11 Electromagnetic Induction

Question 1

electromagnetic induction

Answer 1

the process by which an emf is induced when a conductor experiences a changing magnetic field

Electromagnetic induction may involve a
- moving magnet
- moving conductor
- magnetic field around a changing current

Question 2

Describe the production of an induced emf by a changing magnetic flux and within a uniform magnetic field

Answer 2

moving charges (electrons) in the conductor across a magnetic field → experience magnetic forces →
one side gains -ve charge, the other side loses -ve charge and becomes more +ve → emf (p.d.) is induced

Question 3

Describe the electromagnetic inductions when D.C. and A.C. batteries are used

Answer 3

1. magnetic field due to D.C. current changes from zero to non-zero at the moment when battery is turned on → induces momentary emf
2. magnetic field due to A.C. current constantly changes → induces continuous alternating emf

Question 4

magnetic flux

Answer 4

Φ = BA cosθ
product of normal component of magnetic field strength and area that it links

Question 5

magnetic flux linkage

Answer 5

NΦ = NBA cosθ
product of number of turns in a coil and the magnetic flux through the coil

Question 6

magnetic field strength, B

Answer 6

magnetic flux density
(magnetic flux per unit area)

Question 7

Faraday’s law

Answer 7

ε = -N (dΦ/dt)
the magnitude of induced emf is equal to the rate of change of magnetic flux linkage

Question 8

Lenz’s law

bonus: what is the logic behind this?

Answer 8

induced emf/current acts in a direction;
to oppose the change that caused it

Lenz’s law is based on the law of conservation of energy. Because any induced current has had energy transferred to it, and this energy must have been taken from the original energy in the system????

Question 9

Explain why

1. There is NO induced emf when a magnet is placed next to a circular coil
2. There is induced emf when the magnet moves towards the coil

Answer 9

1. When the magnet is simply placed next to the circular coil, there’s NO CHANGE in the magnetic flux linking the coil → no induced emf
2. When the magnet moves towards the coil, there’s an INCREASE in the magnetic flux linking the coil → induces emf

Question 10

The diagram shows a step-up transformer. Explain why the lamp is off when the switch is closed and the lamp illuminates for a SHORT TIME when the switch is suddenly opened.

Answer 10

There is a current in the primary coil when the switch is closed. This current creates a
magnetic flux in the primary coil, which also links the secondary coil due to the soft iron ring. With the switch closed, there is no change in the magnetic flux linkage
at the secondary coil → lamp is not lit. [1]

When the switch is opened, the magnetic flux suddenly decreases/changes to zero → induces an e.m.f. and the lamp illuminates for a short time. [1]

Eventually there is no magnetic flux at either the primary or the secondary coil → no e.m.f. induced → lamp stays off. [1]

Question 11

A conducting loop enters a region of magnetic field. Describe and explain the SPEED of the loop while it is INSIDE and after it EXITS the magnetic field.

Answer 11

INSIDE: speed decreases as the change in flux induces current and creates magnetic force on the loop, which acts to oppose its motion due to Lenz’s law

OUTSIDE: no more change in flux and magnetic force → constant speed

Question 12

Explain the operation of an A.C. generator, including changing the generator frequency

Question 13

Why are the coils in generators are usually wound on iron cores?

Answer 13

increases the strength of the magnetic field

Question 14

How can you make the output emf of a A.C. generator as large as possible?

Answer 14

• increase the strength of magnetic field (eg. wound on iron core)
• increase number of coils
• increase the area (within the field!)

Question 15

An electrical supply is rated at 110V ac 50Hz.
What is the maximum value of this voltage?

Answer 15

Vrms = 110 = Vmax/√2 → Vmax=156

Question 16

Describe how transformers work

Answer 16

1. alternating voltage drives alternating current through primary coil
2. creates alternating magnetic flux linkage which passes through the iron core to secondary coil
3. induces emf in secondary coil

Question 17

Explain the use of transformers
in A.C. electrical power distribution

Answer 17

Significant amount of energy will be lost (transferred to internal energy) in transmission lines if the currents are too high (P = I²R), so transformers are used to step up voltage to decrease current (P=IV) in transmission lines, thus reducing power loss

Question 18

How can we increase the efficiency of transformers?

Answer 18

laminate the core to prevent the flow of eddy current, which transfer some of the energy into internal energy

Question 19

Why is electrical power usually transferred over large distances using alternating currents?

Answer 19

Need transformers to step up/down voltage to reduce current and power loss. DC can’t produce the necessary changing magnetic flux for the operation of a transformer.

Question 20

rectification

Answer 20

converting ac to dc

Question 21

What device is used to rectify a current?

Answer 21

diode (only allows current to pass in one direction)

Question 22

What is capacitor used for?

Answer 22

smooth the voltage of a rectifier so there is less variation

Question 23

Sketch the V-I characteristic of a diode

Answer 23

RHS: I increases as V increases
LHS: I=0

Question 24

capacitance

Answer 24

the ability of an object to store electric charge

Question 25

for a q-V diagram, how do you find
1. capacitance
2. energy stored in the capacitor

Answer 25

GRADIENT = q/V = CAPACITANCE
AREA = qV/2 = CV^2/2 = ENERGY STORED

Question 26

Describe the exponential discharge of a capacitor

Answer 26

highest rate of discharge at the beginning as there’s the greatest amount of charges on the plate → free electrons experience the greatest force;

the rate of discharge decreases with time, but theoretically never reaches zero

Question 27

time constant, τ

Answer 27

τ = RC = resistance x capacitance

the time that it takes for the q/V/I to decrease to 1/e × the original value (37%)