Magnets, Electromagnet Induction, Electric Motors And Transformers

Question 1

What materials experience magnetic properties

Answer 1

Iron, steel, nickel and cobalt

Question 2

What is the region around a magnet called

Answer 2

A magnetic field

Question 3

Ends of magnets

Answer 3

These are called poles, north and south and the magnetic fields are strongest here

Question 4

Law of attraction

Answer 4

Same poles repel, opposite poles repel

Question 5

Repelling magnets

Answer 5

There is an area in the middle of the poles where the fields cancel wach other out and there is no overall magnetic field

Question 6

Attracting magnets

Answer 6

Question 7

How do you make an electromagent

Answer 7

When a current flows through a wire, a weak magnetic field is created around a wire

Question 8

How to change the strength of the magnetic field line around a current carrying wire

Answer 8

You can increase the strength by

1. Increasing the current in the wire
2. Wrapping the wire into a coil (a solenoid)

Question 9

Magnetic field lines around a solenoid

Answer 9

They are the same as around a bar magnet

Question 10

How can you increase the strength of the magnetic field around the solenoid

Answer 10

1. Increase the current
2. Increase the number of turns (amount of spins in the coil)
3. Wrapping the solenoid around a magnetically soft core such as iron, this is called an electromagnet

Question 11

Using electromagnets

Answer 11

1. The electric bell
2. The relay switch
3. Circuit breakers

Question 12

The electric bell

Answer 12

Switch closed, circuit complete

Electric current flows through

Soft iron core is magnetised

This attracts the steel armature (bar)and the wire which is atttached to it

This pulls the hammer, which hits bell and creates a sound

The circuit is now broken though so the electromagnet is demagnetised and as the wire is springy, it springs back

The process repeats

Question 13

The relay switch

Answer 13

Switch is closed and circuit is complete

Low current passes through and soft iron core is electromagnetised

This means the iron arm is attracted to the magnet and as it moves it pushes the contacts together

This completes the second circuit and current passes through it, powering the motor which begins to spin

When the switch is opened in circuit 1, the iron arm returns to its original position as it is on a spring, breaking circuit 2 and prohibiting the motor from turning

Question 14

How does the electric motor work

Answer 14

1. Electricty put into the circuit
2. Left side= upward right side= downward (hand rule) currents are moving in different directions
3. Split ring commutator means rotating forces can switch every half turn
4. Brushes complete circuit and connect coil to electric supply without prohibiting the split ring commutator to spin

Question 15

Why are the brushes in an electric motor normally made of carbon or graphite?

Answer 15

They both conduct electricity due to free electrons and they are soft so there is little friction between the bristles and the split ring commutator

Question 16

Why does the coil spin through a vertical position on an electric motor/why wouldnt it?

Answer 16

It wouldnt because at vertical the forces are equal to the right and left side sono resultant force

However it builds up momentum and the split ring commutator allows it to spin through vertical and then the rotating forces switch

Question 17

What affects the speed of the motor?

Answer 17

1. Friction bewteen brushes and split ring commutator
2. Strength of magnets
3. Conductivity of wire

Question 18

What is electromagnetic induction?

Answer 18

When a wire is moved thoguh a magnetic field, it cuts through the flux lines and a potential difference is induced. When the wire is connected to an (a.c) circuit, a current will flow through it

Question 19

How does electromagnetic differ from the motor effect?

Answer 19

Motor effect= put electricity in and get motion out

Electromagnetic induction= put motion in and get electricty out creates a generator

Question 20

Why is the current a.c?

Answer 20

When the magnet is stationary inside the solenoid, no current flows as it has to be moving a constantly cutting the field lines

Question 21

Why is both a negative and positive voltage induced when a magnet is dropped through a coil

Answer 21

Because the potential difference is different a both ends of the coil so there are postive and negative charges

Question 22

How can you increase the current the generator produces?

Answer 22

1. Move the magnet faster
2. Use a stronger magnet
3. Use a coil with more turns
4. Use a coil with a larger cross sectional area

Question 23

What is the purpose of step up tranformers?

Answer 23

Increase voltage

Decrease current

Decrease heat

Decrease efficiency

Question 24

Describe how this transformer works

Answer 24

1. When an ac current is passed through a wire, it creates a magentic field. This magnetic field created around the primary coil will cut through the secondary coil and induce a current
2. More coils = lower current so there is a higher current in the primary coil
3. Because P=IV and Pout = Pin, the voltage of the secondary coil will be higher

Question 25

Benefits to the iron used

Answer 25

1. It is a soft iron core allowing it to be magnetised and demagnetised easily
2. It is laminated to reduce heat loss –> reduce energy loss –> increase efficiency

Question 26

Equations for transformers

Answer 26

Pin = IpVp = Pout = IsVs

and

Np/Ns = Vp/Vs = Is/Ip

Question 27

Rule for current and voltage in transformers

Answer 27

Whatever you do to the voltage, do the oppostie to the current

Question 28

UK National Grid and relevance

Answer 28

Transformers are used so that electricity is transmitted as low currents and high voltages to reduce enegy loss by heat loss

Question 29

Process done by UK national grid

Answer 29

1. Generation of electricty in alternators
2. Step up transformers, voltage = 400kV
3. Cables buried under towns and cities
4. Step down trasnformers, voltage= 230V