chapter 21 - emi and cro

Question 1

lenz’s law

Answer 1

direction of induced emf, and hence the induced current in a closed circuit, is always such that its magnetic effect opposes the motion or change producing it

Question 2

faraday’s law

Answer 2

magnitude of the induced emf in a circuit is directly proportional to the rate of change of magnetic flux in the circuit.

Question 3

purpose of soft iron core

Answer 3

concentrate magnetic field to provide
better magnetic flux linkage to produce larger induced emf

Question 4

transformer

Answer 4

device that is used to change high voltages to low voltages or vice-versa

Question 5

deflection of galvanometer

Answer 5

when a bar magnet is stationary inside the solenoid :
no deflection in galvanometer, no induced current

when south pole is pushed in the solenoid :
galvanometer deflected towards left, current induced flowing from right to left

when south pole pulled away from solenoid :
galvanometer deflected towards right, current induced flowing from left to right

when solenoid pushed towards south pole :
galvanometer deflected towards left, current induced flowing from right to left

Question 6

inducing a larger current

Answer 6

• increase the relative speed between the coil and the magnet
• increase number of turns of the coil
• increase strength of magnet

Question 7

why is there current?

Answer 7

changng magnetic flux linkage, induce emf, induce current

Question 8

factors affecting size of induced emf

Answer 8

• speed of rotation doubled : induced emf and frequency doubled
• number of turns doubled : induced emf doubles, frequency remains constant
• strength of magnet doubled : induced emf doubles, frequency remains constant

Question 9

step up vs step down transformers

Answer 9

step up transformers (generators in power stations) : primary coil < secondary coil

step down transformers (chargers) : secondary coil < primary coil

Question 10

working prinicple of transformers

Answer 10

the ac input voltage produces an alternating magnetic field in the primary coil that is linked to the secondary coil through the soft iron core. thus the secondary coil experiences changing magnetic flux, and by farday’s law, this induces a ac. current

Question 11

why is it not possible to ensure a 100% efficient transformer?

Answer 11

energy could be lost in the form of heat produced by current flowing in the coils which have resistance, therefore thick wires are used to reduce resistance, that reduces energy lost as heat. furthermore, energy can be lost as eddy currents produced in the iron core during the process of mutual induction, where a current is also produced in the soft iron core itself. therefore, the soft iron core is normally laminated

Question 12

formula for turns ratio

Answer 12

Ns/Np = Vs/Vpo

Question 13

formula for power across transformer

Answer 13

Vp x Ip = Vs x Is