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Magnetism and Electromagnetism

AQA Physics topic 7

7.1 Magnetism and Electromagetism 21/11/2017

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Magnetic field around a bar magnet

The force is the strongest at each pole

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Attracting and Repelling What would the

magnetic field lines look like here?

Magnetic attraction and repulsion are examples of

non-contact forces.

Permanent vs Induced Magnetism 21/11/2017

Permanent magnets Induced magnets

• Always have a magnetic field around them

• Can attract or repel other magnets

• Become magnetic only when in a magnetic field

• Can only attract magnetic metals, not repel them

• Loses its magnetism quickly

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Magnetic fields in more detail

In this experiment the magnets only start attracting each other when they are within each other’s magnetic fields.

Definition of “field” – “the region in which a force will act”

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The direction of the field is drawn to show what the field will do to a north pole at that point. This is why the arrows point AWAY from a north pole.

Magnetic Metals 21/11/2017

Q. In this experiment a magnet is held towards coins made in different years. Some of the coins were attracted, some weren’t. What does this prove?

Conclusion 1 – some coins are made of magnetic metals, some aren’t. Conclusion 2 – the magnetic metals can only be attracted by a magnetic field, not repelled.

The main magnetic metals are iron, steel, nickel and cobalt

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The Earth’s Magnetic Field Q. How does a compass work?

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Plotting a Magnetic Field

How could you use a compass to plot a magnetic field?

7.2 The Motor Effect 21/11/2017

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Magnetic Field around a current-carrying wire

“Right hand corkscrew” or “right hand grip” rule

Notice that the field gets weaker as you get further away (or if the current is reduced)

Making an electromagnet 21/11/2017

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Electromagnets Experiment

Number of coils Number of paper clips picked up

5

10

15

20

25

Voltage on powerpack/V Number of paper clips picked up

1

2

3

4

5

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Making an electromagnet Task: To make your own electromagnet

Method: How will you do this?

Diagram of apparatus:

Conclusion:

Did it work?

What could you do to make it a better electromagnet? Name two different things you could do:

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N S Magnet:

Coil of wire:

Why did the nail become magnetised? Compare the magnetic field around a bar magnet and around a coil of wire (also called a “solenoid”):

The field inside here is strong and uniform

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Electromagnets The strength of an electromagnet can be increased by

doing three things:

1) Increasing the voltage

2) Increasing the number of coils

3) Inserting an iron core

How could you reverse the

direction of the magnetic field?

21/11/2017 Examples of electromagnets (Physics only): The Relay

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The Motor Effect (HT only)

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1) What will happen to this wire?

2) How can you make it move with more force/faster?

3) How can you make it move in a different direction?

21/11/2017 Force on a current-carrying wire in a magnetic field

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F = Force

B = Magnetic

field

I = Current

Q. Where will this wire go?

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Magnetic force on a wire Clearly, the size of the force on this wire depends on these

things:

1) The strength of the magnetic field

2) The current in the wire

3) The length of the wire (in the field)

These things are related by the simple formula…

F = BIL

B is called “magnetic flux density” and is measured in Teslas. To use this equation, make sure length is in metres and current in Amps!

You do NOT need to learn this equation!

Example questions 21/11/2017

1) A 2m long wire is placed in a magnetic field of strength 5T. If the current through the wire is 0.5A how much force will act on the wire?

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N 2) This wire experiences a force of 0.5N on it. What it the value of the magnetic field strength and which direction would the wire move in?

5N

0.2m

0.1A

25N, into the page

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Using Motors The following devices are just some examples of devices that use electric motors:

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Electric Motors (HT only)

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Electric Motors

The “split ring commutator” changes the current every half rotation

How loudspeakers work (Physics only) 21/11/2017

AC supply

Task: explain how the speaker works in your own words.

7.3 Induced Potential, Transformers and the National Grid

(HT only)

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Electromagnetic Induction

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The direction of the induced current is reversed if…

1) The wire is moved in the opposite direction

2) The field is reversed

The size of the induced current can be increased by:

1) Increasing the speed of movement

2) Increasing the magnet strength

21/11/2017 Electromagnetic induction – the

“generator effect” The direction of the induced current is

reversed if…

1) The magnet is moved in the opposite direction

2) The other pole is inserted first

The size of the induced current can be increased by:

1) Increasing the speed of movement

2) Increasing the magnet strength

3) Increasing the number of turns on the coil

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Direction of the Generated Current Consider a magnet in a solenoid:

The current induced by the magnet induces a north pole that repels the magnet again.

Lenz (1804-1865)

This is called Lenz’s Law, by the way!

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AC Generators

Magnetic Field

Slip rings and brushes

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Questions on the AC Generator

1) How does the generator work?

2) How would you increase its output? Give two answers

3) How would you increase the frequency?

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AC Generator

Induced current can be increased in 4 ways:

1) Increasing the speed of movement

2) Increasing the magnetic field strength

3) Increasing the number of turns on the coil

4) Increasing the area of the coil

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PD generated

Time

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Generators (dynamos) The coil already has an iron

core, but the induced current could still be increased by:

1) Increasing the speed of movement

2) Increasing the magnetic field strength

3) Increasing the number of turns on the coil

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Microphones (HT only)

In a microphone the pressure variations from the sound waves cause a diaphragm to move. The

coil moves in relation to the magnetic field, inducing a current

through the wire.

Signal out

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DC and AC recap

DC stands for “Direct Current” – the current only flows in one direction and a common example is a battery:

AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz).

1/50th s

230V

V

V

Time

T

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Transformers (HT only)

A transformer is basically a device made of two coils of wire around an iron core. Iron is used as it is easily magnetised:

The circuit diagram for a transformer:

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Transformers

Time

Time

Time

Current through primary

Magnetic field

Voltage induced in secondary

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Transformers Transformers are used to _____ __ or step down _______. They only work on AC because an ________ current in the primary coil causes a constantly alternating _______ ______. This will “_____” an alternating current in the secondary coil.

Words – alternating, magnetic field, induce, step up, voltage

We can work out how much a transformer will step up or step down a voltage:

Voltage across primary (Vp)

No. of turns on secondary (Ns) Voltage across secondary (Vs)

No. of turns on primary (Np)

You do NOT need to remember this equation!

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Simple transformer questions

Primary voltage

Vp

Secondary voltage

Vs

No. of turns on primary

Np

No. of turns on secondary

Ns

Step up or step down?

12V 24V 100 ? ?

400V 200V 20 ? ?

25,000V 50,000V 1,000 ? ?

23V 230V 150 ? ?

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More example questions Primary voltage

Vp

Secondary voltage

Vs

No. of turns on primary

Np

No. of turns on secondary

Ns

Step up or step down?

6V 24V 100 ? ?

400,000V 200V ? 1,000 ?

25,000V ? 20,000 20 ?

? 230V 150 1,500 ?

1) A transformer increases voltage from 10V to 30V. What is the ratio of the number of turns on the primary coil to the number of turns on the secondary coil?

2) A current of 0.5A is supplied to a transformer that steps down a voltage from 230V to 12V. What is the current from the secondary coil?

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Transformers and Power If this transformer is 100% efficient then the power output from the secondary coil is the same as the power input to the primary coil.

Power = voltage x current

in W in V in A

In previous units we came across this equation:

If we apply this equation to transformers and assume 100% efficiency we conclude:

Vp x Ip = Vs x Is

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Example questions

1) A transformer connected to the mains supply in the UK runs on a voltage of 230V and a current of 0.1A. If its output voltage is 12V what is its output current?

2) Another step down transformer outputs 120V and 2A. If it runs from the mains what is its input current?

3) A step up transformer near a power station would convert 20,000V and 10A into 300,000V and what current?

1.9A

1.04A

0.07A