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Published on: 15/11/2019
Electromagnetic Induction and Alternating Currents
Download CBSE Class 12th Standard CBSE Physics question papers, sample papers, important questions, and previous year solved papers in PDF format. Get free study materials, NCERT solutions, and exam preparation resources for Class 12th Standard CBSE Physics
Questions + Answers key
Take MCQ Physics Test

1.
Can a.c. source be connected to a circuit and yet deliver no power to it? If so, under what circumstances?
2.
In India, domestic power supply is at 220 V, 50 hz, while in U.S.A, it is 110 V, 60 hz. Give one advantage and one disadvantages of 220 V supply over 110 V supply.
3.
A circular brass loop of radius a and resistance R is placed with its plane perpendicular to a magnetic field, which varies with time as \(B={ B }_{ 0 }sin\omega t.\) Obtain the expression for the induced current in the loop.
4.
Show that the rate of change of magnetic flux has the same units as induced e.m.f.
5.
An e.m.f. is produced in a coil, which is not connected to an external voltage source. This can be due to
the coil being in a time varying magnetic field
the coil moving in a time varying magnetic field
the coil moving in a constant magnetic field
the coil is stationary in external spatially varying magnetic field, which does not change with time
6.
The relation \(\frac { { E }_{ s } }{ { E }_{ p } } =\frac { { n }_{ s } }{ { n }_{ p } } \) is applied only to
a.c. generator
d.c. generator
induction coil
step up/step down transformer
7.
Phase difference between voltage across L and C in series is
\({ 0 }^{ \circ }\)
\({9 0 }^{ \circ }\)
\({180 }^{ \circ }\)
\({ 360 }^{ \circ }\)
8.
Out of the following, choose the correct relation
1henry = \(\frac{1\ volt}{1\ ampere}\)
1henry = \(\frac{1\ amp}{1\ volt}\)
1 henry = \(\frac{1volt}{1\ amp/sec}\)
1 henry = \(\frac{1volt}{1\ amp\ .\ sec}\)
9.
In the relation \(\phi \) = BA cos \(\theta \), \(\theta \) is angle........
which normal to surface area makes with the direction of magnetic field
which magnetic field makes with the surface
which is never constant
none of the above
10.
Raj who is in class XII was demonstrated with an experiment of Faraday's laws of electromagnetic induction by his Physics teacher. While his teacher was explaining the experiment, her lecture and raised a question that "Is there any possibility of induced emf due to earth's magnetism"? After listening his question, the Physics teacher was stunned for a moment and without giving answer, discusses this question in group discussion. In the group discussion, students came out with correct answer.
(i) Write the values that you learnt from this incident.
(ii) What can be the reason for Rays questions?
11.
Draw a labelled diagram of a step-down transformer. State the principle of its working.
Express the turn ratio in terms of voltages.
Find the ratio of primary and secondary currents in terms of turn ratio in an ideal transformer.
How much current is drawn by the primary of a transformer Connected to 220 V supply when it delivers power to a 110 V-550 W refrigerator?
12.
Magnetic field of 2\(\times\)10-2 Wb m-2 is acting at right angle to a coil of 1000 cm having 50 turns. The coil is removed from the field in \(\frac { 1 }{ 10 } \)second. Find the magnitude of induced e.m.f.
13.
A 100 resistor is connected to a 220V, 50Hz ac supply.
(a) What is the rms value of current in the circuit?
(b) What is the net power consumed over a full cycle?
1.
Yes, this would happen when phase difference between alternating voltage and alternating current is \({ 90 }^{ \circ }.\) It can happen when the circuit contains pure L or pure C.
2.
For transfer of power \(\left( =V\times I \right) \) at higher voltage (220 V instead of 110 V), current carried by wires is just half. Therefore, such wires need not be very thick, saving lot of transmission material and reducing the cost of transmission. This is one advantage of 220 V supply. But to design a device of particular wattage, \(P=\frac { { V }^{ 2 } }{ R } \) , as \({ V }^{ 2 }\) is 4 times, R must be four times. If not, the dissipation of power in the form of heat will be larger on 220 V supply. This is one disadvantage of this supply.
3.
Induced current,
\(I=\frac { induced \ e.m.f/ }{ resistance } =\frac { e }{ R } \\ =\frac { d\Phi /dt }{ R } =\frac { -1 }{ R } \frac { d }{ dt } (BA \ cos{ 0 }^{ \circ })\\ I=-\frac { A }{ R } \frac { d }{ dt } ({ B }_{ 0 }sin\omega t)=-\frac { { AB }_{ 0 } }{ R } cos\omega t(\omega )\\ =-\frac { { A\omega B }_{ 0 } }{ R } cos\omega t\)
4.
Induced e.m.f., \(e=V=\frac { work }{ charge } =\frac { M{ L }^{ 2 }{ T }^{ -2 } }{ AT } \)
\(=[{ M }^{ 1 }{ L }^{ 2 }{ T }^{ -3 }{ A }^{ -1 }]\)
Rate of change of magnetic flux\(=\frac { d\Phi }{ dt } =\frac { BA }{ t } \)
\(=\frac { FA }{ q\upsilon t } =\frac { { (MLT }^{ -2 })({ L }^{ 2 }) }{ (AT)({ L }T^{ -1 })(T) } \ (\because F=Bq\upsilon )\\ \ \ =[{ M }^{ 1 }{ L }^{ 2 }T^{ -3 }A^{ -1 }]\)
Both have the same units/dimensions.
5.
(b)
the coil moving in a time varying magnetic field
6.
(d)
step up/step down transformer
7.
(c)
\({180 }^{ \circ }\)
8.
(c)
1 henry = \(\frac{1volt}{1\ amp/sec}\)
9.
(a)
which normal to surface area makes with the direction of magnetic field
10.
(i) Team spirit and curiosity.
(ii) When the wire in N-S direction is dropped feely, none of the components of earth's magnetic field is intercepted. So, no induced emf is produced. When the wire is dropped freely in E-W direction horizontal component of earth's magnetic field is intercepted. So emf is induced in the coil.
11.
Step down transformer:

Principle: When the current flowing through the primary coil changes, an emf is induced in the secondary coil due to the change in magnetic flux linked with it i.e., it works on the principle of mutual induction. For step down transformer,
Ns < Np, hence \({\epsilon}_{s}<{\epsilon}_{p}\).
\({ { {\epsilon}_{s} }\over{ {\epsilon}_{p} } }={{{N}_{s}}\over{{N}_{p}}}\)
For an ideal transformer, Pin = Pout
\({\epsilon }_{p }{ I}_{p }={ \epsilon}_{s }{I }_{s }\Rightarrow{{{I}_{p}}\over{{I}_{s}}}={{{\epsilon}_{s}}\over{{\epsilon}_{p}}}={{{N}_{s}}\over{{N}_{p}}}\)
Pin = Pout = 550 W = \({\epsilon}_{p}{I}_{p}\) = 550
\(220\times{I}_{p}=550\Rightarrow{I}_{p}={{550}\over{220}}={{5}\over{2}}=2.5 \ A\)
12.
Here, B1 = 2\(\times\)10-2 Wb m-2, \(\theta ={ 0 }^{ \circ }\),
A = 1000 cm2 = 1000\(\times\)10-4m2 = 10-1m2, N = 50,
B2 = 0, dt = \(\frac { 1 }{ 10 } \)s, e = ?
\(e=\frac { -d\phi }{ dt } =-\frac { N\left( { B }_{ 2 }-{ B }_{ 1 } \right) A }{ dt } =-\frac { 50\left( 0-2\times { 10 }^{ -2 } \right) { 10 }^{ -1 } }{ 1/10 } =1.0 \ V\)
13.
\(Here, \ R=100\Omega , \ { E }_{ v }=220V, \ v=50Hz\)
\((a) \ { I }_{ v }=?, \ { I }_{ v }=\frac { { E }_{ v } }{ R } =\frac { 220 }{ 100 } =2.2A\)
\((b) \ Net \ power \ consumed \ over \ a \ full \ cycle,\)
\(P={ E }_{ v }{ I }_{ v }=220\times 2.2=484W\)
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