Topic 12- Magnetism and the Motor Effect

The Law of Magnetism

two like poles repel each other

two unlike poles attract each other

magnetic materials

iron

steel

cobalt

nickel

magnetic materials will always be ___ to magnets (regardless of north or south pole)

attracted

how to tests whether a material is a magnet

bring close to a known magnet

if it can be repelled than it is a magnet

if it can only be attracted it is a magnetic material

Permanent magnets

made out of permanent magnetic materials

produces its own magnetic field

will not lose its magnetism

induced magnet

magnetic material temporarily turned into a magnet in another magnetic field

one of end of the material becomes a north/south pole

the end of the magnetic material closest to the magnet will have the opposite pole to the closet pole of the magnet

when the magnetic field is removed from the field it will lose its magnetism quickly

magnetic field

the region around a magnet where a force acts on another magnet or on a magnetic material.

magnetic fields are also known as…

B fields

measure of magnetic field strength

magnetic flux density

symbol B

unit Tesla

magnetic field lines always go…

from North to South

magnetic field lines show the direction of a…

north-seeking pole

where are uniform magnetic fields found?

the space between two opposite poles

uniform magnetic field (definition)

a magnetic field that has the same strength and direction at all points

(shown by equal spacing between field lines)

magnetic field is strongest…

at the poles

what happens as distance from a magnet increases

magnetic flux density (and therefore field strength) decreases

how to investigate shape and direction of magnetic field

plotting compasses

arrow points towards south pole

place a bar magnet on a piece of paper

plot direction of compass and draw points

connect the dots

repeat to create several field lines

evidence that the core of the earth is magnetic

in the absence of a magnet, a magnetic compass will always point towards geographic North Pole (actually the magnetic South), showing that the core’s magnetic field is similar to that of a bar magnet.

the shape of the magnetic field around a current-carrying wire is…

in concentric circles, indicating there are no poles

how to work out the direction of a magnetic field around a wire?

right hand thumb rule

factors affecting strength of field around a wire

size of current → bigger = stronger

distance from wire → further = weaker

as you get further from a wire, the field lines get…

further apart (exaggerate if asked to draw in exam), showing the field strength getting weaker

path of field lines when a current carrying wire is in a coil

they go through and around the coil, strongest on the inside

how does a solenoid increase the strength of the magnetic field of a coil

fields from individual ‘turns’ combine through the centre of a solenoid to produce a very strong, almost uniform field

outside the solenoid, they cancel to give a weaker field.

how to work out the poles of a solenoid

N = current travels anticlockwise

S = current travels clockwise

how to increase the magnetic field strength of a solenoid

• bigger current

• iron core

• more turns in the coil

motor effect (def)

a wire with a current flowing through it is placed in another magnetic field it experiences a force. Caused by the interaction of 2 magnetic fields. An equal and opposite force acts on the magnet.

how to increase magnetic forces

• stronger mag field with a stronger magnet

• increase current to produce stronger mag field around wire

• place wire at 90° to direction of magnet’s field, so most field lines are cut (if parallel there will be no force)

Fleming’s Left Hand Rule

MOTHER FUCKING CUNT

(Motion, force, current)

(thumb, forefinger, middle finger)

Equation for calculating force on a current-carrying conductor (motor effect)

F = BIL

Force = Magnetic Flux Density (field strength) x Current x Length of conductor within the magnetic field

force in Newtons N

current in amps A

magnetic flux density in tesla N/Am

length in metres m

how is the motor effect used in electric motors?

• a coil of wire which is free to rotate positioned in a uniform magnetic field

• when the current flows at 90° to the direction of the mag field, the current creates a magnetic field around the coil, which interacts with the uniform mag field produced by magnets

• force is exerted on the coil (direction determined by Fleming’s LHR). current flows in opposite directions either side of coil, so one side pushed up and one down

• split ring commutator swaps the sliding contacts as the coil rotates, reversing the current every half turn.

• this reverses the direction of the force exerted on each end of the coil, so it keeps rotating in the same direction