electromagnetism

Define the motor effect

A current-carrying wire in the presence of a magnetic field will experience a force

To experience the full force, the current-carrying wire must be placed _______ to the magnetic field

Perpendicular/at right angles

What does A / thumb represent?

Flemming’s Left Hand Rule

Direction of the force

What does B / forefinger represent?

Flemming’s Left Hand Rule

Direction of the magnetic field

What does C / middle finger represent?

Flemming’s Left Hand Rule

Direction of current

Define magnet

A material or object that produces a magnetic field

Define poles of a magnet

The part of a magnet where the magnetic forces are strongest

Field lines always go from…

North to South

The more dense the magnet field lines are, the ____ the magnetic field is in that area

The more dense the magnet field lines are, the stronger the magnetic field is in that area

Like poles…

repel

Opposite poles…

attract

Magnetic field of a bar magnet

Describe how to plot the magnetic field pattern of a magnet

1. Place the plotting compass near the magnet on a piece of paper

2. Mark the direction the compass needle points

3. Move the plotting compass to many different positions in the magnetic field, marking the needle direction each time

4. Join the points to show the field lines

How does a compass work?

• A magnetic compass contains a small bar magnet

• Earth has a magnetic field

• The compass needle points in the direction of the Earth’s magnetic field

Define permanent magnet

Produces its own magnetic field all the time

Define induced magnet

Only becomes a magnet when placed in a magnetic field

Examples of magnetic materials

• Iron

• Cobalt

• Nickel

Define magnetic material

Any object that can be influenced by magnetic fields and has the potential to become a magnet

The force between a permanent and induced magnet will always be….

attractive

What happens when an induced magnet is removed from a magnetic field?

It loses most/all of its magnetism quickly

Define electromagnet

Solenoid with an iron core

When an electrical current flows through a material, a magnetic field is always produced

True or False?

True

What happens when current flows through a conducting wire?

A magnetic field is produced

Describe the magnetic field inside a solenoid

• Strong

• Uniform

How to increase the strength of the magnetic field created by a current flowing through wire?

Shaping the wire to form a solenoid

What two things does the magnetic field of a wire depend on?

• Current flowing through wire

• Distance from the wire (strongest = closest)

Direction of magnetic field depends on…

direction of current

(right hand rule)

Why are electromagnets useful?

• Can be turned on and off

• Can reverse direction of magnetic field (by reversing direction of current)

How can you increase electromagnet strength?

• Increase current

• Increase number of coils (keeping length the same)

• Decrease length (keeping number of coils the same)

• Adding a soft iron core

Magnetic field of solenoid

(Similar to bar magnet)

Explain how a solenoid arrangement can increase the magnetic effect of the current

The small magnetic fields caused by the current in each turn of the coil add together to make a stronger overall magnetic field

What do we call the blue device labelled A in the diagram below?

Split-ring commutator

Explain how a split-ring commutator ensures the coil rotates in one direction

1. The split-ring commutator swaps the positive and negative connections every half turn

2. Therefore, the direction of the current through the coil reverses every half turn

3. This means that the forces acting on the coil will always be acting in the same direction

4. This causes the coil to rotate continuously in one direction

Electric motors use _____ to create a _____ ___

Electric motors use electricity to create a rotational motion

Give three ways to increase the speed of rotation in an electric motor

• Add more turns to the coil

• Increase the magnetic field strength

• Increase size of current in the coil

Define brushes

Make electrical contact between a circuit and the moving parts of the motor

Define slip rings

Makes sure each side of the coil is connected to the same section of the circuit

Define relay

A type of switch operated by an electromagnet

Explain how an electromagnetic device works

1. When we switch on the low voltage circuit, current can now flow around the circuit which means there is a magnetic field around the electromagnet

2. The magnetic field now attracts the iron block in the high voltage circuit. This causes the contacts to close and switches on the high voltage circuit

3. If we switch off the low voltage circuit then now there is no magnetic field

4. The contacts now spring apart and the high voltage circuit is switched off

Explain how a doorbell works

1. The switch is closed when the buzzer is pressed, causing a current to flow around the circuit

2. A magnetic field is produced by the electromagnet

3. The iron contact is now attracted towards the magnetic field and moves towards it, causing the clapper to hit the bell

4. This breaks the circuit so current can no longer flow, and therefore no magnetic field

5. The iron contact now springs back to its original position, completing the circuit again so current can once again flow

6. Process is repeated

What does A.C. output look like?

What does D.C. output look like?

What type of generator is this?

Dynamo

How do transformers work?

1. An alternating potential difference is applied across the primary coil, causing a current to flow

2. This generates a magnetic field around the coil, which induces an alternating magnetic field in the iron core because iron is a magnetic material

3. This induces a potential difference in the secondary coil

4. Because the transformer is part of a complete circuit, current will also flow in the secondary coil

What would happen if transformers were 100% efficient?

The electrical power output would equal the electrical power input

Why has p.d. doubled?

The number of coils has doubled

Why has p.d. halved?

The number of coils has halved

Transformers: Why can the current not pass straight to the secondary coil?

The wires are insulated in plastic so electricity cannot pass to the iron core

Describe the generator effect

Due to electromagnetic induction, as a conductor cuts magnetic field lines an electrical potential difference is induced

Is a split-ring commutator part of the motor or generator effect?

Motor

Are slip rings part of the motor or generator effect?

Generator

Energy changes in motor effect

Electrical energy → kinetic energy

Explain how a loudspeaker works

1. Alternating current passes through coil

2. Alternating current produces an alternating magnetic field around the coil

3. The alternating magnetic field of the coil and the constant magnetic field of the permanent magnet interact

4. This moves the coil in and out

5. As paper cone is attached to coil it moves in and out, producing a sound wave

Explain how a microphone works

1. Sound wave hits the diaphragm and moves it in and out

2. The coil attached to the diaphragm moves in and out

3. As the coil cuts the magnetic field lines of the permanent magnet a potential difference is induced

4. As the coil is part of a complete signal a current flows

5. As the direction of movement changes, the direction of the induced potential difference changes, producing an alternating current

Explain how the motor effect works

1. When a current passes through the coil the wire will experience a force

2. As the current in opposite sides of the coil are in opposite directions they experience forces in opposite directions causing the coil to rotate

3. Each half turn the current in the coil reverses due to the split ring commutator

4. This ensures the forces on the coil are always in the same direction

Explain how the generator effect works: steps 1-4

1. A force rotates the coil

2. The long sides cut through the magnetic field produced by the permanent magnet

3. This induces a potential difference

4. As each side is moving in opposite directions it cuts the field in opposite directions, and the induced potential difference is in opposite directions

Explain how the generator effect works: steps 5-7

5. If connected to a complete circuit an alternating current will be induced

6. After half a turn each side of the coil is now moving in the opposite direction, and so induces a potential difference(and current) in the opposite direction

7. This produces an alternating current, the frequency of which is the same as the frequency of the coil rotating