P18: Motors, Generators and Transformers

Question 1

Describe the magnetic field patterns with like poles facing each other.

Answer 1

Bend away from the midpoint.

Question 2

Describe the magnetic field patterns with unlike poles facing each other.

Answer 2

The field between the poles is uniform.

Question 3

What is special about a uniform field?

Answer 3

The magnetic field lines are parallel to each other.

Question 4

What metal is used for the core of electromagnets?

Answer 4

Iron (Easily magnetised and demagnetised)

Question 5

Name for metals that can be magnetised/ demagnetised easily

Answer 5

ferrous metals

Question 6

4 examples of electromagnets

Answer 6

> Scrapyard cranes > Circuit breakers > Electric bells > Relays

Question 7

Increasing _____ makes the magnetic field of an electromagnet stronger.

Answer 7

Current

Question 8

What surrounds an electromagnets core?

Answer 8

A coil of insulated wire

Question 9

How can the size of the electromagnetic force be increased

Answer 9

Increasing the current Using a stronger magnet

Question 10

In the motor effect, when is the force greatest?

Answer 10

When the wire is perpendicular to the magnetic field.

Question 11

What are the magnetic field lines around a wire?

Answer 11

Circles centred on the wire in a plane perpendicular to the wire.

Question 12

Reversing the direction of the current…

Answer 12

…reverses the magnetic field lines

Question 13

The Motor Effect

Answer 13

When a CURRENT is passed along a wire in a magnetic field, A FORCE IS EXERTED on the wire by the magnetic field.

Question 14

In the motor effect, when is the force zero?

Answer 14

When the wire is parallel to the magnetic field.

Question 15

Fleming’s Left Hand Rule

Answer 15

Movement

Field (N to S)

Current

Question 16

Commutator for DC

Answer 16

Split Ring- Reverse the current on every half turn to keep the motor spinning constantly.

Question 17

Commutator for AC

Answer 17

Slip-ring - Doesnt alternate the current because it alternates already.

Question 18

Electromagnetic Induction

Answer 18

The process of inducing a potential difference/voltage in a wire by moving the wire so it cuts across the lines of force of a magnetic field.

Question 19

The Generator Effect

Answer 19

The production of voltage using a changing magnetic field.

Question 20

What never changes in a DC generator?

Answer 20

The Voltage’s Polarity

Question 21

What is a simple AC generator made up of?

Answer 21

A coil that spins in a uniform magnetic field.

Question 22

When is the size of an induced p.d/voltage on an AC generator zero?

Answer 22

When the coil is perpendicular to the magnetic field lines.

Question 23

When is the size of the induced p.d/voltage on an AC generator greatest?

Answer 23

When the coild is parallel to the direction of the magnetic field.

Question 24

What types of current does a generator work with?

Answer 24

AC

Question 25

The Solenoid Rule

Answer 25

Direction of coil tells us which pole it corresponds to.

North- iN

sOUTh- OUT

Question 26

4 facts about electromagnetic induction.

Answer 26

1. No current when the wire is stationary
2. A current is generated when the magnet moves but not the wire.
3. Faster movement = More current
4. Current reverses when the direction of motion reverses.

Question 27

What must there be for electromagnetic induction to occur?

Answer 27

A changing magnetic field.

Question 28

3 Acronyms for Transformers

Answer 28

Vicky needs 5 A’s

Voltage Can Make Vicky Cry

PC Ian Can’t Solve Crime

A A A A A

V C M V C

PC IC SC

Question 29

3 steps of a transformer based on the acronyms

Answer 29

1. An alternating voltage in the primary coil creates an alternating current there.
2. This creates an alternating magnetic field in the iron core
3. This induces an alternating voltage in the secondary coil, which induces an alternating current.

Question 30

A step up transformer…

Answer 30

Has more turns on the secondary coil, so increases voltage.

Question 31

A step down transformer…

Answer 31

Has less turns on the secondary coil, so decreases voltage.

Question 32

The higher the grid voltage…

Answer 32

… the better the efficiency of the transformer.

Question 33

Equasion for transformer efficiency:

Answer 33

V_p / Ip = V_s / I_s

V = Voltage

I = Current

Question 34

The transformer equation:

Answer 34

V_p / V_s = (No. of turns) N_p / N_s

Question 35

The faster the coil rotates…

Answer 35

…The greater the frequency of the AC current

+

The larger the peak value of the AC current.

Question 36

Switch mode transformer.

Answer 36

A transformer that works between 50 000 Hz and 200 000 Hz

Question 37

What is the core of a switch mode transformer made of?

Answer 37

Ferrite

Question 38

Advantages of a switch mode transformer

Answer 38

> Lighter and Smaller

> Higher frequency

Question 39

Voltage of the National Grid

Answer 39

132 000 V

Question 40

Why does the voltage of the national grid being higher make it more efficient.

Answer 40

If the voltage was lower more curren would be needed for the same amount of power to be transferred. The cables would overheat and result in more wasted energy.

Question 41

What direction are the magnetic field lines in?

Answer 41

North to South