The Motor Effect

Charge deflection by a magnetic field

Electric charges are deflected by magnetic fields provided they are not travelling parallel to the field lines.

Positive and negative charges are deflected in opposite directions.

S

N

+-

+ -+-

+-

The motor effectWhen a current carrying conductor carrying an electric current is placed in a magnetic field, it will experience a force provided that the conductor is not placed parallel to the field lines.

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.

Motor effect - Fendt

Fleming’s 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

Electromagnetic inductionIf an electrical conductor cuts through magnetic field lines, a voltage 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 called electromagnetic induction and is sometimes called the generator effect.

Generator - Fendt

If a magnet is moved into a coil of wire, a voltage 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 voltage and the induced current are also reversed.

Electromagnetic induction also occurs if the magnetic field is stationary and the coil is moved.

Generator - Fendt

The size of the induced voltage 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 voltage 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 how the voltage of a generator varies in time. Using the same set of axes show how the voltage would vary if the rotational speed of the generator was doubled.

V

time

The new voltage 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 voltage ________.

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

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.

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

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.

Vp = Np

Vs Ns

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.

Vp = Np

Vs Ns

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

Transformer power transfer equation

If a transformer is 100% efficient then the power input to the primary coil is equalled by the power output from the secondary coil.

as power = current x voltage

then:

Ip x Vp = Is x Vs

Question 1Calculate the primary current if when a transformer is supplied with 230V the secondary provides 4A at a voltage of 13V. Assume that the transformer is 100% efficient.

Ip x Vp = Is x Vs

Ip x 230V = 4A x 13V

Ip = 52 / 230

Primary current = 0.226 A

Question 2Calculate the secondary current from a transformer supplying a secondary voltage of 6V if the primary is supplied with a current of 0.20A at 230V. Assume that the transformer is 100% efficient.

Ip x Vp = Is x Vs

0.2A x 230V = Is x 6V

Is = 46 / 6

Secondary current = 7.67 A

Complete:

PRIMARY SECONDARY

Np Vp Ip Ns Vp Is

600 200V 0.4 A 30 10V 8 A

100 12V 8 A 4000 480V 0.2 A

300 72V 0.4 A 50 12V 2.4 A

50 25V 10 A 250 125V 2 A

Answers

1 2

4

5

3

6

78

Step-up transformersIn a step-up transformer the voltage across the secondary coil is greater than the voltage 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 voltage across the secondary coil is smaller than the voltage 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

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 ___________ voltage 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

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

Electric Motors and Electromagnetic Induction

1. (a) What is the motor effect? (b) What factors determine the size of the force exerted on a conductor in a magnetic field? (c) With the aid of a diagram show how Fleming’s left-hand rule can be used to find the direction of the force on a conductor.

2. Copy figures 22.5 and 22.6 and explain how a moving coil loudspeaker and electric motor work.

3. (a) Draw diagrams and explain what is meant by ‘electromagnetic induction’? (b) What factors determine the size of the voltage produced?

4. Copy figure 22.12 and use it to explain how a simple generator works.5. Copy figure 22.16 and use it to explain how a transformer works.6. Copy the two transformer equations on pages 193 and 194 and find the

secondary current and voltage for a 100% efficient transformer that has a primary coil of 800 turns supplied with 2A at 40V if the secondary coil has 100 turns.

7. Explain what is meant by step-up and step-down transformers and how they are used in the UK’s National Grid system.

8. Answer the questions on pages 195 and 196.9. Verify that you can do all of the items listed in the end of chapter checklist

on page 195.

Electric Motors and Electromagnetic Induction Notes questions from pages 187 to 196

1. (a) What is the motor effect? (b) What factors determine the size of the force exerted on a conductor in a magnetic field? (c) With the aid of a diagram show how Fleming’s left-hand rule can be used to find the direction of the force on a conductor.

2. Copy figures 22.5 and 22.6 and explain how a moving coil loudspeaker and electric motor work.

3. (a) Draw diagrams and explain what is meant by ‘electromagnetic induction’? (b) What factors determine the size of the voltage produced?

4. Copy figure 22.12 and use it to explain how a simple generator works.

5. Answer questions 1, 2 and 3 on pages 195 and 196.