P7 - magnetism and electromagnetism (pg 227+1)

Question 1

how does a compass work?

Answer 1

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

Compasses always point north. This is because the Earth generates its own magnetic field, which shows that the core of the earth must be magnetic

Question 2

what does a moving charge create?

Answer 2

a magnetic field

Question 3

what happens when a current flows through a wire?

Answer 3

a magnetic field is created around the wire

Question 4

what is the magnetic field of a wire with current flowing through it made up of?

Answer 4

concentric circles (sharing the same centre) perpendicular to the wire, with the wire in the centre

Question 5

does changing the direction of the current change the direction of the magnetic field?

Answer 5

yes

Question 6

how do you work out the direction of a magnetic field from the direction of the current?

Answer 6

using the 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 7

what affects the strength of a magnetic field caused by a moving charge?

Answer 7

the current and the distance from the wire - the larger the current through the wire, or the closer to the wire you are, the stronger the field is.

Question 8

what is a solenoid?

Answer 8

a coil of wire

Question 9

how can you increase the strength of the magnetic field that a wire produces?

Answer 9

by wrapping the wire into a coil called a solenoid

Question 10

why is a solenoid stronger than a single wire?

Answer 10

the field lines around each loop of the wire line up with each other, which results in lots of field lines pointing in the same direction that are very close to each other. The closer together the field lines are, the stronger the field is

Question 11

what is the magnetic field inside a solenoid like?

Answer 11

strong and uniform (it has the same strength and direction at every point in that region)

Question 12

what is the magnetic field of a solenoid like outside the coil?

Answer 12

just like the one round a bar magnet

Question 13

what do the ends of a solenoid act like?

Answer 13

the north pole and south pole of a bar magnet

Question 14

how can you increase the field strength of a solenoid even more?

Answer 14

by putting a block of iron in the centre of the coil. this iron core becomes an induced magnet whenever current is flowing

Question 15

what happens if you stop the current in a solenoid?

Answer 15

the magnetic field disappears

Question 16

what is a solenoid with an iron core called?

Answer 16

an electromagnet

Question 17

what is special about an electromagnet?

Answer 17

its magnetic field can be turned on and off with an electric current

Question 18

when can the motor effect happen?

Answer 18

when you put a current-carrying wire in a magnetic firld

Question 19

what is the motor effect?

Answer 19

when a current-carrying wire (or any other 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 20

in order to get the full force of the motor effect, where does the wire have to be in relation to the magnetic field?

Answer 20

at 90 degrees to it

Question 21

how much force will a wire experience if it runs parallel to the magnetic field?

Answer 21

no force at all

Question 22

what two things does the magnitude of the force of the motor effect increase with?

Answer 22

1. the strength of the magnetic field

2. the amount of current passing through the conductor

Question 23

how can you find the size of the force of the motor effect when the current is at 90 degrees to the magnetic field it is in?

Answer 23

using the equation force (N) = Magnetic flux density (T,telsa) x current (A) x length (m)

Question 24

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

Answer 24

1. the magnetic flux density - how many field (flux) lines there are in a region. This shows the strength of the magnetic field
2. the size of the current through the conductor
3. the length of the conductor that’s in the magnetic field

Question 25

how can you find the direction of the force on a current-carrying wire in a magnetic field?

Answer 25

using Fleming’s left-hand rule

Question 26

what does your first finger represent in Fleming’s left-hand rule?

Answer 26

the magnetic field (F irst finger = magnetic F ield)

Question 27

what does your second finger represent in Fleming’s left-hand rule?

Answer 27

the direction of the current (seCond finger = Current)

Question 28

what does your thumb represent in Fleming’s left-hand rule?

Answer 28

the direction of the force (thuMb = Motion)

Question 29

what does Fleming’s left-hand rule show?

Answer 29

that if either the current or the magnetic field is reversed, then the direction of the force will also be reversed.

Question 30

show Fleming’s left-hand rule on your hand and name what all the fingers mean?

Answer 30

did you do it on your left hand?
first finger = magnetic field
second finger = current
thumb = force

Question 31

what happens to a current-carrying coil of wire in a magnetic field?

Answer 31

it rotates

Question 32

why does a current-carrying coil of wire rotate in a magnetic field?

Answer 32

the forces that act on it are just the usual forces which act on any current in a magnetic field. Because the coil is on a spindle and the forces act one up and one down, it rotates

Question 33

what is a split-ring communicator?

Answer 33

a clever way of swapping the contacts every half turn to keep the motor rotating in the same direction

Question 34

how can the direction of a basic dc motor be reversed?

Answer 34

either by swapping the polarity of the dc supply (reversing the current) or by swapping the magnetic poles over (reversing the firld)

Question 35

how can the speed of a basic dc motor be increased?

Answer 35

either by increasing the current, adding more turns to the coil or increasing the magnetic flux density

Question 36

what direction does current flow in?

Answer 36

from positive to negative