P7

Question 1

Where on a magnet are the magnetic forces the strongest?

Answer 1

The poles.

Question 2

What happens when two magnets are brought close together?

Answer 2

They exert a force on each other (non-contact) and either attract or repel.

Question 3

What does a permanent magnet always produce?

Answer 3

Its own magnetic field.

Question 4

What is an induced magnet?

Answer 4

A material that becomes a magnet when it is placed in a magnetic field. It always causes a force of attraction. When removed from the magnetic field; it loses most / all its magnetism quickly.

Question 5

What is a magnetic field?

Answer 5

The region around a magnet where a force acts on another magnet or on a magnetic material.

Question 6

What are 4 examples of magnetic materials?

Answer 6

Iron. Steel. Cobalt. Nickel

Question 7

What is the force between a magnet and a magnetic material?

Answer 7

Always attraction.

Question 8

What does the strength of the magnetic field depend on?

Answer 8

The distance from the magnet.

Question 9

What is the direction of the force of the magnetic field?

Answer 9

From north (seeking) to south (seeking).

Question 10

How does a compass work?

Answer 10

A magnetic compass contains a small bar magnet. The compass points towards the Earth’s magnetic field.

Question 11

With magnetic field line what does it mean if the lines are closer together?

Answer 11

The magnetic field is stronger.

Question 12

What does “magnetic flux density” mean?

Answer 12

Strength of a magnetic field.

Question 13

What is the unit for magnetic flux density?

Answer 13

Tesla (T).

Question 14

Describe how to plot the magnetic field pattern of a magnet using a compass.

Answer 14

Move a compass around a magnet and plot a point where north is pointing. Draw the path up - it should be going north to south.

Question 15

What is created when a current flows through a wire?

Answer 15

A magnetic field around the wire.

Question 16

What does the strength of the magnetic field (around the wire) depend on?

Answer 16

The strength of the current flowing through the wire and the distance from the wire.

Question 17

When a current flows through a wire, a magnetic field is created around it. What is the shape of the field?

Answer 17

The field is made of concentric circles perpendicular to the wire (with the wire in the centre).

Question 18

What is a solenoid and what does it do?

Answer 18

A coil of current-carrying wire that produces a magnetic field. It increases the strength of the magnetic field - you can also add an iron core.

Question 19

What is an electromagnet?

Answer 19

A solenoid with an iron core.

Question 20

When a current flows through a wire, a magnetic field of concentric circles - perpendicular to the wire - is created around it. How could you find out the direction of the field?

Answer 20

Use the right-hand rule: point your thumb in the direction the current flows. Curl your fingers - the direction they curl is the direction of the magnetic field.

Question 21

Changing the direction of the current flowing through a magnetic field also changes what?

Answer 21

The direction of the magnetic field around it.

Question 22

Describe the shape of the magnetic field around a solenoid.

Answer 22

The magnetic field outside the core of a solenoid has a similar shape to that of a bar magnet. Inside the core, the field is strong and uniform.

Question 23

How are electromagnets used in scrap yards?

Answer 23

They are used in cranes and are turned on to pick up and move objects made from magnetic materials (iron and steel etc) and turned off to drop the object.

Question 24

What is the motor effect?

Answer 24

A current-carrying wire in a magnetic field (and the magnet producing it) will experience a force perpendicular to both the direction of the current and the magnetic field. This is because current-carrying wires produce a magnetic field; making them by definition magnets; and all magnets experience a force when in a magnetic field.

Question 25

How would you use Flemming's left hand rule?

Answer 25

1. The thumb is movement of force. 2. Second finger is magnetic field from north to south. 3. Third finger is current from positive to negative.

Question 26

At what angle to the direction of a magnetic field does a current-carrying wire have to be in order to experience the full force resulting from the motor effect?

Answer 26

90º. For angles in between 0º and 90º the wire experiences some force and for 0º, it experiences none.

Question 27

How would you increase the magnitude of a force resulting from the motor effect? (3)

Answer 27

Increasing the magnetic flux density of the magnetic field that the wire is in. Increasing the current passing through the wire. The length of the conductor (wire).

Question 28

What is the equation for force (in which the conductor is at a right angle to the magnetic field and carries a current)?

Answer 28

force (N) = magnetic flux density (T) x current (A) x length (m)

Question 29

How does an electric motor work?

Answer 29

1. Current in the left-hand side of the coil causes a downward force and current in the right hand side of the coil causes an upward force due to left hand rule. 2. The coil rotates anticlockwise because of the forces are in opposite directions. 3. Each side of the coil is now near the opposite magnetic pole.

Question 30

How can the direction of the rotation be reversed in an electric motor? (2)

Answer 30

Reverse the direction of the current OR Reverse the direction of the magnetic field (changing over the north and south poles).

Question 31

How can the speed of the rotation be increased in an electric motor? (4)

Answer 31

Increasing the size of the current. Using a stronger magnet. Increasing the number of turns of wire in the coil. Reducing friction between the coil and the axel it rotates on.

Question 32

Explain how loudspeakers/headphones (which are just tiny loudspeakers) work.

Answer 32

1. The coil around the cone (and permanent magnet) in the speaker is attached to a circuit to allow an alternating current to flow through. 2. As the current passes through; it generates a magnetic field. 3. This magnetic field interacts with the magnetic field from the permanent magnet. 4. They attract or repel which creates a resultant force making the cone move. 5. When the current switches direction; the direction of the force changes which generates sound waves.

Question 33

What happens if you increase the frequency in loudspeakers or headphones?

Answer 33

Higher pitch.

Question 34

What happens if you increase the size of the current in loudspeakers or headphones?

Answer 34

You increase the amplitude and therefore the loudness.

Question 35

What is the generator effect?

Answer 35

If a conductor and magnetic field move in relation to each other, a potential difference is induced in the conductor. If the conductor is part of a complete circuit, a current is also induced.

Question 36

In what 2 ways could you induce a pd (by the generator effect)?

Answer 36

Moving a conductor in a magnetic field. Moving a magnet in a coil of wire.

Question 37

How would you increase the size of a pd (or current) induced by the generator effect?

Answer 37

You would need to change the rate at which the magnetic field is changing, by: Increasing the speed of movement (of the magnet or conductor). Increasing the magnetic flux density.

Question 38

How could you generate an alternating current with a magnet?

Answer 38

You need a complete circuit. Either: Move the magnet backwards and forwards inside a coil of wire. Rotate the magnet inside a coil of wire. Continually reverse the polarity of the magnet.

Question 39

How could you generate an alternating current by turning a magnet in a coil of wire?

Answer 39

As the magnet turns, the magnetic field through the coil changes inducing a pd (generator effect), so a current flows through the wire. Each time the magnet turns 180º, the direction of the magnetic field reverses so the current in the coil reverses direction. As the magnet keeps rotating an ac is produced.

Question 40

Why does an induced current always oppose the change that makes it?

Answer 40

The generator effect means a change in magnetic field generates a current in a wire. And, when a current flows through a wire, another magnetic field is generated around the wire. This latter magnetic field acts against the change that produced it - e.g. the movement of the wire / change in magnetic field.

Question 41

Explain how microphones convert sound waves into electrical current.

Answer 41

Basically loudspeakers in reverse: Sound waves hit the diaphragm, causing the coil to move. The coil is in the magnetic field - so a current is generated. The size of the current increases with the volume of the sound wave. So microphones convert the pressure variations of sound waves into electrical current.

Question 42

Alternators generate __ current. Dynamos generate __ current.

Answer 42

Alternating. Direct.

Question 43

How do alternators generate alternating current?

Answer 43

1. Generators either rotate a coil in a magnetic field or a magnet in a coil. 2. As the coil/magnet spins a current is induced in the coil which reverses direction every half turn. 3. They have slip-rings and brushes which provide a continuous connection between the coil and ac meter. 4. This means an alternating pd (and current) is produced.

Question 44

How do dynamos generate direct current?

Answer 44

Dynamos either rotate a coil in a magnetic field or a magnet in a coil. As the coil/magnet spins a current is induced in the coil which reverses direction every half turn. They have split-ring commutators (rather than slip-rings and brushes) swapping the connection every half turn. This keeps the current flowing in the same direction - i.e. a direct current.

Question 45

What is the function of transformers?

Answer 45

To change the magnitude of the pd of an alternating current.

Question 46

What do step up and step down transformers do?

Answer 46

Step up transformers increase the pd. Step down transformers decrease the pd.

Question 47

Describe how step-up transformers increase the pd.

Answer 47

Two coils of wire are joined with an iron core. When an alternating current/pd is applied across the primary coil, the iron core alternately magnetises and demagnetises. The changing magnetic field induces an alternating pd in the secondary coil - which is part of a complete circuit - so an ac is induced. Step-up transformers have more turns on the secondary coil than on the primary meaning the pd is increased.

Question 48

Explain how step-down transformers decrease the pd.

Answer 48

Two coils of wire are joined with an iron core. When an alternating current/pd is applied across the primary coil the iron core alternately magnetises and demagnetises. The changing magnetic field induces an alternating pd in the secondary coil - which is part of a complete circuit - so an ac is induced. Step-up transformers have more turns on the secondary coil than on the primary meaning the pd is increased. The changing magnetic field induces an alternating pd in the secondary coil - which is part of a complete circuit - so an ac is induced. Step-down transformers have fewer turns on the secondary coil than on the primary meaning the pd is decreased.

Question 49

Why are iron cores used in electromagnets and transformers?

Answer 49

Because iron is easily magnetised.

Question 50

What can this (given) equation be used to find?

Answer 50

Input pd, Output pd, # turns on primary coil, # turns on secondary coil

Question 51

In transformers, the ratio between the primary and secondary __ is equal to the ratio between the __.

Answer 51

In transformers, the ratio between the primary and secondary potential differences is equal to the ratio between the number of turns on the primary and secondary coils.