magnet

produces its own magnetic field

explain induced magnet

becomes a magnet when it is placed in a magnetic field

• causes a force of attraction

• when removed from the magnetic field an induced magnet loses most/all of its magnetism quickly.

explain magnetic fields

• region around a magnet where a force acts on another magnet or on a magnetic material

1. force between a magnet and a magnetic material is always one of attraction

2. field is strongest at the poles of the magnet

explain electromagnetism

• when a current flows through a conducting wire a magnetic field is produced around the wire

• strength of the magnetic field depends on the current through the wire and the distance from the wire.

explain a solenoid

• increases the strength of the magnetic field created by a current through the wire

• magnetic field: strong and unifrom

• adding an iron core increases the strength of the magnetic field of a solenoid

explain an electromagnet

• solenoid with an iron core

uses:

1. attract and pick up things made from materials like iron and steel - scrap yards

2. used within other circuits to act as switches

explain motor effect

• when a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each othe

explain factors that affect the size of force on a conductor

1. magnetic flux densiity

2. size of current

3. length of conductor thats in a magnetic field

explain electric motors

• coil of wire carrying a current in a magnetic field tends to rotate

1. two bar magnets are held so that the north pole of one magnet faces the south pole of the other magnet

2. coil of wire is mounted in the gap between the two magnets

3. split rings make electrical contact with the coil and reverse the current every half turm

4. when electric current flows through the coil, a force is exerted on the coil, causing it to spin

explain loudspeakers and headphones

• use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves

1. a current in the coil creates an electromagnetic field

2. the electromagnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards

3. the current is made to flow in the opposite direction

4. the direction of the electromagnetic field reverses

5. the force on the cone now pulls it back in

6. repeatedly alternating the current direction makes the cone vibrate in and out

7. the cone vibrations cause pressure variations in the air, which are sound waves

explain the generator effect

• used in an alternator to generate ac and in a dynamo to generate dc

explain how the generator effect is used in an alternator to generate ac

1. coil in-between a magnet rotates due to the magnetic field.

2. As the coil rotates, a current is induced across it (the generator effect)

3. Every half rotation the current changes direction, hence producing an alternating current and potential difference.

explain how the generator effect is used in an alternator to generate dc in a dynamo

dynamo has a split-ring commutator

1. This swaps the connection at every half rotation, allowing the current to continue moving in the same direction even as the coil rotates.

2. Because the current continues moving in the same direction, a direct current is generated.

explain microphones

• use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits.

1. pressure variations in sound waves cause the flexible diaphragm to vibrate

2. the vibrations of the diaphragm cause vibrations in the coil

3. the coil moves relative to a permanent magnet, so a potential difference is induced in the coil

4. the coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit

5. the changing size and direction of the induced current matches the vibrations of the coil

6. the electrical signals generated match the pressure variations in the sound waves

explain transformers

• basic transformer consists of a primary coil and a secondary coil wound on an iron core

• Iron is used as it is easily magnetised

• transformers were 100% efficient, the electrical power output would equal the electrical power input.

• step up: increase p.d and have more turns on secondary cpil than primary coil

• step down: decrease p,d and have more turns on primary coil than secondary coil