P15: Electromagnetism (2/2, generator effect)

What does this image show?

A wire below a MF

What happens if a wire is moved up through a MF?

A PD is induced across the ends of the wire

What happens when the wire stops moving?

The PD is lost

What happens if a wire is moved back down through the MF?

PD is induced again but in the reverse direction

• Induced potential

Induced potential / electromagnetic induction

When a PD is created across a conductor (eg a wire) due to a change in the MF

What happens if the conductor is part of a complete, closed circuit?

A current is induced in the conductor

When is a PD induced across the ends of the conductor?

• If an electrical conductor moves relative to a MF

• Or if there is a change in the MF around a conductor (wire still, but move the MF)

Generator effect

• When a PD is induced across the ends of the conductor (moving thru MF) part of a complete circuit

• So a current is induced in the conductor

Direction of current switches when…

Direction of movement switches

Current stops when…

Movement stops

The generator effect is only seen if?

The wire passes thru the MF

What happens if the wire moves along the MF?

No induced PD / current

How to increase size of induced PD / current

1. Use stronger MF

2. Move wire quicker

3. Shape wire into coil

Link betw no. of turns on the coil + size of induced PD and current

The greater the no. of turns on the coil… the greater the induced PD / current

Does a magnet moving in + out of a coil of wire induce a PD / current?

Yes

Factors affecting the direction of the induced PD / induced current

1. Direction of movement (MF or conductor)

2. Direction of poles of magnet

Direction of current changes when…

1. Direction of movement changes (MF or conductor)

2. Switch poles of magnet

What happens when we move a magnet into a coil of wire?

Current is induced in the wire

Induced current creates its own…

MF

The MF induced current creates…

Opposes the og change

• Movement of the conductor

• Or the change in MF

  • eg movement of magnet

What happens when North pole of a magnet is inserted into a coil?

1. That end of the coil also becomes a N pole

2. This repels the magnet

3. So harder to push magnet in

What happens when North pole of a magnet is pulled out of the coil coil?

1. That end of the coil becomes a S pole

2. This attracts the magnet

3. So harder to pull magnet out

Induced current makes it harder to?

Move the magnet

What does it mean if the induced current makes it harder to move the magnet thru the coil?

Work is done

• ET from movement of magnet into movement of current

What is the generator effect used in?

1. Alternators

2. Dynamos

What does an alternator generate?

AC

What does a dynamo generate?

DC

Alternator

• Coil of wire rotating in a MF

• Generates AC

Set up of an alternator

Coil of wire rotating in a MF

1. Coil connected to 2 commutators (metal rings)

2. PD induced when wire passes thru MF

What does the commutator allow in an alternator?

Current to pass out of the coil

Explain this PD time graph for an alternator in terms of:

1. Max PD

2. 0 PD

3. PD reversed

1. Max PD, when coil is horizontal

   • Wire sweeping directly thru MF lines at the fastest possible rate

   • Red moves down, orange moves up

2. 0 PD, when coil is vertical

   • Coil moving parallel to MF → not cutting thru MF lines

3. PD reversed, as coil continues moving around

   • 2 sides of coil moving in diff direction to before

   • Red moves up, orange moves down

In an alternator:

1. The red side of wire is always connected to…

2. The orange side of wire is always connected to…

1. Ring A

2. Ring B

How to increase size of AC?

Increase

1. Strength of MF

2. No. of turns of coil/ area of coil

3. Rotation speed of coil

How to increase freq of AC?

Increase rotation speed of coil

Why does an alternator generate AC?

2 sides of coil are attached to 2 diff rings

What does a dynamo contain?

1. Split ring commutator

   • SRC has 2 sides separated by a gap (A + B)

In a dynamo, the wire connected to 1 half of the SRC is always moving…

In the same direction

• Red wire moving down → connected to part A of SRC

• Orange wire moving down → connected to part A of SRC

Explain this PD time graph for a dynamo in terms of:

1. Max PD

2. 0 PD

1. Max PD when coil is horizontal

   • Wire sweeping directly thru MF lines at fastest possible rate

   • Red moves down, orange moves up

2. PD 0 when coil is vertical

   • Moving parallel to MF

Why is PD not reversed in a dynamo?

Side of coil moving down still connected to the same part of SRC

How many peaks is in 1 full rotation of the coil + why?

2 peaks

• Each side of the coil passes thru MF 2x during each cycle of rotation

• Once up, once down

What effect do microphones use? (moving coil microphone)

Generator effect

What do microphones use the generator effect to do?

Convert the pressure variations in sound waves into variations in current in electrical circuits.

What current do microphones produce?

AC

• PD reverses

What are transformers used to do?

Change the PD of an electrical supply

What does a basic transformer consist of?

A primary coil + a secondary coil wound on an iron core

The primary + secondary coil are…

Separate

• Electrical current can’t pass directly from 1 coil to another

Why is an iron core used?

Iron is easily magnetized

What is the primary coil connected to?

AC

What happens as current flows thru the primary coil?

1. Generates a changing MF

2. The MF is transmitted along the iron core

3. + Passes thru the secondary coil + induces a PD

When doing the changing MF induce a PD?

When it passes thru the secondary coil

What does the iron core do?

Increase strength of MF

What type of current do transformers only work with and why?

AC

• Bc changing MF needed to induce a PD

Why does DC not work in transformers?

Produces constant MF

In this transformer what do the primary + secondary coil have in common? What does this mean?

Same no of turns

• PD induced in secondary coil same as PD in primary coil

• ONLY IF TRANSFORMER IS 100% EFFICIENT (no energy wasted)

What happens if there are more turns in the secondary coil than in the primary coil?

PD induced in the secondary coil greater than PD in primary coil

• Step up transformer

The ratio of the PDs across the primary + secondary coils of a transformer Vp and Vs depends on?

The ratio of the no. of turns on each coil, np and ns

Step up transformer

PD induced in the secondary coil greater than PD in primary coil

• So increase PD of electricity supply

Step down transformer

PD induced in the secondary coil smaller than PD in primary coil

• Decrease PD of electricity supply

Why does a step down transformer decrease PD?

Fewer turns in secondary coil than primary coil

Equation to calculate PD in secondary coil

What if transformers were 100% efficient?

Electrical power output equals the electrical power input

In transformers…

Power is always conserved

Power equation in a transformer

Assumes transformer is 100% efficient (no energy wasted)

How is electrical power transmitted from power stations to homes?

Thru high voltage cables

Power equation

Power = current x voltage

• P = IV

What is used to transmit a large amount of power?

Large current or large voltage

Problems of the national grid?

Power wasted in transmission cables as heat

What does the amount of power wasted as heat depend on?

Current²

If a large current is used in transmission cables…

A huge amount of power would be wasted as heat

• So a large PD is used

How is electricity transported in the national grid?

1. Electricity from PS passes thru SU transformer

2. Electrical power transmitted down high voltage cables

3. Electrical power passed thru SD transformer (reduces PD, 230 V)

What does the SU transformer increase PD to?

400 000 V

Using transformers helps…

Reduce power wastage by the national grid