Electric Motors and Electromagnetic Induction

Question 1

What happens when a wire carrying the current is put into a magnetic field?

Answer 1

A FORCE (motor effect)

Question 2

Why is there the motor effect?

Answer 2

This is because the charged particles (electrons in a current) moving through a magnetic field will experience a force, as long as they are not moving parallel to the field lines

Question 3

Describe the motor effect, and how does the force on a current-carrying conductor in a magnetic field increase with the strength of the field and with the current?

Answer 3

1. To experience the full force, the wire has to be at 90 degrees to the magnetic field. If the wire runs along the magnetic field it will not experience any force at all. At angle in between it will experience some force.
2. The force always acts in the same direction relative to the magnetic field of the magnets and the direction of the current in the wire
3. The strength of the force increases with the strength of the magnetic field
4. The force also increases with the amount of current passing through the conductor
5. Reversing the current or the magnetic field also reverses the direction of the force

Question 4

How do you speed a motor up?

Answer 4

1. Increase current flowing through the wire
2. Increase number of turns on the coil
3. Increase strength of magnetic field
4. Use a soft iron core in the coil

Question 5

Describe how a motor works

Answer 5

1. Current passes through a loop of wire, one side will experience a force pushing it upwards and the other side will experience a force pushing it downwards causing the loop to rotate
2. The split ring commutator means that when the loop is vertical, the connections to the supply through the brushes swap over, so that the current flowing through each side of the loop changes direction (it swaps the contacts every half-turn to keep the motor rotating in the same direction)
3. The wire at the bottom is now pushes upwards and the wire at the top is now pushed downwards , so the loop carries on turning
4. The arrangement of the brushes and split ring changes the direction of the current flowing through the loop every half turn, which means that the rotation can be continuous

Question 6

How can a motor be adapted?

Answer 6

1. The direction of the motor can be reversed by swapping the polarity of the d.c. supply or swapping the magnetic poles over
2. Permanent magnets can be replaced with curved electromagnets capable of producing very string magnetic fields (practical motors)
3. The single loop is replaced with several coils of wire wrapped on the same axis, this makes the motor more powerful and allows it to run more smoothly (practical motors)
4. The coils are wrapped on a laminated soft iron core. This makes the motor more efficient and more powerful (practical motors)
5. The speed can be increased by adding more turns to the coil, increasing the current, increasing the strength of the magnetic field or by adding a soft iron core

Question 7

How can you work out which way the coil will turn?

Answer 7

The left hand rule

Question 8

Describe how loudspeakers work

Answer 8

1. They use the motor effect
2. A.c signals from an amplifier are fed to a coil of wire in the speaker, which is wrapped around the base of a cone
3. The coil is surrounded by a permanent magnet, so the a.c. signals cause a force on the coil and make it move back and forth
4. These movements make the cone vibrate and this creates sounds

Question 9

What is electromagnetic induction?

Answer 9

When voltage is induced (and maybe current if the circuit is complete) in a wire/conductor/coil which is experiencing a change in magnetic field, or when it moves through a magnetic field

Question 10

What is the dynamo effect?

Answer 10

Using electromagnetic induction to transform kinetic energy into electrical energy

Question 11

What are the two situations of electromagnetic induction?

Answer 11

a) When an electric conductor (a coil of wire is often used) moves through a magnetic field
b) When the magnetic field through an electric conductor changes (gets bigger or smaller or reverses)

Question 12

Describe how a generator works

Answer 12

1. If a wire is moved across a magnetic field at right angles a voltage is induced (or generated) in the wire
2. If the wire is part of a complete circuit then current flows
3. This is called electromagnetic induction
4. Generators use movement to make electricity
5. The voltage and current are generated when a conductor such as a wire cuts through the magnetic field lines
6. The faster the lines are cut, the larger the induced voltage and current
7. This can also be achieved by moving a magnetic through a coil of wire
   - The generation of electricity is by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field

Question 13

What is Faraday’s Law of Electromagnetic Induction?

Answer 13

The size of the induced voltage across the ends of a wire (coil) is directly proportional to the rate at which the magnetic field lines of flux are being cut

Question 14

How can the size of the induced voltage (and current) be increased by in a generator (move wire)?

Answer 14

1. Move wire more quickly
2. Using a stronger magnet
3. Wrapping the wire into a coil so that more pieces of wire move through the magnetic field

Question 15

How can the size of the induced voltage (and current) be increased by in a generator (move magnet)?

Answer 15

1. Move magnet more quickly
2. Using a stronger magnet
3. Using a coil with more turns
4. Using a coil with a larger cross-sectional area (as cuts more field lines)

Question 16

How can you test that the generator works both ways (a and b)?

Answer 16

• By connecting an ammeter to a conductor and moving the conductor through a magnetic field (or moving a magnet through the conductor)
• The ammeter will show the magnitude and direction of the induced current
• If the direction of the movement is reversed, the induced voltage/current will be reversed too

Question 17

Give an example of how generators are used in everyday life

Answer 17

1. As the cyclist pedals, the wheel rotates and a small magnet within the dynamo spins around
2. As the magnet turns its magnetic field cuts through the surrounding coil, inducing a current in it.
3. This current can be used to work the cyclist’s lights.

Question 18

What is a generator that produces an alternating current called?

Answer 18

An alternator

Question 19

What are the properties of an alternator?

Answer 19

1. They rotate a coil in a magnetic field or a magnet in a coil, and as the coil spins, a current is induced in the coil. This current changes every half turn
2. They have slip rings and brushes so that the contacts don’t swap every half turn, instead of a split-ring commutator
3. This means that they produce a.c. voltage. The faster revolutions produce not only more peaks, but higher overall voltage too
   - E.G power stations use alternators to produce electricity, they just get the energy needed to turn the coil or magnetic field in different ways
   - The frequency of an alternating current is the number of complete cycles it makes each second. If an alternator rotates twice in a second, the frequency of the alternating current it produces is 2Hz. The frequency of the UK mains supply is 50Hz

Question 20

What are the similarities of a motor and generator?

Answer 20

• Both use magnets
• Both use a coil
• Both transfer energy

Question 21

What are the differences of a motor and generator?

Answer 21

They have the reverse process of each other
Motor: electrical energy into kinetic energy
Generator: kinetic energy into electrical energy

Question 22

Describe a transformer

Answer 22

They all have two coils, the primary and secondary joined with an iron core

Question 23

What do transformers do?

Answer 23

They change the size of an alternating voltage by having different numbers of turns on the input and output sides

Question 24

Describe how a transformer works

Answer 24

1. When an alternating current is passed through a coil, the magnetic field around it is continuously changing (reversing direction and as the magnetic field is continuously changing direction the voltage produced is an alternating voltage)
2. As the size of the current in the coil increases the field grows and as the size of the current decreases the field collapses
3. If a second coil is placed near the first, this changing magnetic field will pass through it and as it cuts through the wires of the second coil, a voltage is induced across that coil
4. The size and direction of the induced voltage changes as the voltage applied to the first coil changes (and the number of turns on each coil)
5. An alternating voltage is applied across the primary coil therefore produces an alternating voltage across the secondary coil
6. This combination of two magnetically linked coils is called a transformer

Question 25

Where are transformers used?

Answer 25

• Mobile phone chargers

- National grid to transfer current from power stations to power lines to our homes and buildings

Question 26

What is a step up transformer?

Answer 26

A transformer that is used to increase voltage (more turns on the secondary coil than the primary coil)

Question 27

What is a step down transformer?

Answer 27

A transformer that is used to decrease voltages (more turns on the primary coil than the secondary coil)

Question 28

How do the coils affect a transformer?

Answer 28

The ratio between the primary and secondary voltages is the same as the ratio between the number of turns on the primary and secondary coils

Question 29

What is the transformer equation?

Answer 29

input voltage/output voltage is equal to number of turns on primary coil/number of turns on secondary coil

Vp/Vs (Volts) = Np/Ns (no units)

Question 30

What is the efficiency of a transformer?

Answer 30

Assume transformers are 100% efficient
Pin=Pout
Vp x Ip = Vs x Is

Question 31

How are transformers used in the national grid?

Answer 31

• Step up and step down transformers are used when transmitting electricity across the country
• Transformers are used in the National Grid so that so that the electricity is transmitted at low currents and high voltages (so less energy lost in the form of heat)
  1. The voltage produced by power stations is too low to be transmitted efficiently. P=VI so the lower the voltage the higher the current for a given amount of power and the current causes the wire to heat up
  2. Therefore a step-up transformer is used to boost the voltage before it is transmitted (increased to approximately 400 kV but very dangerous so are supported high above the ground of pylons)
  3. As cables enter cities and towns they are buried underground. Close to where the energy is needed the supply is passed through a step-down transformer. It is used at the end of the journey to reduce the voltage (to approximately 230 V, whilst at the same time increasing the current) so that it is more useful and safer to use.

Question 32

How does an electric motor work when switch is closed? (marks scheme)

Answer 32

1. Current in coil
2. (Creates) a magnetic field (around the wires of the coil)
3. Interaction of this field with that of permanent magnets
4. There is a force on the wore of coil
5. Reference to LHR
6. Force up on one side and down on other side

Question 33

How does a transformer change the size of an alternating voltage?

Answer 33

By having different numbers of turns on the input and output sides

Question 34

What is the relationship between input (primary) voltage and output (secondary) voltage and the turns ratio for a transformer?

Answer 34

input (primary) voltage / output (secondary) voltage = primary turns / secondary turns
Vp/Vs = np / ns

Question 35

What is the relationship between input power and output power?

Answer 35

input power = output power
VpIp = VsIs
for 100% efficiency

Question 36

What is an electromagnet?

Answer 36

When a solenoid is wrapped around a magnetically soft core such as iron (combination of soft iron core and solenoid)

Question 37

Describe the structure of a transformer

Answer 37

1. They have two coils, the primary and the secondary joined with an iron core
2. When an alternating voltage is applied across the primary coil, the magnetically soft iron core magnetises and demagnetises quickly and this induces an alternating voltage in the secondary coil
3. The ratio between the primary and secondary voltages is the same as the ratio between the number of turns on the primary and secondary coils