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Questions Related to physics

The inductance is measured in 

introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

  1. ohm

  2. farad

  3. henery

  4. none of these

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Correct Option: C
Explanation:

The henry (symbolized H) is the Standard International ( SI ) unit of inductance and it is used to measure inductance.

Therefore, C is correct option.

Alternating current is flowing in inductance L and resistance R. The frequency of source is $\displaystyle\frac{\omega}{2\pi}$. Which of the following statement is correct.

introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

  1. For low frequency the limiting value of impedance is L

  2. For high frequency the limiting value of impedance is $L\omega$

  3. For high frequency the limiting value of impedance is R

  4. For low frequency the limiting value of impedance is $L\omega$

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Correct Option: A
Explanation:

$\begin{array}{l} \, \, As\, frequency\, approaches\, zero\, or\, DC,\, the\, inducators\, reac\tan  ce\, would\, decrease\, tozero\, , \ acting\, like\, a\, short\, circuit.\, this\, means\, inductive\, reac\tan  ce\, is\, proportional\, to\, fequency \ \, \, \, \, \, \, \, \, \, \, \, \, \, so\, ,\, for\, low\, frequency\, the\, { { limimiting } }\, \, value\, of\, impedance\, is\, L,\, and\, \, alternating\,  \ current\, is\, flowing\, in\, inductance\, L\, and\, resistance\, R.\, \, The\, frequency\, of\, source\, is\, \frac { \omega  }{ { 2\pi  } } . \ so\, the\, correct\, option\, is\, A. \end{array}$

A student measures the terminal potential difference (V) of a cell (of emf $\varepsilon$ and internal) resistance r) as a function of the current (I) flowing through it. The slope and intercept of the graph between V and I, then respectively equal to :



introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

  1. $-\in \;and\;r$

  2. $\in \;and\;-r$

  3. $-r\;and\;\in \;$

  4. $r\;and\;-\in$

Show answer
Correct Option: C
Explanation:

$E = V + Ir $
$\Rightarrow V=E-Ir$ 
$Comparing\;with\; y = mx + c$ 
$Slope = - r, intercept = E$




If circular coil with $N _{1}$ turns is changed in to a coil of $N _{2}$ turns. What will be the ratio of self inductances in both cases.

introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

  1. $\dfrac {N _{1}}{N _{2}}$

  2. $\dfrac {N _{2}}{N _{1}}$

  3. $\dfrac {N _{1}^{2}}{N _{2}^{2}}$

  4. $\sqrt {\dfrac {N _{1}}{N _{2}}}$

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Correct Option: A
Explanation:

If circular coil with N1 turns is changed in to a coil of N2 turns. 

$L=\dfrac{Nd \phi}{dt} $
$L _1=\dfrac{N _1d \phi}{dt} $
$L _2=\dfrac{N _2d \phi}{dt} $ 
 L is in Henries
        N is the Number of Turns
        Φ is the Magnetic Flux
        Ι  is in Amperes
$\dfrac{L _1}{L _2}=\dfrac{N _1}{N _2} $

If the number of turns per unit length of a coil of a solenoid is doubled the self-inductance of the solenoid will:

introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

  1. remain unchanged

  2. be halved

  3. be doubled

  4. become four times

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Correct Option: D

Henry, the SI unit of inductance can be written as :

introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

  1. weber ampere$^{-1}$

  2. volt second ampere$^{-1}$

  3. joule ampere$^{-1}$

  4. ohm s$^{-1}$

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Correct Option: B
Explanation:

The SI unit of inductance is Henry,
$\displaystyle L =-\dfrac{e}{\dfrac{di}{dt}}$
SI unit $= \displaystyle \dfrac{volt}{A} \times s$
$=volt \times second \times  ampere^{-1}$

Multiple Correct Answers Type
The SI unit of inductance, henry, can be written as

introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

  1. Weber/ampere

  2. Volt-second / ampere

  3. $Joule / (ampere)^2$

  4. Ohm-second

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Correct Option: A,B,C,D
Explanation:

$L=\cfrac{\phi}{i}$

$L=\cfrac{weber}{Ampere}$
$V=L\cfrac{Ldi}{dt}$
$L\to$ volt.second/ampere
$E=\cfrac{1}{2}Li^2$
$L\to $joule/(ampere$)^2$
$\omega L=X _L$
$L\to$ohm.second
Hence all are correct.

A lossless coaxial cable has a capacitance of $7\times { 10 }^{ -11 }$ F and an inductance of $0.39\mu H$. Calculate characteristic impedance of the cable.

introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

  1. 65

  2. 75

  3. 66

  4. 77

Show answer
Correct Option: B
Explanation:

Here,$C=7\times { 10 }^{ -11 }F$,
        $L=0.39\times { 10 }^{ -6 }H$

         ${ Z } _{ o }$ As the cable is lossless,
        $\therefore { Z } _{ o }\sqrt { \dfrac { L }{ C }  } =\sqrt { \dfrac { 0.39\times { 10 }^{ -6 } }{ 7\times { 10 }^{ -11 } }  } =75ohm$

A source of 220 V is applied in an A C circuit . The value of resistance is 220 $\Omega$. Frequency & inductance are 50Hz & 0.7 H then wattless current is 

introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

  1. 0.5 amp

  2. 0.7 amp

  3. 1.0 amp

  4. None

Show answer
Correct Option: A
Explanation:

A source= $220V$

The value of resistance= $220 \Omega$
Frequency= $50 Hz$
Inductance= $0.7H$
Find the wattless current= ?
Wattless component of current is $i=i _v\sin \theta$
                                                            $=\cfrac {Ev}{z}\sin \theta$
where, $z=$ impedance of $L-R$ circuit
                $=\sqrt {R^2+L^2W^2}$ so,
$i=\cfrac {220}{\sqrt {R^2+L^2+W^2}}\sin \theta$ from impedance triangle,
$\sin \theta= \cfrac {LW}{\sqrt {R^2+L^2W^2}}$
$\Rightarrow i=\cfrac {220}{\sqrt {R^2+L^2W^2}}\cfrac {LW}{\sqrt {R^2+L^2W^2}}$
        $=\cfrac {220}{R^2+L^2W^2}LW$
        $=\cfrac {220 \times 0.7 \times 2 \Pi \times 50}{(220)^2+(0.7\times 2\Pi \times 50)^2}$
        $=\cfrac {220 \times 220}{(220)^2+(220)^2}$
        $=0.5 A$ .

The time constant of a circuit is 10 sec, When a resistance of $ 100 \Omega $ is connected in series in a previous circuit then time constant becomes 2 second,then the self inductance of the circuit is;-

introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

  1. $250 H$

  2. $50H$

  3. $150 H$

  4. $25 H$

Show answer
Correct Option: A
Explanation:

In LR circuit,

The time constant $\tau=\dfrac{L}{R}$
$10=\dfrac{L}{R}$
$L=10R$. . . . . . .(1)
When Resistance $100\Omega $ is connect in series, than the time constant is
$\tau'=\dfrac{L}{R+100}=2s$
$L=2R+200$. . . . . . .(2)
Equating equation (1 ) and (2), we get
$2R+200=10R$
$8R=200$
$R=25\Omega$
From equation (1),
$L=10R=10\times 25$
$L=250H$