P13 revision

Question 1

Magnetic field

Answer 1

A region where other magnets or magnetic materials experience a non-contact force

Question 2

How do magnetic field lines go?

Answer 2

Go from north to south

Question 3

What does it mean the closer together the magnetic field lines are?

Answer 3

The closer together the lines are, the stronger the magnetic field. The further away from a magnet you get, the weaker the field is

Question 4

Like poles

Answer 4

Two poles that are the same

Question 5

Unlike poles

Answer 5

Two poles that aren’t the same

Question 6

What will two like poles do to each other?

Answer 6

They’ll repel each other

Question 7

What will two unlike poles do?

Answer 7

Attract each other

Question 8

How attractive is the force between a magnet and a magnetic material?

Answer 8

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

Question 9

Magnetic field in a bar magnet?

Answer 9

The north pole of the tiny bar magnet inside a compass is attracted to the south pope of any other magnet it’s near, so the compass points in the direction of the magnetic field it’s in

Question 10

Where do compasses point when they’re not near a magnet?

Answer 10

When they’re not near a magnet, compasses always point north. This is because the Earth generates its own magnetic field, whoch shows the inside(core) of the Earth must be magnetic.

Question 11

Permanent magnets

Answer 11

Produce their own magnetic field

Question 12

Induced magnets

Answer 12

Magnetic materials that turn into a magnet when they’re put into a magnetic field.

Question 13

What is the force between permanent and induced magnets?

Answer 13

The force between permanent and induced magnets is always attractive.

Question 14

What happens when you take away the magnetic field of induced magnets?

Answer 14

When you take away the magnetic field, induced magnets quickly lose their magnetism and stop producing a magnetic field.

Question 15

What happens when a current flows through a wire?

Answer 15

A magnetic field is created around the wire. The field is made up of concentric circles perpendicular to the aire, with the wire in the centre. You can see this by placing a compass near the wire that’s carrying a current. As you move the compass, it will trace the direction of the magnetic field.

Question 16

Right-hand thumb rule

Answer 16

Changing the direction of the current changes the direction of the magnetic field, with the right-hand thumb rule used to work out which way it goes.
Right-Hand Thumb rule: Using your right hand, point your thumb in the direction of current and curl your fingers. The direction of your fingers is the direction of the field.

Question 17

When a moving charge creates a magnetic field, what affects the strength of the magnetic field?

Answer 17

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

Question 18

Solenoid

Answer 18

A coil used to wrap the wire and increase the strength of the magnetic field a wire produces.

Question 19

How does a solenoid increase the strength of the magnetic field?

Answer 19

This results in lots of field lines pointing in the same direction that are very close to each other.

Question 20

How does a magnetic field work in a solenoid?

Answer 20

The magnetic field inside a solenoid is strong and uniform(it has the same strength and direction at every point in that region). Outside the coil, the magnetic field is like the one round a bar magnet, meaning the ends of a solenoid act like the north pole and south pole of a bar magnet. You can work out which end of the solenoid is the north pole and which is the south pole using the right-hand rule shown above.

Question 21

How to increase the field strength of the solenoid even more?

Answer 21

By putting a block of iron in the centre of the coil. The iron core becomes an induced magnet whenever current’s flowing. However, if you stop the current, the magnetic field disappears.

Question 22

Electromagnet

Answer 22

A solenoid with an iron core(a magnet whose magnetic field can be turned on and off with an electric current)

Question 23

Motor effect

Answer 23

When a current-carrying wire is put between magnetic poles, the magnetic field around the wire interacts with the magnetic field it’s been placed on, causing the magnet and conductor to exert a force on each other, which causes the wire to move.

Question 24

In the motor effect, how can a current in a magnetic field experience a force?

Answer 24

1) To experience the full force, the wire has to be 90 degrees to the magnetic field. If the wire runs parallel to the magnetic field, it won’t experience any force at all. At angles in between, it’ll feel some force.
2) The force always acts at right angles to the magnetic field of the magnets and the direction of the current in the wire.
3) One wah to show the direction of the force is to apply a current to a set of rails inside a horseshoe magnet. A bar is placed on the rails, which completes the circuit. This generated a force rolling the bar along the rails.

Question 25

In a motor effect, what affects the magnitude of the force?

Answer 25

The magnitude of the force increases with the strength of the magnetic field and the amount of current passing through the conductor.

Question 26

What affects the force acting on a conductor in a magnetic field?

Answer 26

Magnetic flux density
Size of current through conductor
Length of conductor in magnetic field

Question 27

Magnetic flux density

Answer 27

How many field(flux) lines are in the region, showing the strength of the magnetic field.

Question 28

How to find the direction of the force exerted on any current-carrying conductor in a magnetic field.

Answer 28

Can use Fleming’s left-hand rule:
1) Using your left hand, point your first finger in the direction of the magnetic field.
2) Point your second finger in the direction of the current.
3) Your thumb will then point in the direction of the force.
Fleming’s left-hand rule shows if either the current or the magnetic field is reversed, the direction of the force will also be reversed.

Question 29

How to find the force acting on a conductor in a magnetic field:

Answer 29

When the current’s at 90 degrees to the magnetic field it’s in, the force acting on it can be found using the equation:
Force = Magnetic flux density x current x length

Question 30

How is the rotation of a current-carrying coil of wire impacted in a magnetic field?

Answer 30

1) In a basic dc motor, forces act on the two side arms of a coil of wire that’s carrying a current.
2) These forces are just the usual forces which act on any current in a magnetic field.
3) Because the coil’s on a spindle and the forces act one up and one down, it rotates.
4) The split-ring commutator swaps the contacts every half turn to keep the motor rotating in the same direction.
5) The direction of the motor can be reversed either by swapping the polarity of the dc supply(reversing the current) or swapping the magnetic poles over(reversing the field).
6) The speed of the motor can be increased by increasing the current, adding more turns to the coil or increasing the magnetic flux density.

Question 31

How to use Fleming’s left hand rule to work out which way a current carrying coil of wire will turn in a magnetic field?

Answer 31

1) Draw in current arrows(from positive to negative).
2) Use Fleming’s left-hand rule on one branch.
3) Point your first finger in the direction of the magnetic field(north to south).
4) Point yoursecond finger in the direction of the current.
5) Draw in the direction of motion(the direction your thumb is pointing in).

Question 32

Magnetic field strength=

Answer 32

Force(N)/Current x length(m)

Question 33

What objects can be magnetised and demagnetised?

Answer 33

Iron
Steel
Cobalt
Nickel

Question 34

What are permanent magnets made of?

Answer 34

Steel, magnetised steel doesn’t lose its magnetism easily

Question 35

Where are magnetic field lines most concentrated?

Answer 35

At the poles - the field is strongest here

Question 36

1uT=

Answer 36

10-6T

Question 37

1mT=

Answer 37

10-3T