Topic 12 - Magnetism and the Motor Effect

Question 1

SP12a - What is a permanent magnet?

Answer 1

A magnet that always has a magnetic field

Question 2

SP12a - What is a temporary magnet?

Answer 2

A magnet material that only has a magnetic field when in the field of another magnet.

Question 3

SP12a -When a temporary magnet enter a magnetic field and becomes magnetised, what can you say it is?

Answer 3

It is now an induced magnet

Question 4

SP12a - What are the four magnetic materials?

Answer 4

• Iron
• Steel
• Nickel
• Cobalt

Question 5

SP12a - What piece of equipment can be used to find the magnetic field of a bar magnet?

Answer 5

A plotting compass

Question 6

SP12a - What do two opposite parallel magnetic plates create?

Answer 6

A uniform parallel magntic field

Question 7

SP12a - What do the field lines aorund a bar magnet look?

Answer 7

• They are going form north to south, increasingly spaced out the further away from the magnet.
• They go in curves around the outside

Question 8

SP12a - How do we know that the earth has a magnetic core?

Answer 8

• It has field lines similar to that of a bar magnet around it.
• We can see this through compasses.
• It is a core made of nickel and iron.

Question 9

SP12a - What is the key thing to remember about the earth’s poles?

Answer 9

Its geographic north pole is its magnetic south pole and vice versa

Question 10

SP12b - What does the direction of the magnetic field around a wire depend on?

Answer 10

The direction of the current

Question 11

SP12b - What can we use to figure out the direction of the current and how can this be done?

Answer 11

• Curl your right hand into a fist and stick out your thumb.
• Point your thumb in the direction of the current and the direction your finger curl aorund is the direction of the magnetic field?

Question 12

SP12b - When a magnetic field is shown what does a cross or a dot represent?

Answer 12

• Cross means that the current is haeding away from you
• Dot means the current is going towards you

Question 13

SP12b - What is he shape of a magnetic field around a wire?

Answer 13

Circular

Question 14

SP12b - When you coil a wire, what is it called?

Answer 14

A solenoid

Question 15

SP12b - The magnetic field of a solenoid is similar to that of _______

Answer 15

A bar magnet

Question 16

SP12b - What is the magnetic fieldat the centre of the solenoid similar to?

Answer 16

A uniform parallel field

Question 17

SP12b - What do you call a solenoid with a current flowing thorugh it?

Answer 17

An electromagnet

Question 18

SP12b - What are the ways you can make an electromagnet stronger?

Answer 18

• Adding an iron core (a temporary magnet)
• Increasing the current

Question 19

SP12b - What is the strength of the magnetic field around a wire dependant on?

Answer 19

• The strength of the current (Stronger when higher)
• The distance from the wire (stronger when closer)

Question 20

SP12c - When does the motor effect occur?

Answer 20

When a wire carrying a current enters a uniform parallel magnetic field

Question 21

SP12c - Why does the motor effect occur and what does it produce?

Answer 21

The magnetic field from the wire acting against the magnetic field from the uniform parallel field produces a force that acts on the wire.

Question 22

SP12c - What can be used to work out the direction of the force in the motor effect?

Answer 22

The left hand rule:

• Set your hand so your thumb is up and your first and second fingers are at right angles.
• Your first finger represents the direction of the field.
• Your second finger represents the direction of the current.
• Once these are lined up, the way your thumb is pointing will be direction of the force

Question 23

SP12c - How do you calculate the force exerted on a wire in the motor effect.

Answer 23

F = B I L

Force (N) =

Magnetic field strength (T) x Current (A) x Length of wire inside the UP field (m)

Question 24

SP12c - What are the two units and the alternate name for magnetic field strength?

Answer 24

Magnetic flux density:

• T - Telsa
• N/Am - Newton per amp metre

Question 25

SP12c - How do motors spin?

Answer 25

• The motor effect is set up so that the wire returns back to where is started.
• This means there are two sections of the wire inside the UP field with current travelling in different directions in each.
• This means that the direction of force on one will be up and down on the other.
• The wires are centered around a pivot which allows these forces to cause a turning motion.
• This is the motor effect.

Question 26

SP12c - What is a split - ring commutator used for?

Answer 26

• Once the side of the motor’s wire that is moving upwards has made a half turn it will be on the other side but still experiencing an upward force.
• This would mean that the motor keeps spinning back and forth around that point without ever making a full turn.
• The SR commutator means that once it has made a half turn the current will be ‘reset’ and change direction so the wire that was previously having a current going forward will now have it going back.
• So the wire experiencing an upward force will now experience a downward force and vice versa.