Year 12 Physics - EMF

what is Lenz’s law

the direction of the induced emf in a conductor opposes the change which produces it

define an eddy current

a current produced which produces a magentic field, opposing the change in flux which created it

what is a transformer

a device used to change voltage/current of a circuit using electromagnetic induction

what are the parts of a transformer

primary coil

secondary coil

iron core

describe each coil in a transformer

primary → has the electrical supply

secondary → has electricity induced

how is electricity induced in a trasnformer

the primary coil has an alternating current which produces a changing magnetic flux

this induces an emf in the second coil

describe a step up transormer

more turns in the secondary coil

voltage increases

current decreases

describe a step down transformer

fewer turns in secondary coil

voltage decreases

current increases

what stays constant across transformers

power

how is transformer efficiency increased

soft iron core → increases flux density by focusing field

laminate iron core to minimise eddy currents

(lots of small currents better than 1 big current)

how is energy lost and how do we minimise this

electrons collide with metal ions and lose energy

decreasing current decreases collisions and so we get more energy

increasing wire cross section also decreases resistance to decrease loss

describe induction stoves

use eddy currents

copper coil in cooktop is supplied with an alternating current, producing a changing flux

flux change induces an eddy current in the cookware

resistance in the pot transforms electrical energy into heat

describe regenerative braking

the kinetic energy usually lost when braking is recaptured and turned into electrical energy

when braking, the motor generates negative torque and behaves like a generator, recovering energy

how does a dc motor work

dc supply moves armature through a field

current and field result in a force on the armature

as the coil reaches perpendicular to the field, the forces stop unless current direction changed

use split ring commutuator to allow stationary terminals but switch over over wire end so current in each swaps direction

force changes direction and motion continues for 360*

how does an AC motor work

AC supply already has a changing current

uses slip rings to have constant contact with the same terminal

how does an AC generator (alternator work)

armature moved through field by mechanical means

emf results

emf produces an alternating current in the armature

slip rings maintain constant contact with terminals

AC current runs through circuit

how does a DC generator work

same principles as AC

uses split ring commutator instead

maintains a constant output direction in the circuit by changing wire connection to terminal each half rotation

what is back emf

the opposing emf induced when an electric motor is running, which reduces the current in the circuit so it doesn’t burn out

what results in back emf

the current produced when a conducting coil moves through a magnetic field

what does back emf oppose

the change in flux which created it

current of back emf opposes current direction which made it

why does motor speed increase back emf

because the faster it spins the quicker the change in flux so more emf produced

what happens if a load is applied to the motor

the load slows the motor

less back emf produced

net emf increases

what happens if the motor is held stationary

no back emf produced as no change in flux

full voltage and full current run through circuit

circuit burns out