Topic 12&13:Magnetism And The Motor Effect

Question 1

What is a permanent magnet?

Answer 1

A magnet that is always magnetic, such as a bar magnet.

Question 2

What are the 4 magnetic materials?

Answer 2

• iron
• nickel
• cobalt
• steel

They will always be attracted by magnetic materials.

Question 3

Define the term magnetic field:

Answer 3

The space around a magnet where it can attract magnetic materials.

Question 4

What is an induced magnet?

Answer 4

When a piece of magnetic material in a magnetic field become such a magnet itself. It stops being magnetic when it is taken out of the field again.

Question 5

How are magnets used?

Answer 5

• electric motors
• generators
• loudspeakers
• door latches
• knife holders

Question 6

How can the shape of a magnetic field be found?

Answer 6

Using a plotting compass. We represent magnetic field using lines that show how a single North Pole would move (from north to south). The field is strongest when the lines are closest together.

Question 7

What is a plotting compass?

Answer 7

A small compass used to find the shape of a magnetic field.

Question 8

Why are compass needles weighted?

Answer 8

The needle of a plotting compass is a very small magnet. Compass needles are weighted at one end to keep them level.

Question 9

What evidence shows that the earth has a magnetic field?

Answer 9

The behaviour of a compass is evidence that the Earth has a magnetic field, which is similar in shape to the magnetic field of a bar magnet.

Question 10

What causes the Earths magnetic field?

Answer 10

It is thought to be caused by electric currents in the molten outer core, which is made from a mixture of iron and nickel.

Question 11

What does a current flowing though a wire cause?

Answer 11

A magnetic field, electric motors and many other devices depend on the magnetic effect of electric currents.

Question 12

How does a compass work?

Answer 12

• compass needles point in the direction of Earths magnetic field.
• a compass points north because all magnets ave 2 poles. The north of one pole is attracted to the south of another pole.
• the earth is a magnet that interact with other magnets, so the north end of a compass magnet is drawn to align with the earths magnetic field.
• needle of a compass weighted to keep it level.

Question 13

Where is Earths magnetic field strongest?

Answer 13

At the poles. Earths magnetic field has the same shale as field lines around a bar magnet.

Question 14

Why are compass needles weighted?

Answer 14

The needles are weighted at one end to keep them level.

Question 15

How can you test to see if a current is flowing through a wire?

Answer 15

• put the wire through a piece of card.
• put iron fillings on the card.
• if current flows through the wire, iron fillings in the card make circular patterns on the card.
• this shows the current is causing a magnetic field, because iron fillings are lining up with the direction of the magnetic field.

Question 16

How can you use a plotting compass to find out the direction of the magnetic field?

Answer 16

-the direction of the magnetic field depends on the direction of the current. If the current changes direction so does the direction of the magnetic field.

Question 17

What does the strength of a magnetic field depend on (in a current flowing through a wire)?

Answer 17

• the size of the current (the higher the current the stronger the magnetic field.
• distance to wire. Magnetic field is strongest closer to the wire.

Question 18

What is a solenoid?

Answer 18

A coil of wire with electricity flowing through it. This is also called an electromagnet.

Question 19

Why is magnetic field inside a solenoid stringer than outside the solenoid?

Answer 19

The field from individual coils add together to form a very string field inside the solenoid.
Outside the solenoid the fields from one side of the coil tends to cancel out the fields from the other side to give a weaker field outside.

Question 20

What is an electromagnet?

Answer 20

A coil of wire with a current flowing through it.

Question 21

How can the magnetic field of an electromagnet be made stronger?

Answer 21

The field of an electromagnet can be made stronger by putting a piece of iron (iron core) inside the coil. The iron becomes a temporary magnet - it is only magnetic while the field from the electromagnet is affecting it.

Question 22

What is the motor effect?

Answer 22

A term used when a current-carrying wire in the presence of magnetic field experiences a force.

Question 23

Describe the motor effect:

Answer 23

A wire carrying a current experiences a force when it is placed between 2 magnets. The current in the wire creates a magnetic field around the wire and this interacts with with the magnetic field between the magnets.

The force is greatest when the wire is at right angles to the magnetic field and is 0 when wire is in same direction, there are equal and opposite forces acting on the magnets.

Question 24

What is Flemings left-hand rule?

Answer 24

A way of remembering the direction of the force when a current flows in a magnetic field.

Question 25

What does the direction of the force in Flemings left hand rule depend on?

Answer 25

The direction of the magnetic field and the current.

Question 26

What does the size of the force in Flemings left hand rule depend on?

Answer 26

The size of the force depends on the magnetic field strength, the current and the length of wire in the field.

Question 27

What is the magnetic flux density and what is it measured in?

Answer 27

This is the strength of the magnetic field. It is measured in newtons per amp metre (N/Am) alcove called tesla (T) and is given as the symbol B.

Question 28

What equation links force, magnetic field strength (flux density), current and length?

Answer 28

Force on conductor carrying current at right angles to the magnetic field = magnitude field strength x current x length
N N/Am or T A m

F= B x I x L

Question 29

What is the use of the split-ring commutator in an electric motor?

Answer 29

Ensures the force in the coil always turns it in the same direction.

Question 30

What is the size of the carbon brush in an electric motor?

Answer 30

Makes electrical contacts between the circuit and the motor.

It also ensures that the current changes direction every half turn producing an alternating current.

Question 31

How does am electric motor work?

Answer 31

The force on a conductor in the magnetic field is used to cause rotation in electric motors. There is a force on each part of the wore carrying a current in the magnetic field and so using a coil with many turns of wire increases the total force turning the coil.

Question 32

What can a changing magnetic field induce?

Answer 32

It can induce a voltage/potential difference (p.d) in a wire which causes a current to flow.

Question 33

How can voltage/p.d be induced?

Answer 33

• a changing magnetic field.
• if a wire is moved into a magnetic field. (A wire is often made into coil of wire so that there is more wire in the chaining magnetic field).

Question 34

What does the size of the induced potential difference depend on?

Answer 34

-depends on the number of turns in a coil of wire, the strength of the magnetic field and how fast the magnetic field changes or moves past the coil. Reversing the direction reversed the direction of the induced potential difference.

Question 35

How does a generator work?

Answer 35

A generator consists of a coil of wire that is rotated inside a magnetic field. As the coil turns, a voltage is induced in the wire. The ends of the coil are connected to slip rings. Electrical contact with an external circuit is made using carbon brushes, which press on the slip rings. A generator like this produces a.c and can be called an alternator.

Question 36

What is needed for electrical appliances to produce direct current?

Answer 36

A commentator switches over the connections every half turn of the coil and so princes a form of direct current.

Question 37

What is a generator with a commutator called?

Answer 37

A dynamo

Question 38

What type of current does a dynamo produce?

Answer 38

d.c

Question 39

How does a generator produce alternating current?

Answer 39

• the rotation in the generator produces a.c in the coil.
• the slip rings provide contact with coil (touching it) and rotates around it. (Coil moves around slip rings passing current to ring and then passing it to carbon brushes.

Question 40

How do microphones do?

Answer 40

Microphones convert the pressure variations in sound waves into variations in current and electrical currents.

Question 41

How do microphones work?

Answer 41

1- sound waves cause variation in air pressure.
2- the pressure variations make a diaphragm vibrate.
3- the diaphragm moves a coil of wire backwards and forwards.

(Some microphones use electromagnetic indication to produce a caring current).

Question 42

What do loudspeakers do?

Answer 42

Covert variations in an electrical current into sound waves.

Question 43

How does does a loudspeaker work?

Answer 43

• They varying current flows through the coil that is in a magnetic field.
• This causes the force on the coil, which moves backwards and forwards as the current varies.
• the coil is connected to a diaphragm, and the movements of the diaphragm produce sound waves.

Question 44

What are the similarities between microphones and loudspeakers?

Answer 44

• both have a diaphragm.
• both have a coil that can move.
• both induce a magnet.

Question 45

What are the differences between microphones and loudspeakers?

Answer 45

• microphones convert variations in air pressure into variations in current.
• loud speakers convert variations in in current init variations in air pressure.

The microphone uses movement to include a current whereas, the loudspeaker uses current variations to produce movement.

Question 46

What is a transformer?

Answer 46

A device which is used to change potential difference (voltage) of an electricity supply.

Question 47

Why do we use transformers?

Answer 47

• because not all appliances can use 230V.

- factories, hospitals (etc) need p.d more than 230V.

Question 48

What is the assumption we make about transformers?

Answer 48

Assume they are 100% efficient. Power that goes into transformer = power that leaves the transformer.

Question 49

What is the National grid?

Answer 49

A network of wires and cables that electricity is sent to from power stations that provide electricity to homes, school, factories, hospitals (etc).

Question 50

If the voltage of the electricity passing through a wire is increased what happens to its current?

Answer 50

The current is decreased.

Voltage increased = current decreased
Voltage decrease = current increased.

Question 51

What is a step-up transformer?

Answer 51

Increases the voltage and decrease the current at the same time.

Question 52

What is a step-down transformer?

Answer 52

Decreases voltage and makes the current higher.

Question 53

What is the order of voltages from the national grid starting from the power station?

Answer 53

25kV, 400kV, 11kV, 33kV, 230v.

Question 54

What are transmission lines?

Answer 54

The wires over head or underground that take electricity from power stations to towns and cities.

Question 55

How is a transformer made?

Answer 55

It is made using 2 coils of insulated wire wound onto an iron core. There is no electrical connections between the 2 coils of wire.

Question 56

Why are transformers used in the national grid?

Answer 56

When current is smaller (caused by transfers) less energy is transferred by heating and the efficiency is improved.

The amount of heating wasted in wires inside the buildings is quite small, but the amount is significant for the transmission lines in the National grid.

Question 57

What current to transformers need to work.

Answer 57

Only work with a.c.

Question 58

The potential difference across the coils of a transformer be worked out using the equation:

Answer 58

Potential difference across the primary coil/potential difference across the secondary coil = number of turns in primary coil/number of turns in secondary coil.

Vp/Vs=Np/Ns

Question 59

Define the term potential difference:

Answer 59

A measure of the energy transferred by each coulomb of charge that flows through a wire.

Question 60

What equation links electrical power, current and potential difference?

Answer 60

Electrical power = current x potential difference
(W) (A) (V)

P = I x V

Question 61

What equation links potential difference and current across primary and secondary coils?

Answer 61

Potential differences across primary coil(V) x current in primary coil (A) = Potential differences across secondary coil(V) x current in secondary coil (A)

Vp x Ip = Vs x Is

Question 62

Why must the power supplied to a transformer in a primary coil be equal to the power transferred away from the transformer in the secondary coil?

Answer 62

Energy cannot be created or destroyed so must be transferred. If a transformer is 100% efficient the the power is the primary and secondary coil must be equal.