Magnetism and magnetic fields - PowerPoint Presentation

Slide1

Magnetism and magnetic fieldsSlide2
Slide3

The Shape of Magnetic Fields

Where is the magnet stronger?

Which direction do the arrows go?

What does this tell you about the Earth’s poles?Slide4
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Slide6
Slide7

Induced and Permanent Magnets

Induced Magnets

A magnet that is produced when a when a magnetic material is

placed in a magnetic field,

which may or may not

stay magnetic when the field is removed. Permanent Magnets A magnet that stays magnetic when other magnets or an electric current is removed.Slide8
Slide9

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

Electromagnetism

RH Screw RuleSlide10

Electromagnets

What happens if we make the wire into a coil?

We call this coil a

solenoid

.Slide11

Electromagnets

How do you make them stronger?

Add an iron core.

Add more turns of wire.

Increase current through coil.

Large CS Area or iron core.Shorter length of iron core per turn.Slide12

Electromagnets Experiment

Plan an experiment to test how these factors affect the strength of the magnet.Slide13

Forces on a wire in a magnetic fieldSlide14

This allows us to predict the direction of the force acting on the wire if we know the direction of the current and the magnetic field..

Fleming’s left-hand rule

thu

M

b =

M

otion

F

irst finger = magnetic

F

ield

se

C

ond finger =

C

urrentSlide15

A wire carrying a current will experience a force in a magnetic field. This can be reversed by:

reversing the current

reversing the magnetic fieldSlide16
Slide17
Slide18

The

size of the force can be increased by:

Increasing

the current

Increasing

the magnet strength

Using

a longer length of wire,

eg

a coil. – This forms the basis of the electric

motor

Force

= Magnetic Flux Density x Current x Length of wire

(N) (T) (A) (m)

Newton Tesla Amps metre Slide19
Slide20

What are electric motors?

An

electric motor

is a device that converts electrical energy into mechanical energy to produce a turning effect.

Most motors are powered using

direct current

(DC), which is produced by cells and batteries.

Motors powered by mains electricity use

alternating current

(AC). These motors use electromagnets rather than permanent magnets.Slide21

Coil in a magnetic fieldSlide22

How does an electric motor work?Slide23

DC electric motor simulationSlide24

Notes on how a motor works

1: In the diagram the force on the right hand side of the coil will be ……….. and the force on the left hand side will be ……. (from Flemings Left Hand Rule).

This produces a turning effect.

2: When the coil is in the vertical position the turning effect will be ....... However the coil continues to move due to its ............

3: When the coil has gone past the vertical position, each side of the coil is now touching the other b........

and so the direction of the current in the coil is ............... The coil is therefore able to keep turning in the same direction. Slide25

Every

half turn the brushes are in contact with different halves of the

commutator

- This is the reason for the split ring

commutators

!Slide26

Effect of:

1: More current – This would produce ........

2: Stronger magnet – This would ...............

3: More turns – This would .................

4: Changing direction of current – This would .......Slide27

One important last thing:

Practical motors have a radial field produced by curved pole pieces so that the field is always at right angles to the coil.Slide28

How electricity is made (generated)

We have seen how a magnet and a current in a coil produces movement (motors).

We are now going to look at how a magnet and movement produces a current in a coil.Slide29

Inducing current in a wireSlide30

Electromagnetic induction

occurs when a wire moves relative to a magnet and produces a current in the wire.

Induction also occurs if the magnet does the moving.

In both cases the wire and magnetic field move

perpendicular

to each other. If they move

parallel

to each other, no current is induced.Slide31

Inducing current in a coilSlide32

- Used to predict the direction of the induced current

Fleming’s right-hand rule?

F

irst finger = magnetic

F

ield

se

C

ond finger =

C

urrent

thu

M

b =

M

otionSlide33

If the direction of the magnetic field is reversed so is the voltage.Slide34

How do DC generators work

? Dynamo

A DC generator is very similar to the DC motor.

The only difference is that the movement of the coil produces a direct current instead of the direct current producing movement.

So the drawing looks the same, - we just have to get rid of the batterySlide35

AC generators

however are slightly differentSlide36

AC generator: AlternatorSlide37

How do AC generators work?Slide38

Notes on how an AC generator works

1: In the diagram the current in the right hand side of the coil will be ...................... the

slip ring

(from Flemings Right Hand Rule).

2: When the coil is in the horizontal position the voltage generated will be at a maximum as it cuts through the magnetic field lines.

3: When the

r.h.s

. of the coil has rotated 180 degrees the current will going .........................the slip ring.

towards

Towards away from

away from Slide39

4: The direction of the current at each of the brushes is therefore changing each time the coil rotates 180 degrees. This is an AC generator. Slide40

Note: A magnet could be rotated inside a coil of wire to produce the same effect. This is how electricity is generated in a power station.(produces alternating current)

As before: - Varying number of turns and speed of rotation

More turns – produces higher voltage.

Rotate faster – produces higher voltage and higher frequencySlide41

What do the brushes and slip rings do?

(past exam question)

Brushes: Collect current from coil

Slip rings: Allow coil to spin freely or so that wires do not become twisted.

Effect of varying number of turns and speed of rotation

More turns – produces higher voltage.

Rotate faster – produces higher voltage and higher frequency

(normal frequency in UK is 50 Hertz)Slide42

Transformers are used to change the size of an alternating voltage.

Step-up transformers – used in power stations to increase voltage.

Step-down, - used to decrease voltage before being used by consumers.

TransformersSlide43

A

B

Suppose a current is passed through coil A.

What will be produced

around

coil A?

A magnetic field

Now suppose that the current supplied is an alternating current – this means that the magnetic field produced will be constantly changing.

What will happen to coil B when it is in a constantly changing

magnetic field ?

A current will flow through B

N

SSlide44

By passing a current through A, we produce a current in B.

A

B

This is a transformer and is used to either increase or decrease voltages between A (the primary coil) and B (the secondary coil)Slide45

How a transformer works

Label your drawing to show primary and secondary coils and also the iron core.

1: An alternating current is passed through the

primary coil.

This produces a ……….. ………. around the primary

coil that is constantly changing.

2: The secondary coil is inside the magnetic field

produced by the primary coil.

An i…….. c........... is therefore produced in the

secondary coil. (This is also alternating)

This is how voltages can be increased or decreased.Slide46

Vp

=

Np

Vs Ns

Where v is voltage, N is number of turns and p and s refer

to primary and secondary

If you prefer words to symbols:-

primary voltage

secondary voltage

primary turns

secondary turns

=

The frequency of the current in the secondary circuit will always match the primary circuit.

The size of the voltages in the 2 coils depends on the number of turns on each of the coils and are linked by the equation: -Slide47

Calculations

1: A transformer has 100 turns on its primary coil. It has an input voltage of 35 V and an output voltage of 175 V.

= 500 turns

=

V

s

=

N

s

175

=

N

s

V

p

N

p

V

s

N

s

× N

p

V

p

× 100

35

How many turns are on the secondary coil?

=

V

s

N

s

V

p

N

p

=

N

s

V

s

N

p

V

pSlide48

= 230

V

=

N

s

=

V

s

50

=

V

s

V

p

N

p

V

s

N

s

× V

p

N

p

× 920

200

=

V

s

N

s

V

p

N

p

2: A transformer has 200 turns on its primary coil and 50 turns on its secondary coil. The input voltage is 920 V.

What is the output voltage?Slide49

A step-up transformer

The power in the secondary circuit cannot be greater than the power in the primary circuit, (or the transformer would be more than 100% efficient!)

P = V × I

So a step-up transformer increases voltage, but reduces current.

Power and Power LossSlide50

Power loss

power = current

2

× resistance

There is a power loss in cables which is related to the amount of current flowing:

P = I

2

× RSlide51

Vp × Ip = Vs × Is

primary

secondary

Vp

Vs

Ip

Is

power in = power out

Pp = PsSlide52

Transformer power example

primary

secondary

V

p

V

s

A transformer has a primary voltage of 1000

V and a primary current of 0.5

A.

V

p

×

I

p

= V

s

×

I

s

I

p

I

s

If the secondary circuit has a current of 0.01A flowing, what is the secondary voltage?

= 50000

V

0.01

0.5

1000 ×

=

I

s

I

p

V

p

×

V

s

=Slide53
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Research

Loudspeakers:

-Find a diagram which shows how a coil and magnet move in a loudspeaker.

-How do they work? How is electricity converted into sound?

Microphones