electromagnetism Flashcards
what does a simple transformer consist of?
- core
- primary coil
- secondary coil
what is a step up transformer?
- Number of turns on primary coil is less than number of turns on secondary coil
what is a step down transformer?
- when number of turns on primary coil is greater than number of turns on secondary coil
name the main sources of inefficiency in the core
- eddy currents in the core
- heat loss through the coils
- not all flux from the primary coil going through the secondary coil
- magnetising and demagnetising the core
how can we reduce the sources of inefficiency?
- laminate the core
- use thick copper wires with low resistance
- use a soft iron core
- make the core from a soft magnetic material
what is the transformer equation?
Ns/Np = Vs/Vp = Ip/Is
what is the equation for efficiency of a transformer
Is Vs / Ip Vp
what is the power loss in a wire given by?
I^2 x R
how do transformers work
- we put a step up transformer between power stations and the transmission lines. reducing current, reducing power lost as heat.
- we have a step down transformer before peoples homes to reduce the voltage to a safe level
what are eddy currents?
- a change in magnetic flux through a conductor enduces an emf
- if the conductor is solid, this emf causes currents to flow in loops within the conductor
what is Faraday’s Law?
- the magnitude of the emf induced in a coil is equal to the rate of change of flux linkage through the coil
what is Lenz’s Law?
- the direction of the induced emf opposes the change that caused it
what is magnetic flux?
- magnetic flux density multiplied by the area of the surface, where A is perpendicular to the lines of flux
BAcos€
how can we induce an EMF in a coil?
- change the flux density
- change the area of the coil
- rotate the coil
- movr a coil into or out of a magnetic field
how can we calculate the EMF from a graph of flux linkage against time?
- gradient is the EMF
what is 1 tesla defined as
the magnetic field strength that produces a force of 1N on a 1 metre wire carrying a current of 1 amp
what happens when current passes through a wire?
- a magnetic field is induced
how can we use FLHR to calculate the direction of the force
ThuMb - direction of force
First finger - direction of the field
seCond finger - direction of conventional current
- remember opposite direction to electron flow
what is meant by conventional current?
- current that flows from positive to negative
why does the particle experience circular motion?
- the force induced by the magnetic field acts as a centripetal force
what is the equation for the radius of the particle’s circular path?
r = mv/BQ
r = radius
m = mass
v = velocity
Q = charge
B = magnetig flux density
what is a cyclotron and what is it used for?
- a cyclotron is a particle accelerator that accelerates charged particles through a spiral path using a fixed magnetic firld and an alternating potential difference
what is meant by peak voltage?
- half the peak-to-peak voltage, and is equivalent to the amplitude of the wave form
what is meany br root mean square voltage
- average of all the squares of the possible voltages
- Vrms = Vo/root 2
what is meant by root mean square current
- average of all the squares of possible currents
- Irms = Io/root2
what happens when a charged particle moves at right angles to a uniform flux?
- particle moves in a circle, because force is always perpendicular to the motion
what happens when the wire lies parallel to the flux lines
there is no force on the wire as sin theta is zero
