25 Electromagnetic induction

Question 1

electromagnetic induction

Answer 1

the generation of an emf when the magnetic flux linkage through a coil changes or a conductor cuts across the magnetic field

Question 2

lens’s law

Answer 2

when a current is induced by electromagnetic induction, the direction of the induced current is always such to oppose the change that causes the current

Question 3

Faraday’s law of electromagnetic induction

Answer 3

the induced emf in a circuit is equal to the rate of change of magnetic flux linkage through the circuit. for changing magnetic field in a fixed coil of area

Question 4

back emf

Answer 4

emf induced in the spinning of an electric motor or in any coil in which the current is changing. back emf acts against the change of applied pd

Question 5

transformer

Answer 5

converts the amplitude of an alternating pd to a different value. it consists of two insulated coils, the primary and secondary coil wound round a soft iron laminated core

Question 6

step-down transformer

Answer 6

a transformer in which the rms pd across the secondary coil is less than the rms pd applied to primary coil

Ns > Np and Vs > Vp

Question 7

step-up transformer

Answer 7

a transformer in which the rms pd across the secondary coil is greater than the rms pd applied to the primary coil

Ns < Np and Vs < Vp

Question 8

eddy current

Answer 8

induced currents in the metal parts of ac machines

Question 9

induced current can be increased by

Answer 9

moving the wire faster
stronger magnet
making the wire into a coil

Question 10

Fleming’s right hand rule (dynamo rule)

Answer 10

rule that relates the directions of the force, magnetic field and current on a current-carrying conductor in a magnetic field

Question 11

induced emf

Answer 11

E = BA / t
E = BLv
E = E0 sin 2πft

Question 12

back emf

low speed

Answer 12

current is high because the induced emf is small

Question 13

back emf

high speed

Answer 13

current is low because the induced emf is high

Question 14

power station alternating

Answer 14

three sets of coils at 120 degrees to each other. each set produces three alternating currents out of phase by 120

Question 15

effeciency of an electric motor

Answer 15

mechanical power output / electrical power supplied X 100%

Question 16

heating effect of an alternating current

Answer 16

P=IV=I^2R

Question 17

root mean square

Answer 17

value of alternating current is the value of direct that would give the same heating effect as the alternating current in the same resistor
(Irms)^2 R = 0.5I0^2R

Vrms = 1/√2

Question 18

transformer rule

Answer 18

Vs / Vp = Ns / Np

Question 19

soft iron

Answer 19

easily magnestised and demagnetised

reduces power wasted

Question 20

laminated core

Answer 20

separated layers of iron separated by layers of insulation
reduces eddy currents
heating effect is reduced

Question 21

efficiency of a transformer

Answer 21

IsVs / IpVp X 100 %

Question 22

grid system

Answer 22

national grid
frequency of 50 Hz  (25Kv)
step-up = 400 Kv
step-down = 230v
transmission of electrical power over long distances is much more efficient at high voltage than low voltage

Question 23

magnitude of induced emf

Answer 23

= rate of change of flux linkage

Question 24

emf induced in a coil orating uniformly in a magnetic field

Answer 24

E = BAN ω sin ω