electromagnetism

Question 1

what does a simple transformer consist of?

Answer 1

• core
• primary coil
• secondary coil

Question 2

what is a step up transformer?

Answer 2

• Number of turns on primary coil is less than number of turns on secondary coil

Question 3

what is a step down transformer?

Answer 3

• when number of turns on primary coil is greater than number of turns on secondary coil

Question 4

name the main sources of inefficiency in the core

Answer 4

• 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

Question 5

how can we reduce the sources of inefficiency?

Answer 5

• laminate the core
• use thick copper wires with low resistance
• use a soft iron core
• make the core from a soft magnetic material

Question 6

what is the transformer equation?

Answer 6

Ns/Np = Vs/Vp = Ip/Is

Question 7

what is the equation for efficiency of a transformer

Answer 7

Is Vs / Ip Vp

Question 8

what is the power loss in a wire given by?

Answer 8

I^2 x R

Question 9

how do transformers work

Answer 9

• 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

Question 10

what are eddy currents?

Answer 10

• 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

Question 11

what is Faraday’s Law?

Answer 11

• the magnitude of the emf induced in a coil is equal to the rate of change of flux linkage through the coil

Question 12

what is Lenz’s Law?

Answer 12

• the direction of the induced emf opposes the change that caused it

Question 13

what is magnetic flux?

Answer 13

• magnetic flux density multiplied by the area of the surface, where A is perpendicular to the lines of flux

BAcos€

Question 14

how can we induce an EMF in a coil?

Answer 14

• change the flux density
• change the area of the coil
• rotate the coil
• movr a coil into or out of a magnetic field

Question 15

how can we calculate the EMF from a graph of flux linkage against time?

Answer 15

• gradient is the EMF

Question 16

what is 1 tesla defined as

Answer 16

the magnetic field strength that produces a force of 1N on a 1 metre wire carrying a current of 1 amp

Question 17

what happens when current passes through a wire?

Answer 17

• a magnetic field is induced

Question 18

how can we use FLHR to calculate the direction of the force

Answer 18

ThuMb - direction of force
First finger - direction of the field
seCond finger - direction of conventional current

• remember opposite direction to electron flow

Question 19

what is meant by conventional current?

Answer 19

• current that flows from positive to negative

Question 20

why does the particle experience circular motion?

Answer 20

• the force induced by the magnetic field acts as a centripetal force

Question 21

what is the equation for the radius of the particle’s circular path?

Answer 21

r = mv/BQ

r = radius
m = mass
v = velocity
Q = charge
B = magnetig flux density

Question 22

what is a cyclotron and what is it used for?

Answer 22

• a cyclotron is a particle accelerator that accelerates charged particles through a spiral path using a fixed magnetic firld and an alternating potential difference

Question 23

what is meant by peak voltage?

Answer 23

• half the peak-to-peak voltage, and is equivalent to the amplitude of the wave form

Question 24

what is meany br root mean square voltage

Answer 24

• average of all the squares of the possible voltages
• Vrms = Vo/root 2

Question 25

what is meant by root mean square current

Answer 25

• average of all the squares of possible currents
• Irms = Io/root2

Question 26

what happens when a charged particle moves at right angles to a uniform flux?

Answer 26

• particle moves in a circle, because force is always perpendicular to the motion

Question 27

what happens when the wire lies parallel to the flux lines

Answer 27

there is no force on the wire as sin theta is zero