AQA GCSE Physics 3-3

Electromagnetism

GCSE Physics pages 254 to 265

April 10th 2010

AQA GCSE SpecificationTHE MOTOR EFFECT13.7 How can electricity be used to make things move?

Using skills, knowledge and understanding of how science works:• to explain how the motor effect is used in simple devices.

Skills, knowledge and understanding of how science works set in the context of:• When a conductor carrying an electric current is placed in a magnetic field, it may experience a force.• The size of the force can be increased by:– increasing the strength of the magnetic field– increasing the size of the current.• The conductor will not experience a force if it is parallel to the magnetic field.• The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed.

ELECTRICAL GENERATORS13.8 How do generators work?

Using skills, knowledge and understanding of how science works:• to explain from a diagram how an a.c. generator works, including the purpose of the slip rings and brushes.

Skills, knowledge and understanding of how science works set in the context of:• If an electrical conductor .cuts. through magnetic field lines, an electrical potential difference is induced across the ends of the conductor.• If a magnet is moved into a coil of wire, an electrical potential difference is induced across the ends of the coil.• If the wire is part of a complete circuit, a current is induced in the wire.• If the direction of motion, or the polarity of the magnet, is reversed, the direction of the induced potential difference and the induced current is reversed.• The generator effect also occurs if the magnetic field is stationary and the coil is moved.• The size of the induced potential difference increases when:– the speed of the movement increases– the strength of the magnetic field increases– the number of turns on the coil increases– the area of the coil is greater.

TRANSFORMERS13.9 How do transformers work?

Using skills, knowledge and understanding of how science works:• to determine which type of transformer should be used for a

particular application.

Skills, knowledge and understanding of how science works set in the context of:

• The basic structure of the transformer.• An alternating current in the primary coil produces a changing

magnetic field in the iron core and hence in the secondary coil. This induces an alternating potential difference across the ends of the

secondary coil.• The potential difference (p.d.) across the primary and secondary

coils of a transformer are related by the equation:p.d. across primary / p.d. across secondary = number of turns on

primary / number of turns on secondary• In a step-up transformer the potential difference across the

secondary coil is greater than the potential difference across the primary coil.

• In a step-down transformer the potential difference across the secondary coil is less than the potential difference across the primary coil.

• The uses of step-up and step-down transformers in the National Grid.

+ -+-

+-

The motor effectWhen a conductor carrying an electric current is placed in a magnetic field, it may experience a force.This is called the motor effect.

S

N

+ -

Motor effect - Fendt

The force increases if:– the strength of the magnetic field is increased– the current is increased

The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed.

The conductor will not experience a force if it is parallel to the magnetic field.

Motor effect - Fendt

The left-hand motor rule

Note:

Magnetic field direction is from NORTH to SOUTH

Current direction is from PLUS to MINUS

Motor effect - Fendt

Insert the missing information

Note: means current out of the page

means current into the page

N S S N

N S

Q1. Force direction ? Q2 Current direction ?

Q3 N and S poles ?Q4 Force directions ?

N S

Motor effect - Fendt

The electric motor

Electric current flowing around the coil of the electric motor produces oppositely directed forces on each side of the coil.

These forces cause the coil to rotate.

Every half revolution the split ring commutator causes the current in the coil to reverse otherwise the coil would stop in the vertical position.

Electric motor - Fendt

N

+

S

Brushes lose contact with the split ring commutator.

Current no longer flows through the motor coil.

The coil will continue to rotate clockwise due to its momentum.

Brushes in contact with the split ring commutator.

Current flows through the motor coil.

Forces exert a clockwise turning effect on the coil

Brushes regain contact with the split ring commutator.

Current flows through the motor coil but in the opposite direction.

Forces exert a clockwise turning effect on the coil.

Brushes lose contact with the split ring commutator.

Current no longer flows through the motor coil.

The coil will continue to rotate clockwise due to its momentum.

Brushes regain contact with the split ring commutator.

Current flows through the motor coil but in the original direction.

Forces exert a clockwise turning effect on the coil.

split-ring commutator

contact brush

rotation axis

Electric motor - Fendt

Model electric motor

Electric motor - Fendt

The loudspeakerThe sound signal consists of an alternating current supplied by the amplifier.

This current flows through the coil of the loudspeaker.

Due to the motor effect, the magnetic field around the coil causes the coil to vibrate in step with the alternating current.

The coil causes the diaphragm (speaker cone) to vibrate in step with the original sound signal.

The diaphragm causes air to vibrate and so produces a sound wave.

QuestionChoose appropriate words to fill in the gaps below:

The motor effect occurs when a _______ carrying wire is placed inside a ________ field.

The force exerted is __________ when the wire is at 90° to the magnetic field __________ but is zero if the wire is ________ to the field.

The force increases with _________ or current strength, the force __________ in direction if either are reversed.

Applications include the electric motor and ___________.

magneticparallel

maximumdirectionloudspeaker

current

WORD SELECTION:

reverses

magnetic

parallel

maximum

direction

loudspeaker

current

field

reverses

field

The motor effectNotes questions from pages 254 & 255

1. What is the motor effect?2. Copy out the bullet points at the bottom of page 254 listing the factors that

affect the force on a current carrying wire inside a magnetic field.3. Copy and answer question (a) on page 254. 4. Copy Figure 3 on page 255 and explain how a simple electric motor

works. Your account should include the purpose of the split-ring commutator.

5. Copy and answer question (b) on page 255.6. Copy Figure 4 on page 255 and explain how a moving coil loudspeaker

works. 7. Copy and answer question (c) on page 255.8. Copy the ‘Key points’ table on page 255.9. Answer the summary questions on page 255.

Electric motor - FendtMotor effect - Fendt

The motor effect ANSWERS

In text questions:(a) No change, the actions

cancel each other out.(b) The material must conduct

electricity.(c) A direct current will not

produce a changing magnetic field.

Summary questions:1. (a) Current, coil, force, coil.

(b) Current, force, coil.2. (a) The direction of the

current is reversed and so the force on the coil is in the opposite direction.

(b) (i) Faster because the coil is lighter(ii) Faster because the field is much stronger due to the presence of iron.

The generator effectIf an electrical conductor cuts. through magnetic field lines, an electrical potential difference is induced across the ends of the conductor.

If the wire is part of a complete circuit, a current is induced in the wire.

This is also called electromagnetic induction.

Generator - Fendt

If a magnet is moved into a coil of wire, an electrical potential difference is induced across the ends of the coil.

If the direction of motion, or the polarity of the magnet, is reversed, then the direction of the induced potential difference and the induced current are also reversed.

The generator effect also occurs if the magnetic field is stationary and the coil is moved.

Generator - Fendt

The size of the induced potential difference increases when:

– the speed of the movement increases

– the strength of the magnetic field increases

– the number of turns on the coil increases

– the area of the coil is greater.

Generator - Fendt

Alternating Current Generators

Most electricity is produced using the ‘generator effect’.

The simplest generators and the types used in power stations produce alternating current (A.C.)

Generator - Fendt

Moving Coil A.C. Generator

Generator - Fendt

Generator - Fendt

This like an electric motor in reverse.

As the coil is rotated electromagnetic induction occurs.

An alternating voltage is induced in the coil.

An alternating current is drawn off through two slip rings.

The faster the coil is rotated:

- the greater is the amplitude of the voltage and current

- the higher is the frequency of the a.c.

Generator - Fendt

Bicycle generatorWhen the wheel turns the magnet is made to rotate next to the fixed coil of wire.

Electromagnetic induction occurs and a alternating potential difference is induced in the coil.

This causes an alternating current to flow to the light bulb of the bicycle.

Generator - Fendt

Question 1The graph opposite shows the potential difference of a generator varies in time. Using the same set of axes show how the potential difference would vary if the rotational speed of the generator was doubled.

PD

time

The new potential difference will have TWICE the amplitude AND frequency of the original.

Question 2Choose appropriate words to fill in the gaps below:

The _________ effect occurs when a conductor is moved relative to a ____________ field. This is also known as electromagnetic ___________.

The greater the relative __________ of the conductor and magnetic field the _______ is the potential difference ________.

If the conductor is part of a ________ circuit an electric current will flow.

___________ current is produced if the direction of movement is continually _________.

magnetic generatorcomplete

induction movementgreater

WORD SELECTION:

induced

alternating reversed

magnetic

generator

complete

induction

movement

greater induced

alternating

reversed

Electromagnetic induction Notes questions from pages 256 & 257

1. What is induced in a wire because of the dynamo effect?2. Copy and answer question (a) on page 256.3. Copy Figure 2 on page 256 and explain how a cycle dynamo

works.4. Copy and answer questions (b) and (c) on page 256.5. Explain how the alternating current generator on page 257 works.

Your explanation should include a copy of both parts of Figure 4.6. Copy the ‘Key points’ table on page 257.7. Answer the summary questions on page 257.

Generator - Fendt

Electromagnetic induction ANSWERS

In text questions:(a) (i) The current increases.

(ii) The direction of the current reverses.(iii) No current is produced.

(b) The wires leading to the coil would get twisted up. No brushes are needed.

(c) (i) There is no current.(ii) A p.d. is produced in the opposite direction.

Summary questions:1. (a) The pointer would move to

the right but not as far. (b) The pointer returns to zero.

(c) The pointer would move rapidly to the left.

2. (a) Spin the coil faster, use more loops of coil, use stronger magnets.(b) The peak voltage would be lower and the period would be longer.

The transformerA transformer is a device that is used to change one alternating voltage level to another.

Transformer - eChalk

circuit symbol

Structure of a transformerA transformer consists of at least two coils of wire wrapped around a laminated iron core.

Transformer - eChalk

laminated iron core

PRIMARY VOLTAGE Vp

PRIMARY COIL of Np turns

SECONDARY COIL of Ns turns

SECONDARY VOLTAGE Vs

How a transformer worksWhen an alternating voltage, Vp is applied to the primary coil of Np turns it causes an alternating to flow in this coil.

This current causes a changing magnetic field in the laminated iron core which cuts across the secondary coil of Ns turns.

Electromagnetic induction occurs in this coil which produces an alternating voltage, Vs.

Transformer - eChalk

QuestionWhy can a transformer not change the level of the voltage output of a battery?

– A battery produces a steady (DC) voltage.– This voltage would cause a constant direct current in

the primary coil of a transformer.– This current would produce an unchanging magnetic

field in the iron core.– This unchanging magnetic field would NOT cause

electromagnetic induction in the secondary coil.– There would therefore be no secondary voltage.

Transformers Notes questions from pages 258 & 259

1. Copy Figure 1 on page 258 and (a) explain what a transformer is, (b) what a transformer does and (c) how a transformer works.

2. Copy and answer questions (a), (b) and (c) on page 258.3. Copy the circuit symbol for a transformer on page 259 and

explain why the electric current supplied to a transformer must be alternating in order for the transformer to function.

4. Copy and answer question (d) on page 259.5. Copy the ‘Key points’ table on page 259.6. Answer the summary questions on page 259.

Transformer - eChalk

Transformers ANSWERS

In text questions:(a) The magnetic field in the core

would be much weaker because the core is not a magnetic material.

(b) The lamp would be brighter.(c) The lamp would not light up

with direct current in the primary coil.

(d) Iron is easier to magnetise and demagnetise as the alternating current increases and decreases each half cycle.

Summary questions:1. Current, primary, magnetic field,

secondary, p.d., secondary.2. (a) Direct current in the primary

coil would not produce an alternating magnetic field, so no p.d. would be induced in the secondary coil.(b) The current would short-circuit across the wires instead of passing through them. This would cause the coil to overheat if it did not cause the fuse to blow.(c) Iron is a magnetic material, so it makes the magnetic field much stronger. It is easily magnetised and demagnetised when the current alternates.

The transformer equationThe voltages or potential differences across the primary and secondary coils of a transformer are related by the equation:

primary voltage = primary turns

secondary voltage secondary turns

Vp = Np

Vs Ns

Transformer - eChalk

Step-up transformersIn a step-up transformer the potential difference across the secondary coil is greater than the potential difference across the primary coil.

The secondary turns must be greater than the primary turns.

Use: To increase the voltage output from a power station from 25 kV (25 000 V) to up to 400 kV.

Transformer - eChalk

Step-down transformersIn a step-down transformer the potential difference across the secondary coil is smaller than the potential difference across the primary coil.

The secondary turns must be smaller than the primary turns.

Use: To decrease the voltage output from the mains supply from 230V to 18V to power and recharge a lap-top computer.

Transformer - eChalk

Question 1Calculate the secondary voltage of a transformer that has a primary coil of 1200 turns and a secondary of 150 turns if the primary is supplied with 230V.

primary voltage = primary turnssecondary voltage secondary turns

230 / Vs = 1200 / 150

230 / Vs = 8

230 = 8 x Vs

230 / 8 = Vs

Secondary voltage = 28.8 V

Transformer - eChalk

Question 2Calculate the number of turns required for the primary coil of a transformer if secondary has 400 turns and the primary voltage is stepped up from 12V to a secondary voltage of 48V.

primary voltage = primary turnssecondary voltage secondary turns

12 / 48 = Np / 400

0.25 = Np / 400

0.25 x 400 = Np

Primary has 100 turns

Transformer - eChalk

Complete:

PRIMARY SECONDARY

Voltage Turns Voltage Turns

230 V 1000 11.5 V 50

230 V 500 46 V 100

230 V 200 920 V 800

9 V 120 72 V 960

Answers

50

46 V

200

9 V

Transformer - eChalk

Transformers and the National GridThe National Grid is the system of cables used to deliver electrical power from power stations to consumers.

The higher the voltage used, the greater is the efficiency of energy transmission.

Lower voltages result in higher electric currents and greater energy loss to heat due to the resistance of the cables.

At power stations the output voltage of the generators is stepped up by transformers from 25kV to 132kV.

The voltage may be further increased to up to 400 kV for transmission over long distance pylon lines.

The voltage is reduced in stages by step-down transformers to different levels for different types of consumer.

The lowest level is 230V for domestic use. The final step-down transformer will be at sub station within a few hundred metres of each group of houses.

Question 1Why is electrical energy transmitted over the National Grid in the form of alternating current?

– To maximise efficiency high voltages must be used.– Voltage therefore needs to be changed in level.– Transformers are needed to change voltage levels.– Transformers only work with alternating current.

Question 2Choose appropriate words to fill in the gaps below:

Transformers are used to change one ___________ potential difference level to another. They do not work with ____________current.

Step-up transformers _________ the voltage because their ___________ coil has more turns than the primary.

Transformers are used in the __________ Grid. The _______ output of a power station is increased to up to _______. A high voltage reduces the ________ lost to heat due to the _________ of the power lines.

alternating400 kVincrease

energy secondary

25 kV

WORD SELECTION:

direct National resistance

alternating

400 kV

increase

energy

secondary

25 kV

direct

National

resistance

Transformers and the National Grid Notes questions from pages 260 & 261

1. (a) Why are transformers used in the National grid? (b) What is the advantage of using high voltages?

2. Copy the transformer equation on page 260.3. Copy a version of the worked example on page 260 but in your version

change the number of turns on the secondary coil from 60 to 30.4. What is the purpose of (a) step-up and (b) step-down transformers?5. Explain how the number of turns on the coils of a transformer determine

whether a transformer is step-up or step-down.6. State how the currents and voltages associated with the primary and

secondary coils are related to each other with a 100% efficient transformer.

7. Copy and answer questions (a) and (b) on page 261.8. Copy the ‘Key points’ table on page 261.9. Answer the summary questions on page 261.

Transformer - eChalk

Transformers and the National Grid ANSWERS

In text questions:

(a) 60 turns

(b) (i) 6A (ii) 0.26A

Summary questions:

1. (a) (i) Secondary, primary.

(b) Up, down.

2. (a) 2000 turns

(b) (i) 3A (ii) 0.15A

More power to you Notes questions from pages 262 & 263

1. Answer questions 1 and 2 on page 263.

Electromagnetism SimulationsMotor effect - FendtElectric motor - FendtFaraday Electromagnetic Lab – PhET Play with a bar magnet and coils to learn about Faraday's law. Move a bar magnet near one or two coils to make a light bulb glow. View the magnetic field lines. A meter shows the direction and magnitude of the current. View the magnetic field lines or use a meter to show the direction and magnitude of the current. You can also play with electromagnets, generators and transformers!

Faraday's Law - PhET - Light a light bulb by waving a magnet. This demonstration of Faraday's Law shows you how to reduce your power bill at the expense of your grocery bill. Generator - FendtTransformer - load can be changed but not turns ration - netfirms Transformer - eChalk

More power to you ANSWERS

1. (a) They would not need heavy iron magnets.(b) There would be no power wasted in the wires, as the wires would have no resistance.

2. (a) Ionising radiation, carcinogenic (cancer-causing) substances.(b) People are at risk due to other causes. There is an extra risk to those exposed to these magnetic fields.(c) A hypothesis is put forward as an ‘unproven’ theory to be tested by scientific experiments. If lots of experiments are carried out and they all support the hypothesis, it gains scientific credibility and is accepted as a theory. But at any stage, it could be overthrown by any conflicting scientific evidence.

How Science Works ANSWERS

a) The voltmeter was not sensitive enough. It would also not give a read-out of the voltage, so it would be impossible to get an accurate result even if it was sensitive enough.

b) Height on the X-axis, voltage on the Y-axis. Axes fully labelled and plots correctly plotted.

c) In part. The voltage increased as height increased, but it was not directly proportional.

d) 0.01Ve) Not at the greater heights.f) Improve the sensitivity of the

oscilloscope. Repeat his results.

g) By checking it against other data/other similar research/get someone else to repeat his work or calculate theoretical relationships.

h) For example: Measuring the speed of an object through a tube.