4. Electromagnetic Induction and Alternating Current
1.Define “magnetic flux”.
The magnetic flux (φ) linked with a surface held in a magnetic field (B) is defined as the number of magnetic lines of force crossing a closed area (A). If θ is the angle between the direction of the field and normal to the area,
Then φ = B . A φ = BA cos θ.
2.Define :Electromagnetic induction.
Whenever there is a change in the magnetic flux linked with a closed circuit an emf is produced. This emf is known as the induced emf and the current that flows in the closed circuit is called induced current.
The phenomenon of producing an induced emf due to the changes in the magnetic flux associated with a closed circuit is known as
3.State Faraday’s first law of electromagnetic induction.
Whenever the amount of magnetic flux linked with a closed circuit changes, an emf is induced in the circuit. The induced emf lasts
so long as the change in magnetic flux continues.
4.State Faraday’s second law of electromagnetic induction.
The magnitude of emf induced in a closed circuit is directly
proportional to the rate of change of magnetic flux linked with the
circuit.Let φ1 be the magnetic flux linked with the coil initially and φ2 be
the magnetic flux linked with the coil after a time t. Then
Rate of change of magnetic flux = (φ2 − φ1)/t .According to Faraday’s second law, the magnitude of induced emf is, e α = (φ2 − φ1)/t
5.State :Lenz’s law
The Russian scientist H.F. Lenz in 1835 discovered a simple law giving the direction of the induced current produced in a circuit. Lenz’s law states that the induced current produced in a circuit always flows in such a direction that it opposes the change or cause that produces it.
6. State Fleming’s right hand rule
The forefinger, the middle finger and the thumb of the right hand are held in the three mutually perpendicular directions. If the forefinger points along the direction of the magnetic field and the thumb is along
the direction of motion of the conductor, then the middle finger points
in the direction of the induced current. This rule is also called
7.Define :Self Induction
The property of a coil which enables to produce an opposing induced emf in it when the current in the coil changes is called self induction.
8.Define : Coefficient of self induction
The coefficient of self induction of a coil is numerically equal to
the opposing emf induced in the coil when the rate of change of current
through the coil is unity. The unit of self inductance is henry (H).
9.Define :One henry.
One henry is defined as the self-inductance of a coil in which a
change in current of one ampere per second produces an opposing emf
of one volt.
One henry is defined as the coefficient of mutual induction
between a pair of coils when a change of current of one ampere per
second in one coil produces an induced emf of one volt in the other coil.
10.Define :Mutual induction.
Whenever there is a change in the magnetic flux linked with a coil, there is also a change of flux linked with the neighbouring coil, producing an induced emf in the second coil. This phenomenon of producing an induced emf in a coil due to the change in current in the other coil is known as mutual induction.
11.Define:Coefficient of mutual induction.
The coefficient of mutual induction of two coils is numerically equal to the emf induced in one coil when the rate of change of current through the other coil is unity. The unit of coefficient of mutual induction is henry.
12.State the factors of coefficient of mutual induction between pair of coils depends?
The coefficient of mutual induction between a pair of coils
depends on the following factors:
(i) Size and shape of the coils, number of turns and permeability
of material on which the coils are wound.
(ii) proximity of the coils
13.State the methods of producing induced emf
The induced emf is given by the expression e = –d φ/ dt
=d (NBA cos θ)/dt Hence, the induced emf can be produced by changing
(i) the magnetic induction (B)
(ii) area enclosed by the coil (A) and
(iii) the orientation of the coil (θ) with respect to the magnetic field.
14.What is polyphase generator/alternator.
A single phase a.c. generator or alternator has only one armature
winding. If a number of armature windings are used in the alternator
it is known as polyphase alternator. It produces voltage waves equal to
the number of windings or phases
15.What is Eddy currents?
Foucault in the year 1895 observed that “When a mass of metal moves in a magnetic field or when the magnetic field through a stationary mass of metal is altered, induced current is produced in the metal. This induced current flows in the metal in the form of closed loops resembling ‘eddies’ or whirl pool. Hence this current is called eddy current”. The direction of the eddy current is given by Lenz’s law.
16.How Eddy current appliedin Dead beat galvanometer?
When current is passed through a galvanometer, the coil
oscillates about its mean position before it comes to rest. To bring the
coil to rest immediately, the coil is wound on a metallic frame. Now,
when the coil oscillates, eddy currents are set up in the metallic frame,
which opposes further oscillations of the coil. This inturn enables the
coil to attain its equilibrium position almost instantly. Since the
oscillations of the coil die out instantaneously, the galvanometer is
called dead beat galvanometer.
17.How Eddy current applied in Induction furnace?
In an induction furnace, high temperature is produced by
generating eddy currents. The material to be melted is placed in a
varying magnetic field of high frequency. Hence a strong eddy current
is developed inside the metal. Due to the heating effect of the current,
the metal melts.
18.How Eddy current applied in Induction motors?
Eddy currents are produced in a metallic cylinder called rotor,
when it is placed in a rotating magnetic field. The eddy current initially
tries to decrease the relative motion between the cylinder and the
rotating magnetic field. As the magnetic field continues to rotate, the
metallic cylinder is set into rotation. These motors are used in fans.
19.How Eddy current applied in Electro magnetic brakes?
A metallic drum is coupled to the wheels of a train. The drum
rotates along with the wheel when the train is in motion.When the
brake is applied, a strong magnetic field is developed and hence, eddy
currents are produced in the drum which oppose the motion of the
drum. Hence, the train comes to rest.
20.How Eddy current concept applied in Speedometer?
In a speedometer, a magnet rotates according to the speed of the
vehicle. The magnet rotates inside an aluminium cylinder (drum) which
is held in position with the help of hair springs. Eddy currents are
produced in the drum due to the rotation of the magnet and it opposes
the motion of the rotating magnet. The drum inturn experiences a
torque and gets deflected through a certain angle depending on the
speed of the vehicle. A pointer attached to the drum moves over a
calibrated scale which indicates the speed of the vehicle.
21.What is a transformer?
Transformer is an electrical device used for converting low alternating
voltage into high alternating voltage and vice versa. It transfers
electric power from one circuit to another. The transformer is based on
the principle of electromagnetic induction.
22.What is transformer ratio (or) turns ratio?
E s / E P = N s/ N P = I P/I s = k
where k is called transformer ratio.For step up transformer k > 1 and
for step down transformer k < 1.
In a step up transformer Es > Ep implying that Is < Ip. Thus a step up transformer increases the voltage by decreasing the current, a step down transformer decreases the voltage by increasing the current.
23. Define Efficiency of a transformer.
Efficiency of a transformer is defined as the ratio of output power
to the input power.η =output power/input power
E s I s = E P I P
The efficiency η = 1 (ie. 100%), only for an ideal transformer
where there is no power loss. But practically there are numerous
factors leading to energy loss in a transformer and hence the efficiency
is always less than one.
24.Define Hysteresis loss in a transformer.
The repeated magnetisation and demagnetisation of the iron core
caused by the alternating input current, produces loss in energy called
This loss can be minimised by using a core with a
material having the least hysterisis loss. Alloys like mumetal and
silicon steel are used to reduce hysterisis loss.
25.Define Copper loss in a transformer.
The current flowing through the primary and secondary windings
lead to Joule heating effect. Hence some energy is lost in the form of
heat. Thick wires with considerably low resistance are used to minimise
26.Define Eddy current loss in a transformer.
Eddy current loss (Iron loss)
The varying magnetic flux produces eddy current in the core. This leads to the wastage of energy in the form of heat. This loss is minimised by using a laminated core made of stelloy, an alloy of steel.
27.Define Flux loss in a transformer.
The flux produced in the primary coil is not completely linked with the secondary coil due to leakage. This results in the loss of energy.
This loss can be minimised by using a shell type core. In addition to the above losses, due to the vibration of the core, sound is produced, which causes a loss in the energy.
28.Define RMS value of a.c.
The rms value of alternating current is defined as that value of the
steady current, which when passed through a resistor for a given time,
will generate the same amount of heat as generated by an alternating
current when passed through the same resistor for the same time.
The rms value is also called effective value of an a.c. and is
denoted by Irms or Ieff.
29.Define Inductive reactance & Capacitive reactance.
XL = ωL = 2πν L, where ν is the frequency of the a.c. supply
For d.c. ν = 0; XL = 0
1/ωC = XC is the resistance offered by the capacitor. It is called
capacitive reactance. Its unit is ohm .
30.Capacitor allows a.c. but not d.c. Why?
Capacitor offers infinite resistance to d.c. For an a.c. the
capacitive reactance varies inversely as the frequency of a.c. and also
inversely as the capacitance of the capacitor.
31.Define Resonant frequency of the circuit.
The particular frequency νo at which the impedance of the circuit
becomes minimum and therefore the current becomes maximum is
called Resonant frequency of the circuit. Such a circuit which admits
maximum current is called series resonant circuit or acceptor circuit
32.What is an acceptor circuit?
The series resonant circuit is often called an ‘acceptor’ circuit. By
offering minimum impedance to current at the resonant frequency it is
able to select or accept most readily this particular frequency among
many frequencies. In radio receivers the resonant frequency of the circuit is tuned to the frequency of the signal desired to be detected. This is usually done by varying the capacitance of a capacitor.
33.Define Q-factor (or) quality factor.
The selectivity or sharpness of a resonant circuit is measured by
the quality factor or Q factor. In other words it refers to the sharpness
of tuning at resonance.
The Q factor of a series resonant circuit is defined as the ratio of
the voltage across a coil or capacitor to the applied voltage.
Q =voltage across L or C/applied voltage
34.What is a Choke coil ?
A choke coil is an inductance coil of very small resistance used
for controlling current in an a.c. circuit. If a resistance is used to
control current, there is wastage of power due to Joule heating effect
in the resistance. On the other hand there is no dissipation of power
when a current flows through a pure inductor.
35.What are types of choke coil?.
Chokes used in low frequency a.c. circuit have an iron core so
that the inductance may be high. These chokes are known as audio –
frequency (A.F) chokes. For radio frequencies, air chokes are used since
a low inductance is sufficient. These are called radio frequency (R. F)
or high frequency (H.F) chokes and are used in wireless receiver