Unit 2 Magnetism and Electromagnetism

Question 1

What are the four main magnetic materials and which one is used to make permanent magnets?

Answer 1

Iron, steel, nickel and cobalt. Steel is used to make permanent magnets.

Question 2

What is a magnetic field?

Answer 2

The space around a bar magnet in which a magnetic force acts.

Question 3

What two things can be used to show the shape of a magnetic field?

Answer 3

-Iron filings (only shape)
-Plotting compasses (shape and direction)

Question 4

What direction do magnetic field lines run?

Answer 4

From north to south
magnetic field lines never cross- they are closest together at the poles, indicating that the magnetic field is strongest here

Question 5

What is a cylindrical coil of wire called and what happens when current flows through it?

Answer 5

Solenoid- a magnetic field is produced. The solenoid is now an electromagnet because the magnetism was produced by the electricity.

Question 6

Describe the right hand grip rule

Answer 6

When the right hand grips the solenoid so that the fingers curl in the same way as the current, the thumb points to the north pole of the solenoid.

Question 7

When making and testing an Electromagnet why is the coil of wire insulated?

Answer 7

To make sure the current only flows through the coil not the core.

Question 8

Why is the power supply switched off between each reading?

Answer 8

To avoid overheating (keeping the wire/coil a constant temperature).

Question 9

What is the conclusion to the investigation on how the ‘strength of the magnetic field depends on the current in coil’?

Answer 9

As current increases, the electromagnet’s strength increases.
With properly functioning apparatus we would expect the two variables to be directly proportional

Question 10

What is the conclusion to the investigation on how the ‘strength of the magnetic field depends on the number of turns in the coil’?

Answer 10

As the number of turns increases, the strength of the electromagnet (no. of paperclips lifted) increases.
Again, with correctly functioning apparatus we would expect directly proportionality

Question 11

What is the conclusion to the investigation on how the ‘strength of the magnetic field depends on the material used as the core’?

Answer 11

If the core is made from one of the known magnetic materials, it gives a much stronger magnetic field.

Question 12

What happens when you place a current carrying wire inside another magnetic field?

Answer 12

It will experience a force. The force is perpendicular to both the direction of the current and magnetic field.

Question 13

Describe Fleming’s left-hand rule

Answer 13

-thuMb- Movement of wire
-First finger- Field direction (magnetic)
-seCond finger- Current direction

Question 14

How can you change the direction of a force (in an electromagnet)?

Answer 14

-Reverse the magnetic field direction.
-Reverse the current direction.

Question 15

What does the electric motor consist of?

Answer 15

A coil of wire (ABCD) wound on an armature and placed in a magnetic field. The coil is connected to a metal ring (split ring commutator) which makes contact with the carbon brushes that are connected to a d.c. power supply. The coil is also free to rotate.

Question 16

How does the electric motor work?

Answer 16

Sides AB and DC of the coil experience a force because they are carrying currents which cross the magnetic field at right angles (the magnetic fields runs from North to South). The current on side AB runs from A to B while the current on side DC runs from C to D. This causes the left side of the coil to move up and the right side of the coil to move down and therefore the coil rotates clockwise on the pivot. These forces cause the coil to move to the vertical position where it would come to rest. However, due to the action of the split ring commutator, the direction of the current in either side of the coil gets reversed (happens every half turn). This causes a change in direction of the forces and the coil gets pulled further around to the vertical position again and so on. In this way, the coil keeps rotating clockwise, half a turn at a time.

Question 17

Why do the top and bottom of the coil (in the electric motor) experience no force?

Answer 17

Because they are parallel to the magnetic field.

Question 18

How can the speed of the electric motor be increased?

Answer 18

1. Use a larger current in the coil.
2. Increase the magnetic field strength.
3. Increase the length of wire in the magnetic field.
4. Add more turns to the coil.

Question 19

What is direct current (d.c.)?

Answer 19

Direct current flows around a circuit in only one direction and it keeps the same steady value the whole time. You get it from batteries.

Question 20

What piece of equipment is used to see what d.c. looks like?

Answer 20

Cathode Ray Oscilloscope (C.R.O)

Question 21

What is alternating current (a.c.)?

Answer 21

Alternating current flows around a circuit periodically reversing direction. You get it from any mains supply.

Question 22

What is the value for mains voltage?

Answer 22

230V at a frequency of 50Hz.

Question 23

What is electromagnetic induction?

Answer 23

Electromagnetic induction is the generating of an electric current in a conductor by changing a magnetic field.

Question 24

What are the two ways an induced current can be achieved?

Answer 24

1. By moving a magnet in and out of a coil of wire (or moving the coil instead).
2. By changing the current in one of two neighbouring coils.

Question 25

Describe how current can be induced by moving a magnet in and out of a coil of wire

Answer 25

If a permanent magnet is moved in and out of a stationary coil of wire as shown, a current will be induced in the coil. The size of the current will be shown on the ammeter. The ammeter pointer will constantly move left and right as the magnet is moved in and out of the coil. If it flicks to the right when the magnet is moved in, then it will flick to the right when the magnet moves out again. If the magnet does not move relative to the coil, no current will be induced and the ammeter will stay at 0 amps.

Question 26

Why does a current appear in the coil when the magnet moves?

Answer 26

As the magnet moves towards the coil, its magnetic field starts to move through the coil. The coil experiences this as a changing magnetic field. This changing magnetic field induces a current in the coil. When the magnet moves out of the coil, the magnetic field moves away (in the opposite direction) which reverses the direction of the induced current.

Question 27

Describe how current is induced by changing the current in one of two neighbouring coils

Answer 27

If coil 1 (battery attached) is switched on and off, a current will be induced in coil 2. The size of the current will be shown on the ammeter. The ammeter pointer will constantly move left and right as coil 1 is switched on and off. If it briefly flicks to the right when coil 1 is switched on, then it will briefly flicker to the left when coil 1 is switched off. If coil 1 is just left off or just left on, no current will be induced and the ammeter will stay at 0 amps.

Question 28

What does an A.C. Generator do?

Answer 28

It generates or produces alternating current. It works on the principle of electromagnetic induction whereby electricity is produced when an conductor is moved through a magnetic field.

Question 29

How does an A.C. Generator work?

Answer 29

It consists of a wire bent in the shape of a flat rectangular coil placed between the poles of a permanent magnet. The coil is rotated and as it rotates, it cuts through the magnetic field and experiences a changing magnetic field. This causes a current to be induced in the coil.

Now since the coil has two sides and when one side moves up the other side moves down, the current which is induced in the coil, flows in one direction on one side and in the opposite direction on the other side.
This current is allowed to flow out to an external circuit through conducting rings known as slip rings, which are attached to each end of the coil (Y to the left and X to the right), and rotate with the coil. The slip rings are in contact with fixed carbon brushes, which conduct the current produced to the external circuit.

However, after one half turn, the left side of the coil becomes the right side and the right side becomes the left.
The current is still induced in the same directions as current shown in the diagram but since Y is now attached to the right side of the coil and X is attached to the left side, the current flowing out through the slip rings has changed direction, hence an alternating current is produced. If any device e.g. a bulb is connected between X and Y then an alternating current will flow through it.

Question 30

How can the size of the current produced in the A.C. Generator be increased?

Answer 30

1. Increasing the speed of rotation of the coil.
2. Increasing the strength of the magnet.
3. Increasing the number of turns of wire in the coil.

Question 31

How do transformers work?

Answer 31

An A.C. is applied to the primary coil giving it a changing magnetic field around it. This changing magnetic field passes through to the secondary coil and induces an A.C. and voltage in the secondary coil. The induced current has a frequency equal to the frequency of the original A.C.

Question 32

What are the two types of transformers?

Answer 32

Step-Up Transformer- for low to high voltage.
Step-Down Transformer- for high to low voltage.

Question 33

Describe how Step-Up Transformers work

Answer 33

With this transformer the induced output voltage from the secondary coil is greater than the applied input voltage to the primary coil (but the induced output current is less than the applied input current). This is achieved by having a larger number of turns in the secondary coil.

Question 34

Describe how Step-Down Transformers work

Answer 34

With this transformer the induced output voltage from the secondary coil is less than the applied voltage to the primary coil (but the induced output current is greater than the applied input current). This achieved by having a smaller number of turns in the secondary coil.

Question 35

What is the equation linking the no. of turns in each coil to the applied voltage and induced voltage?

Answer 35

No. of primary coils/ no. of secondary coils = primary coil voltage/ secondary coil voltage = secondary coil current/ primary coil current

Question 36

What is the relationship between the electrical power input and electrical power output?

Answer 36

Electrical Power Input = Electrical Power Output

Ip Vp = Is Vs

Question 37

What are the standard values for voltage in power stations, transmission lines and homes?

Answer 37

Power stations- 25 000V
Transmission lines- 275 000V
Homes- 230V

Question 38

How is energy lost as waste in cables?

Answer 38

The current must flow through many cables and in doing so causes these cables to heat up. This means that some of the electrical energy to be transmitted gets wasted as heat. The greater the current to be transmitted, the greater the heat produced and the greater the heat produced and the greater the amount of energy wasted.

Question 39

How is the energy loss reduced in overhead cables?

Answer 39

In order to reduce this energy loss in the overhead cables, transformers are used. A step-up transformer is used at the power station and a step-down transformer is being used after the transmission lines and before the electricity enters our homes.

When current flows in a wire, some of the energy it is carrying is lost as heat energy because of the wire’s high resistance and the wires get hot. A smaller current wastes less energy than a high current.