2 Marks Questions

Question 12 Marks

An electric dipole of dipole moment $2\times10^{-6}cm$ is kept inside a closed surface. What will be the net flux coming out from the surface ?

Answer

Dipole Moment $P=2\times10^{-6}$ cm
Gauss's rule,
$\phi=\frac{q_{in}}{\in_{0}}$ [Net Flux $q_{in}=+q-q=0]$
Or $\phi=\frac{0}{\in_{0}}=0$
$\frac{p}{+q \quad-q}$
So Net Flux will be zero.

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Question 22 Marks

10¹⁹ electrons are placed on an uncharged body. Calculate the charge produced on the body.

Answer

$\theta=n e$
$n=\theta e$
Here, $\theta=1C$ and $e=1.6\times10^{-19}C$
$\therefore n=\frac{1}{1.6\times10^{-19}C}=6.25\times10^{18}$ electron

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Question 32 Marks

Define intensity of electric field at any point. Write down its S.I. unit.

Answer

The amount of force experienced by a unit positive charge placed at any point of the electric field is called the intensity of the electric field at that point. It is a vector quantity. Its direction is that in which the force acts on a unit positive charge. Its Unit is Newton/Coulomb.

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Question 42 Marks

Find the increase in energy of a condenser of capacity 6 µF on changing potential difference from 10 V to 20 V.

Answer

$C=6~\mu F$
$C=6~\mu F$
$V_{1}=10~V$
$V_{2}=20~V$
Increase in energy
$\Delta U=\frac{1}{2}(C{V_{2}}^{2}-{CV_{1}}^{2})$
$=\frac{1}{2}C({V_{2}}^{2}-{V_{1}}^{2})$
$=\frac{1}{2}\times6\times10^{-6}\times[20^{2}-10^{2}]$
$=\frac{1}{2}\times6\times10^{-6}\times300=9\times10^{-4}J$

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Question 52 Marks

A sphere of radius 5 cm has a charge of 31.41 µC. Calculate the surface density of charge.

Answer

$R=5~cm=5\times10^{-2}m$
$q=31.41~\mu c=31.41\times10^{-6}C$
Surface charge density
$\sigma=\frac{q}{4\pi R^{2}}=\frac{31.41\times10^{-6}}{4\pi\times(5\times10^{-2})^{2}}$
$\sigma=10^{-3}c/m^{2}$

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Question 62 Marks

Obtain a relation for equivalent capacitance of the series combination of capacitors.

Answer

Equivalent capacitance of series combination of capacitors : Let three capacitors of capacitance $C_{1}$, $C_{2}$, $C_{3}$ be connected in series as shown in fig.

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Question 72 Marks

Define volume density of charge and write its S.I. unit ?

Answer

Volume charge density: When the charge density is throughout the volume of a body, then the charge per unit volume is called volume charge density and it is represented by
$\rho=\frac{q}{V}$
Hence the unit of $\rho$ is $C/m^{3}$

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Question 82 Marks

What do you mean by electric field intensity ?

Answer

The electric field Intensity at a point is the force experienced by a unit positive charge placed at that point. Electric Field Intensity is a vector quantity. It is denoted by 'E', Electric Field $=F/q,$ Unit of E is NC^-1 or $Vm^{-1}.$

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Question 92 Marks

Define Electric flux and write its S.I. unit.

Answer

Electric flux is defined as the measure of count of number of electric field lines crossing an area.
Electric flux
$\phi=EA~cos~\theta$
SI unit of electric flux is Nm²/C

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Question 102 Marks

A capacitor with capacitance C is changed to potential V. What is its electrical energy ?

Answer

Electrical energy of capacitor
$U=\frac{Q^{2}}{2C}$ ...(1)
Where $Q=CV$
In equation (1) the value of Q
$U=\frac{(CV)^{2}}{2C}$
$U=\frac{1}{2}CV^{2}$

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