Magnetism and Electromagnetism

Question 1

What is a magnetic field

Answer 1

A region where other magnets or magnetic materials experience a force

Question 2

Give three magnetic materials

Answer 2

Iron, steel, nickel and cobalt

Question 3

In what direction do magnetic field lines point

Answer 3

From north to south to show which way a force would act on a north pole if it was put in the field

Question 4

Describe how you could use a compass to show the direction of a bar magnet’s magnetic field line

Answer 4

Inside the compass there is a tiny bar magnet. The north pole of the magnet is attracted to the south pole of any other magnet. The compass points in the direction the field lines are in. So put the compass near a magnet and it points along the field lines

Question 5

Describe the behaviour of a compass that is far away from a magnet

Answer 5

Compasses point north because the Earth generates its own magnetic field showing its core must be magnetic

Question 6

What is the force between a magnet and a magnetic material

Answer 6

Always attractive no matter its pole

Question 7

What happens to an induced magnet when it is moved far away from a permanent magnet

Answer 7

the induced magnets quickly lose their magnetism (or most of it) and stop producing a magnetic field

Question 8

Describe the magnetic field around a current carrying wire

Answer 8

It is made up of concentric circles perpendicular to the wire with the wire in the centre.

Question 9

Why does adding more turns to a solenoid increase the strength of its magnetic field

Answer 9

The field lines around each loop of wire line up with each other. This results in lots of field lines pointing in the same direction that are very close together. Therefore adding more the magnetic field will strengthen because the closer the field lines are the stronger the field is.

Question 10

Describe an electromagnet and give one example of where it could be used

Answer 10

Electromagnets are solenoids with an iron core (becomes induced magnet), because if you stop the current the magnetic field is turned off. They can be used in cranes to pick up things made from magnetic materials, can act like switches and are used in loudspeakers

Question 11

Explain why a current carrying conductor in a magnetic field experiences a force

Answer 11

The magnetic field of the wire interacts with the magnetic field it has been placed in, causing the magnet and the conductor to exert a force on each other

Question 12

State the equation for calculating the size of this force (at 90 degrees to magnetic field)

Answer 12

magnetic field flux density-how many field lines there are in a region
Force(N)=Magnetic flux density(T)xCurrent(A)xlength(m)

Question 13

Name three ways you could increase the force on a current carrying wire in a magnetic field

Answer 13

Increasing the strength of the magnetic field, increasing the amount of current passing through the conductor, putting the wire perpendicular to magnetic field, increasing length of the conductor that in the field

Question 14

What is Fleming’s left hand rule

Answer 14

• point First finger in direction of Field
• point your seCond finger in the direction of the Current
• your thuMb will then point in the direction of the force (Motion)

Question 15

Explain how a basic dc motor works

Answer 15

Forces act on the two side arms of a coil of wire thats carrying a current. Because the coil is on a spoindleand the forces act on up and one down it rotates. The split ring commutator is a clever way of swapping the contacts every half turn to keep the motor rotating in the same direction.

Question 16

Describe how a loudspeaker works

Answer 16

An alternating current is sent through a coil of wire attached to the base of a paper cone. The coil surrounds one pole of a permanent magnet and is surrounded by the other pole so the current causes a force on the coil causing the cone to move. When the current reverses the force reverses causing the cone to move in the opposite direction. Variations in the current make the cone vibrate making the air vibrate creating sound waves. The frequency of the sound wave is the same as the frequency of the ac.

Question 17

Describe how you can induce a current

Answer 17

Cutting magnetic field lines induces a potential difference and a current if there’s a complete circuit. You can do this by moving a magnet in a coil of wire or moving a conductor in a magnetic field

Question 18

Give two ways you could reverse the direction of an induced current

Answer 18

If you move the magnet or conductor in the opposite direction then the potential difference/current will be reversed. If the polarity of the magnet is reversed the potential difference/current will reverse too

Question 19

Do induced currents create magnetic fields that oppose the change that made them

Answer 19

Yes, the magnetic field created by an induced current always acts against the change that made it (trying to returns things to the way they were)

Question 20

Give two ways that you can increase the size of an induced potential difference

Answer 20

Increasing the speed of the movement-cutting more magnetic field lines in a given time
Increasing the strength of the magnetic field-more field lines can be cut.

Question 21

Which type of generator uses slip rings and brushes

Answer 21

Alternators, so they don’t swap every half turn and change direction and produce an alternating potential difference

Question 22

What kind of current do dynamos produce

Answer 22

Direct current

Question 23

Explain how microphones translate sound waves into electrical signals

Answer 23

Sound waves hit a flexible diaphragm that is attached to a coil of wire wrapped around in a magnet like in a speaker. This causes the coil of wire to move in the magnetic field and generate a current. The movement of the coil depends on the properties of the sound wave.

Question 24

What kind of current are transformers used with

Answer 24

Alternating current only

Question 25

Why do transformers have a core of iron

Answer 25

Because iron magnetises and demagnetises quickly

Question 26

Which side has more coils on a step down transformer

Answer 26

The primary coil

Question 27

A transformer has an input pd of 100V and an output pd of 20V what kind of transformer is it

Answer 27

Step down

Question 28

State the transformer equation

Answer 28

Input pd/output pd=number of turns on primary coil/number of turns on secondary coil

Question 29

Write down the equation that relates the input and output currents and pds of transformers and what does this equation assume

Answer 29

secondary pd x secondary current = primary pd x primary current
It assumes they are 100% efficient and no energy is lost

Question 30

Explain how transformers are used to improve efficiency when transmitting electricity

Answer 30

They are used to transmit electricity at a low current by increasing the voltage so less energy is lost by the heating of the wires

Question 31

Explain how transformers work

Answer 31

The alternating current in the primary coil makes the iron core into an electromagnet. As the current is alternating, the magnetic field also ‘moves’ and changes direction.
The moving magnetic field causes a current to be induced in the secondary coil. If the number of turns is higher on the secondary coil the pd will increase.