SP12: magnetism and the motor effect

Question 1

where is a magnet’s force the strongest?

Answer 1

near the magnets pole

Question 2

rules of magnetism

Answer 2

opposites attract, likes repel

Question 3

what type of force is a magnetic force?

Answer 3

a non-contact force

Question 4

magnetic material examples

Answer 4

iron, steel, nickel and cobalt

Question 5

what does it mean if something is magnetic?

Answer 5

able to be magnetised/attracted to a magnet

Question 6

what is a permanent magnet?

Answer 6

-magnet made from a magnetic material
-its magnetism cannot be turned on or off

Question 7

key features of a magnetic material:

Answer 7

-it produces its own magnetic field
-the magnetic field cannot be turned on and off – it is there all the time

Question 8

examples of permanent magnets:

Answer 8

bar magnets and horseshoe magnets

Question 9

what are induced magnets?

Answer 9

-a temporary magnet
-made from a magnetic material placed in a magnetic field
-the induced magnetism is lost when moved out of the magnetic field

Question 10

examples of induced magnets:

Answer 10

iron filings

Question 11

how do you test for magnetism?

Answer 11

a permanent magnet can:
-attract or repel another permanent magnet
-attract a magnetic material (but not repel it)

this means that you can only show that an object is a permanent magnet by checking if it repels another magnet

Question 12

magnetic field

Answer 12

area around a magnet where it’s force can act on another magnet or magnetic material

Question 13

can a magnetic field be seen?

Answer 13

no, it is invisible

Question 14

how can you detect a magnetic field?

Answer 14

-using a magnetic compass
-a compass contains a small bar magnet on a pivot so that it can rotate
-the compass needle points in the direction of the earth’s magnetic field, or the magnetic field of a magnet

Question 15

how to map out a magnetic field using a plotting compass:

Answer 15

1. place the plotting compass near the magnet on a piece of paper
2. mark the direction the compass needle points
3. move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
4. join the points to show the field lines

Question 16

what produces the earth’s magnetic field?

Answer 16

the earth’s core, which is made from iron and nickel

Question 17

direction of the magnetic field of a bar magnet:

Answer 17

from the north pole of the magnet to its south pole

Question 18

direction of the earth’s magnetic field

Answer 18

from south to north

Question 19

what happens when current flows in a wire?

Answer 19

it creates a circular magnetic field around the wire

Question 20

what can the magnetic field created by a wire do?

Answer 20

-deflect the needle of a magnetic compass

Question 21

where is the strength of a magnetic field created by a wire the strongest?

Answer 21

closer to the wire, it increases if the current increases

Question 22

right hand rule

Answer 22

-the direction of the current and magnetic field can be found using the right hand grip rule
-coil the fingers of the right hand as if holding the handlebars of a bicycle, with the thumb pointing away from the hand
-the thumb indicates the direction of the current, and the fingers then indicate the direction of the magnetic field

Question 23

what is a solenoid?

Answer 23

a long, thin coil of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it (the shape of the magnetic field is very similar to the field of a bar magnet)

Question 24

what is the field inside of a solenoid like?

Answer 24

-strong and uniform
-the small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field

Question 25

what is the field outside a solenoid like?

Answer 25

-the small magnetic fields from each wire cancel each other out and the outside field is much weaker than inside

Question 26

the motor effect

Answer 26

-a wire carrying a current creates a magnetic field
-this can interact with another magnetic field, causing a force that pushes the wire at right angles

-there is no motor effect force if the current and magnetic field are parallel to each other

Question 27

equation for the motor effect force

Answer 27

force = magnetic flux density × current × length

Question 28

units for the equation for the motor effect force

Answer 28

-force in newtons (N)
-magnetic flux density in tesla (T)
-current in amperes/amps (A)
-length in metres (m)

Question 29

2A flows through a 50 cm wire. calculate the force acting on the wire when it is placed at right angles in a 0.4 T magnetic field

Answer 29

F = BIl
F = 0.4 x 2 x 0.5
F = 0.4N

Question 30

when does the force on a wire in a magnetic field increase?

Answer 30

-when the current in the wire increases
-when the strength of the magnetic field increases

Question 31

when is the force on a wire in a magnetic field the strongest?

Answer 31

when the direction of the current is 90° to the direction of the magnetic field

Question 32

fleming’s left hand rule

Answer 32

hold your thumb, forefinger and second finger at right angles to each other:

-the thumb shows the direction of the motor effect force on the conductor carrying the current (F)

-the pointer finger is lined up with magnetic field lines pointing from north to south (B)

the middle finger is lined up with the current pointing from positive to negative (I)

Question 33

electric motor (detailed explanation using cognito video)

Answer 33

-a wire/coil with current entering at the positive terminal and exiting at the negative terminal
-one side of the coil experiences an upwards force while another experiences downwards force, this causes the coil to spin (either clockwise or anti-clockwise)

-when the coil has spun 180 degrees, the current is going in the opposite direction, so the forces also swap direction
-the coil then spins in the opposite direction to how it originally spun and goes back to where it stated
-to fix the fact that the coil can never turn 360 degrees, the direction of the current must be changed every half turn, so that the coil can continue to rotate in the same direction

-a split ring commutator is used to do this (it swaps the negative and positive connections every half turn, so that the direction of the current also swaps every half turn, this means the forces acting on the coil will always act in the same direction)

Question 34

how can the speed of an electric motor be increased?

Answer 34

-increase the current that is passing through the wire
-add more turns to the coil