IISER Physics - Electromagnetic induction And Alternating Currents
Exam Duration: 45 Mins Total Questions : 30
A coil of wire is rotated in a uniform magnetic field.
- (a)
A direct current of constant e.m.f is produced
- (b)
An alternating e.m.f of constant amplitude is produced
- (c)
An alternating e.m.f of varying frequency is produced
- (d)
A direct current of varying e.m.f is produced
Lenz's law is a consequence of
- (a)
law of conservation of charge
- (b)
law of conservation of energy
- (c)
retarding force and ohm's law
- (d)
magnetic effect of current
Three metalic loops of copper,platinum and sliver having exactly the same dimension,are rotating in a uniform magnetic field with the same angular velocity.The induced e.m.f is
- (a)
maximum in copper loop
- (b)
maximum in platinum loop
- (c)
maximum in sliver loop
- (d)
same in all the loops
The flux in a coil of 100 turns changes from 0.1wb to 0.5 wb in 4 seconds.the induced voltage in the coil is
- (a)
0.1 volt
- (b)
1.6 volt
- (c)
10 volt
- (d)
160 volt
An inductive load has
- (a)
the current laging behind the e.m.f
- (b)
the e.m.f laging behind the current
- (c)
the current and e.m.f in the same phase
- (d)
the current and e.m.f out of phase
The admittance of an inductor is expressed as
- (a)
\(-\frac { i }{ \omega L } \)
- (b)
\(i\omega L\)
- (c)
\(-i\omega L\)
- (d)
\(\frac { i }{ \omega L } \)
In case of step-up transformer,\({ I }_{ p }\)and \({ I }_{ s }\)are the current flowing in primary and secondary coils. then,
- (a)
\({ I }_{ p }<{ I }_{ s }\)
- (b)
\({ I }_{ p }>{ I }_{ s }\)
- (c)
\({ I }_{ p }={ I }_{ s }\)
- (d)
\({ I }_{ p }\)is independent of \({ I }_{ s }\)
In the case of an ideal step-down transformer
- (a)
The secondary wire is thinner
- (b)
The primary wire is thinner
- (c)
the two have the same thickness
- (d)
the thickness is no consideration
A saturated magnetic induction of one tesla is developed in a homogeneous sample of iron with relative permeability 2000. The energy density of the field with the sample is [\({ \mu }_{ 0 }=4\pi \times { 10 }^{ -7 }H{ m }^{ -1 }\)]
- (a)
\(398\quad J\quad { m }^{ -3 }\)
- (b)
\(1210\quad J\quad { m }^{ -3 }\)
- (c)
\(199\quad J\quad { m }^{ -3 }\)
- (d)
\(1011\quad J\quad { m }^{ -3 }\)
A 25-\(\mu \)F capacitor is left isolated after having been charged.Three hours later it is discovered that the capacitor has lost one half of its charge.The leakage resistance in the dielectric material of the capacitor is
- (a)
623 ohm
- (b)
623 kilo ohm
- (c)
623 mega ohm
- (d)
62.3 ohm
The LRC circuits has L=10mH, C=1\(\mu \)F,R=3.3\(\Omega \) and a.c. supply as E(t)=\(\cos { \omega t } \).At resonance, the current amplitude is
- (a)
3.3 A
- (b)
0.15 A
- (c)
0.30 A
- (d)
None of these
A capacitor C is charged as usual in a circuit which has a resistance R and e.m.f.E.After how many time constants will the energy stored in the capacitor equal 80% of the final value?
- (a)
5.0
- (b)
4.0
- (c)
3.0
- (d)
2.25
5 cm long solenoid having 10 ohm resistance and 5mH inductance isjoined in a 10 V battery.At steady state the current through the solenoid in amperes will be
- (a)
5
- (b)
1
- (c)
2
- (d)
3
The number of turns of primary and secondary coils of a transformer are 5 and 10 respectively and mutual inductance of the transformer is 25H.now, the number of turns in primary and secondary are made 10 and 5 respectively.Mutual inductance of transformer will be
- (a)
25H
- (b)
12.5H
- (c)
50H
- (d)
6.25H
An e.m.f. of 15 V is applied in a circuit coil containing 5 henry inductance and 10 ohm resistance.The ratio of currents at time t=\(\infty \) and t=1 s, is
- (a)
\(\frac { { e }^{ 1/2 } }{ { e }^{ 1/2 }-1 } \)
- (b)
\(\frac { { e }^{ 2 } }{ { e }^{ 2 }-1 } \)
- (c)
1-\( { e }^{ -1 }\)
- (d)
\( { e }^{ -1 }\)
In an induction coil with resistance, the induced e.m.f. will be maximum, when
- (a)
the switch is put on, due to high resistance
- (b)
the switch is put off, due to high resistance
- (c)
the switch is put on, due to low resistance
- (d)
the switch is put off, due to low resistance
Quantity that remains unchanged in a transformer is
- (a)
voltage
- (b)
current
- (c)
frequency
- (d)
none of these
What will be the phase difference between virtual voltage and virtual current, when the current in the circuit is wattless?
- (a)
\({ 90 }^{ \circ }\)
- (b)
\({ 45 }^{ \circ }\)
- (c)
\({ 180 }^{ \circ }\)
- (d)
\({ 60 }^{ \circ }\)
In a pure inductive circuit with a.c.source, the current lags behind e.m.f by
- (a)
\(\pi\)
- (b)
\(2\pi\)
- (c)
\(\frac { \pi } { 2 }\)
- (d)
\(\frac { \pi } { 4 }\)
In an a.c. circuit containing only capacitance, the current
- (a)
leads voltage by \({ 180 }^{ \circ }\)
- (b)
remains in phase with voltage
- (c)
leads voltage by \({ 90 }^{ \circ }\)
- (d)
lags behind voltage by \({ 90 }^{ \circ }\)
An L-C-R circuit is connected to source of a.c. at resonance, the phase difference between voltage and current in the circuit is
- (a)
\(\frac { \pi } { 4 }\)
- (b)
\(\frac { \pi } { 2 }\)
- (c)
\(\pi\)
- (d)
Zero
The resistor \({ R }_ {1 }\)dissipates power p when connected to a generator.If a resistor \({ R } _ { 2 }\)is inserted in series with \({ R }_ {1 }\),the power dissipated by \({ R }_ {1 }\)
- (a)
increases
- (b)
decreases
- (c)
remains the same
- (d)
may decrease or increase depending on values of \({ R }_ {1 }\) and \({ R } _ { 2 }\)
A coil of inductance L is broken into two equal parts. If both parts are placed close to each other such that the flux linkage is complete, the mutual inductance of coil is
- (a)
L
- (b)
\(\frac { L } { 2 }\)
- (c)
\(\sqrt{ { L } /{ 2 }}\)
- (d)
\(\sqrt { 2 } L\)
To induced an emf there must be a A motion between a magnet and a coil. Here, A refers to
- (a)
relative
- (b)
translational
- (c)
rotational
- (d)
None of these
A square loop of side a and resistance R is moved in the region of uniform magnetic field B (loop remaining completely inside field), with a velocity v through a distance x. The work done is
- (a)
\(Bl^2vx\over R\)
- (b)
\(2Bl^2vx\over R\)
- (c)
\(4B^2l^2vx\over R\)
- (d)
None of the above
The output of a step-down transformer is measured to be 24 V when connected to a 12 W light bulb. The value of the peak current is
- (a)
\(1/\sqrt2\ A\)
- (b)
\(\sqrt2\ A\)
- (c)
\(2\ A\)
- (d)
\(2\sqrt2\ A\)
In L-C-R circuit, the power dissipation is through
- (a)
R
- (b)
L
- (c)
C
- (d)
all L, C and R
A closed planar wire loop of area A and arbitrary shape is placed in a uniform magnetic field of magnitude B, with its plane perpendicular to magnetic field. The resistance of the wire loop is R. The loop is now turned upside down by \(180^O\) so that its plane again becomes perpendicular to the magnetic field. The total charge that must have flowed through the wire ring in the process is
- (a)
\(<{AB\over R}\)
- (b)
\({AB\over R}\)
- (c)
\({2AB\over R}\)
- (d)
None of these
A horizontal straight wire 20 m long extending from East to West is falling with a speed of \(50\ ms^{-1}\), at right angles to the horizontal component of the earth's magnetic field \(0.30\times 10^{-4}\ Wbm^{-2}\). The instantaneous value of the emf induced in the wire will be
- (a)
6.0 mV
- (b)
3 mV
- (c)
4.5 mV
- (d)
1.5 mV
Two coaxial solenoids are made by winding thin insulated wire over a pipe of cross-sectional area \(A=10\ cm^2\) and length = 20 cm. If one of the solenoids has 300 turns and the other 400 turns, their mutual inductance is \(\mu_0=\ 4 \pi\times 10^{-7}\ TmA ^{-1}\)
- (a)
\(2.4 \pi \times 10^{-5}\ H\)
- (b)
\(4.8 \pi \times 10^{-4}\ H\)
- (c)
\(4.8 \pi \times 10^{-5}\ H\)
- (d)
\(2.4 \pi \times 10^{-4}\ H\)