Tag: physics

Questions Related to physics

When current breaks in primary coil current reaches to zero in  second. Emf induced in the secondary coil is 20,000V and mutual inductance between the coils is 5H. The maximum current is the primary before the break is

  1. 0.2 amp

  2. 0.4 amp

  3. 4 amp

  4. 2 amp


Correct Option: B

Two conducting circular loops of radii $R _{1}$ and $R _{2}$ are placed in the same plane with their centres coinciding. If $R _{1} \gg R _{2}$, the mutual inductance $M$ between them will be directly proportional to

  1. $R _{1}/R _{2}$

  2. $R _{2}/R _{1}$

  3. $R _{1}^{2}/R _{2}$

  4. $R _{2}^{2}/R _{1}$


Correct Option: D

The mutual inductance $M _{12}$ of coil 1 with respect to coil 2

  1. increases when they are bought nearer.

  2. depends on the current passing through the coils.

  3. increases when one of them is rotated about an axis.

  4. is not same as $M _{21}$ of coil 2 with respect to coil 1.


Correct Option: A

A long solenoid  of diameter $0.1\ m$ has $2 \times {10^4}$ turns per metre.At the centre of the solenoid, a coil of $100$ turns and radius $0.01\ m$ is placed with its axis coinciding with the solenoid axis.The current in the solenoid reduces at a constant rate to $0\ A$ from $4\ A$ in $0.05\ s$. If the resistance of the coil is $10 \ {\pi ^2}\Omega ,$ the total charge flowing through the coil during this time is.

  1. $32\ \pi \mu C$

  2. $16\ \mu C$

  3. $32\ \mu C$

  4. $16\ \pi \mu C$


Correct Option: C
Explanation:
Given,

Number of turns, $n=100$

Radius, $r=0.01\,m$

Resistance, $R=10\pi^2 \Omega$

As we know,

$\epsilon=-N\dfrac{d\phi}{dt}$

$=\dfrac{\epsilon}{R}=-\dfrac NR\dfrac{d\phi}{dt}$,   $\Delta I=-\dfrac NR\dfrac{d\phi}{dt}$

$\dfrac{\Delta}{\Delta t}=-\dfrac NR\dfrac{\Delta\phi}{\Delta t}\implies \Delta q=-[\dfrac NR(\dfrac{\Delta \phi}{\Delta t})]\Delta t$

$-$ve sign shoes that induced emf opposes the change in flux.

$\Delta q=\dfrac{\mu _0 ni\pi r^2}{R}$

$\Delta q=\dfrac{4\pi\times 10^{-7}\times 100\times 4\times \pi\times (0.01)^2}{10\pi^2}=32\mu C$

Two coils, a primary of $400$ turns and a secondary of $20$ turns are wound over an iron core of length $20\pi\ cm$ and cross-section of $2\ cm$ radius. If $\mu _{r}=800$, then the coefficient of mutual induction is approximately

  1. $1.6\times 10^{7}H$

  2. $1.6\times 10^{-2}H$

  3. $1.6\times 10^{3}H$

  4. $1.6\ H$


Correct Option: A

A charge of ${10^{ - 6}}C$ is describing a circular path of radius $1$ cm making $5$ revolution per second . The magnetic induction field at the centre of the circle is 

  1. $\pi \times {10^{ - 10}}T$

  2. $\pi \times {10^{ - 9}}T$

  3. $\frac{\pi }{2} \times {10^{ - 10}}T$

  4. $\frac{\pi }{2} \times {10^{ - 9}}T$


Correct Option: D

Two coils A and B have mutual inductance $2\times { 10 }^{ -2 }$ henry. If the current in the primary is $i=5\sin { \left( 10\pi t \right)  } $ then the maximum value of e.m.f.induced in coil B is 

  1. $\pi \quad volt$

  2. $\pi /2volt$

  3. $\pi /3volt$

  4. $\pi /4volt$


Correct Option: A

When the primary current in the spark-coil of a car changes from $4A$ to zero in $10\mu s$, an emf of $40000$V is induced in the secondary. The mutual inductance between the primary and the secondary winding of the spark-coil will be-

  1. 1 H

  2. 0.1 H

  3. 10 H

  4. zero


Correct Option: A

Two coils A and B have mutual inductance $2\times { 10 }^{ -2 }$ henry. If the current in the primary is $i=5\sin { \left( 10\pi t \right)  } $ then the maximum value of e.m.f. induced in coil B is

  1. $\pi \quad volt$

  2. $\pi /2volt$

  3. $\pi /3volt$

  4. $\pi /4volt$


Correct Option: A

The electric field of an electromagnetic wave is given by, $E=(50N^{-1})\, \sin { \omega  } (t-x/c)$. Find the energy contained in a cylinder of cross section $10cm^2$ and length $50 cm$ along the x-axis.

  1. $5.5\times 10^{-12}J$

  2. $4.5\times 10^{-12}J$

  3. $5\times 10^{-13}J$

  4. $3.5\times 10^{-10}J$


Correct Option: A