CGP P7

Question 1

How can you show a magnetic field

Answer 1

By drawing a magnetic field line

Question 2

Which way do magnetic field lines go

Answer 2

. From North to South
(Left to Right)

Question 3

How do magnetic field lines demonstrate the strength of a magnetic field

Answer 3

. The closer the lines are, the stronger the magnetic field

Question 4

When are magnetic fields the strongest

Answer 4

. At the poles of the magnet
. This means the magnetic forces are also the strongest at the poles

. The further away from a magnet you get, the weaker the field is

Question 5

Forces between a magnet and magnetic material

Answer 5

. The force between a magnet and a magnetic material is always attractive no matter the pole

Question 6

Compasses

Answer 6

. Compasses contain tiny bar magnets. The north pole of this magnet is attracted to the south pole of any magnet it is close to
. So the compass points to the direction of the magnetic field it is in

Question 7

Where do compasses point to when they’re not near a magnetic field

Answer 7

. They always point north because the Earth generates it’s own magnetic field
. This means the Earth’s core must be magnetic

Question 8

What are the two types of magnets

Answer 8

. Permanent
. Induced

Question 9

. Permanent magnets

Answer 9

. Produce their own magnetic field

Question 10

. Induced magnets

Answer 10

. Induced magnets are magnetic materials that turn into a magnet when they’re put into a magnetic field
. When you take away the magnetic field, they lose their magnetism and stop producing a magnetic field

Question 11

Force between permanent and induced magnets

Answer 11

. The force between permanent and induced magnets is always attractive

Question 12

Current carrying wire

Answer 12

. When a current flows through a wire, a magnetic field is created around the wire

. The field is made up of concentric circles perpendicular to the wire in the center
(demonstrated by a compass)

Question 13

Changing the direction of current in
a carrying wire

Answer 13

. Changing the direction of a current changes the direction of the magnetic field
. You can use the right-hand thumb rule to work out the way it goes

Question 14

Right-hand thumb rule

Answer 14

. Point your thumb up and curl your fingers
. The thumb represents the direction of the current
. The fingers represent the direction of the field

Question 15

How does the strength of the magnetic field change (current-carrying wire)

Answer 15

. The strength of the magnetic field produced changes with the current and the distance from the wire

. The larger the current through the wire or closer to the wire you are, the stronger the field is

Question 16

. How can you increase the strength of the magnetic field of a current-carrying wire

Answer 16

. By wrapping the wire into a coil, called a solenoid
. This happens because the field lines around each loop line up with each other . Resulting in lots of field lines pointing in the same direction and being close to each other

Question 17

Magnetic field inside a solenoid

Answer 17

The magnetic field inside a solenoid is strong and uniform
(Has the same strength and direction at every point in that region)

Question 18

Magnetic field outside of a solenoid

Answer 18

Outside the coil the magnetic field is the same as a bar magnet

Question 19

How can we increase the field strength of a solenoid

Answer 19

. We can increase the field strength of a solenoid even more by putting a block of iron in the center of the coil.
. This iron core becomes an induced magnet whenever the current is flowing
. If you stop the current, the magnetic field disappears

Question 20

What is an electromagnet

Answer 20

A solenoid with an iron core

Question 21

Why are electromagnets useful

Answer 21

. You can switch the magnets on and off
. They’re usually used because they’re quick to turn on and off
. And they can create a varying force

Question 22

Where can electromagnets be used

Answer 22

. Cranes in scrap yards
. Within other circuits to act as switches

Question 23

Electromagnets in scrap yards

Answer 23

. Electromagnets are used in cranes to pick up stuff made from iron or steel
. Using an electromagnet means the magnet can be turned on to pick up stuff and turned off to drop stuff

Question 24

Electromagnets in circuits

Answer 24

. Electromagnets can be used to act as switches
. When the switch is closed, the electromagnet is turned on attracting the iron contact on the rocker
. The rocker pivots and closes the contacts completing circuit two and turning on the motor

Question 25

What is the motor effect

Answer 25

. When a current-carrying wire (or any conductor) is put between magnetic poles, the magnetic field around the wire interacts with the magnetic field it has been placed in.

. This causes the magnet and the conductor to exert a force on each other

. This is called the motor effect and can cause the wire to move

Question 26

What conditions need to be present for the wire to feel force

Answer 26

. To experience full force, the wire has to be at 90° to the magnetic field
. If the wire runs parallel, it won’t feel any force at all
. At angles in-between, it will feel some force

Question 27

How can we demonstrate the direction of the force of the motor effect

Answer 27

. We can do this by putting a set of rails inside a horseshoe magnet
. A bar is placed on the rails which completes the circuit
. This generates a force that rolls the bar along the rails

Question 28

How can we increase the force of the motor effect

Answer 28

. Increasing the strength of the magnetic field
. Increasing the current of the conductor

Question 29

What three things does the force acting on a conductor in a magnetic field depend on

Answer 29

. Magnetic flux density (how many field (flux) lines there are in a region, this also shows the strength of a magnetic field)
. The size of the current through the conductor
. The length of the conductor that’s in the magnetic field

Question 30

When the current is 90° to the magnetic field (motor effect), how can we calculate the force

Answer 30

. When it is only 90° to the magnetic field, we can use the equation :
F = BIL
F = Force (N)

B = Magnetic flux density (T)
I = Current (A)
L - Length (m)

Question 31

How can we find out which way a force is acting when a current-carrying wire is 90° to the magnetic field (motor effect)

Answer 31

By using Fleming’s left-hand rule, using your left hand:
. Point your pointer finger forwards, representing the direction of the field
. Point your middle finger left (90°), representing the direction of the current
. Stick your thumb upwards, representing the direction of the force

Question 32

What does Fleming’s left-hand rule show

Answer 32

Fleming’s left-hand rule shows that if either current or the magnetic field is reversed, the direction of the force will also be reversed

Question 33

Electric motors (D.C. motor)

Answer 33

. Forces act on the two side arms of the coil of wire that’s carrying a current
. These forces are just the usual which act on any current in a magnetic field
. Because the coil is on a spindle and the forces act in opposite directions, it rotates

. The split-ring commutator swaps the contacts at every half-turn to keep the motor rotating in the same direction

Question 34

How can the direction of the motor be reversed

Answer 34

. Swapping the polarity of the dc supply (reversing the current)
. Swapping the magnetic poles (reversing the field)

. We can use Fleming’s left-hand rule to work out which way the coil will turn

Question 35

Loudspeakers

Answer 35

. An alternating current is sent through the coil of wire attached to the base of the 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 acts in the opposite direction, causing the cone to move in the opposite direction

. Variations in the current make the cone vibrate, making the air around the cone vibrate creating variations in pressure that causes a sound wave
. The frequency of the sound wave is the same as the frequency of the ac, so by controlling the frequency of the ac, you can altar the sound waves produced

Question 36

How is electricity generated

Answer 36

Using the generator effect (electromagnetic induction)

Question 37

What is the generator effect

Answer 37

. The induction of a potential difference (and current if the circuit is complete) in a wire which is moving relative to a magnetic field, or experiencing change in a magnetic field

Question 38

How can we create the generator effect

Answer 38

. Moving a magnet in a coil of wire or moving a conductor in a magnetic field
. Shifting the magnet from side to side creates a little “blip” of current if the conductor is part of a complete circuit
. If you move the magnet (or conductor) in the opposite direction, then the potential difference/ current will be reversed (same if the polarity of the magnet is reversed)

. If you keep the magnet (or coil) moving backwards and forwards, you produce a potential difference that keeps swapping direction - an alternating current

Question 39

Reminder

Answer 39

. Page on the generator effect
. Generators and microphones
. Transformers