P7 - Magnetism And Electromagnetism

Question 1

Where are the magnetic forces the strongest in a bar magnet?

Answer 1

The magnetic forces are the strongest at the poles of the magnet

Question 2

Describe the attraction and repulsion between unlike and like pole for permanent magnets

Answer 2

Two magnets are brought together and they exert a force on each other

• Two LIKE poles repel each other
• Two UNLIKE poles attract each other

Question 3

Attraction and repulsion between two magnetic poles are examples of what type of force?

Answer 3

Non-contact force

Question 4

What is the difference between permanent and induced magnets?

Answer 4

A permanent magnet (like a bar magnet) produces its own magnetic field, whereas an induced magnet is a material that becomes a magnet when it is placed in a magnetic field

Question 5

Explain when an induced magnet loses most of its magnetism

Answer 5

Induced magnetism always causes a force of attraction
When removed from the M.F. an induced magnet loses most/all of its magnetism quickly because it is no longer placed in a M.F.

-> This means it can be used for picking up and dropping metallic materials

Question 6

What are the magnetic materials?

Answer 6

Iron, steel, cobalt and nickel

Question 7

What is the magnetic field?

Answer 7

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

Question 8

The force between a magnet and a magnetic material is always one of …

Answer 8

Attraction

Question 9

What does the strength of a magnetic field depend on?

Answer 9

Distance from the magnet (strongest at the poles)

Question 10

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

Answer 10

Place the compass near the North Pole of the bar magnet
Mark a cross at the North Pole of the compass
Move the compass so the South Pole of the compass is on the cross
Again, mark another cross at the North Pole of the compass
Continue until a complete magnetic field line is plotted
Connect all the dots with a circular line and the direction of the field line is shown with an arrow
Repeat this starting at different points around the North Pole of the bar magnet

Question 11

What is the direction of the magnetic field at any point given by?

Answer 11

Direction of the force that would act on another North Pole placed at that point

Question 12

Explain how the behaviour of a magnetic compass is related to evidence that the core of the Earth must be magnetic

Answer 12

The compass contains a small bar magnet
if we hold the compass away from any magnets, the compass needle points in the direction of the Earth’s M.F.
This means the earth has a magnetic field which is due to the earth’s core

Question 13

When a current flows through a _____ wire, a ________ _______ is produced around the wire

Answer 13

Conducting
Magnetic field

Question 14

What does the strength of the magnetic field through a conducting wire depend on?

Answer 14

Current through the wire
Distance from the wire

Question 15

Explain how a solenoid arrangement can increase the magnetic effect of the current

Answer 15

Shaping a wire to form a solenoid increases the strength of the MF created by a current through a conducting wire

The MF inside a solenoid is strong and uniform

Can be increased by:
- increase current as increases strength of MF
- increase turns of the coil
- add an iron core as increases strength of MF

Question 16

Describe how to use Fleming’s left-hand rule

Question 17

What is an electromagnet?

Answer 17

A solenoid containing an iron core

Question 18

Describe the motor effect

Answer 18

When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor interact and exert a force on each other

Question 19

What are the factors that affect the size of the force on the conductor?

Answer 19

Increase current
Use stronger magnet
Greatest when the wire is perpendicular to the magnetic field; zero when the wire is parallel to the MF lines

Question 20

What are the SI units for the equation: F = BIL

Answer 20

F (force) in newtons, N
B (magnetic flux density) in tesla, T -> measure of strength of MF
I (current) in amperes, A
L (length) in metres, m

Question 21

Explain why the coil of a simple electric motor rotates continuously when the motor is connected to a battery

Answer 21

A force acts on each side of the coil due to the motor effect
The force on one side is in the opposite direction to the force on the other side
The split-ring commutator reverses the current around the coil every half turn of the coil. As the sides swap over, the coil is pushed in the same direction every half-turn

Question 22

How do loudspeakers and headphones use the motor effect?

Answer 22

Convert variations in current in electrical circuits to the pressure variations in sound waves

Question 23

Explain how a moving-coil loudspeaker/headphones works

Answer 23

Current passes through the coil and generates a MF
MF from coil interacts with the MF from permanent magnet
This produces a resultant force that causes the cone to move
When the AC current switches direction, the direction of the force on the cone reverses
Causing the cone to move in and out, generating sound waves

Question 24

How can the frequency and amplitude of a loudspeaker/microphone be changed?

Answer 24

Changing frequency of AC supply will change frequency of cone vibration. Higher frequency = higher pitch sound

Increasing size of current increases amplitude of the vibration. This increases the volume of the sound

Question 25

Describe the generator effect

Answer 25

If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If it is part of a complete circuit, a current is induced in the conductor

[Generator effect is the effect of inducing a pd using a magnetic field]

Question 26

What are the factors that affect the size of the induced pd/induced current

Answer 26

Use a stronger magnetic field from a stronger magnet
Move the conducting wire more quickly
Shape the wire into a coil for greater turns

Question 27

What are the factors that affect the direction of the induced potential/induced current?

Question 28

An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in the magnetic field

Question 29

What is a use of the generator effect?

Answer 29

Alternator to generate an ac
Dynamo to generate a dc