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21.

Two infinitely long parallel plates of equal areas 6 cm^{2} are separated by a distance of 1 cm. While one of the plates has a charge of + 10 nC and the other has − 10 nC. The magnitude of the electric field between the plates, if ${\mathrm{\epsilon}}_{0}=\frac{{10}^{-9}}{36\mathrm{\pi}}$ F/m is

0.6 π kV/m

6 π kV/m

600 π kV/m

60 π V/m

22.

The electric field strength in NC^{-1} that is required to just prevent a water drop carrying a charge 1.6 x10^{-19} C from falling under gravity is (g = 9.8 ms^{-2}, mass of water drop = 0.0016 g)

9.8 × 10

^{-16}9.8 × 10

^{16}9.8 × 10

^{-13}9.8 × 10

^{13}

23.

A cylinder of radius r and length l is placed in a uniform electric field of intensity E acting parallel to the axis of the cylinder. The total flux over curved surface area is

$2\mathrm{\pi rE}$

$\left(\frac{2\mathrm{\pi}}{\mathrm{l}}\right)\mathrm{E}$

Zero

$\frac{\mathrm{E}}{2\mathrm{\pi rl}}$

24.

Electric lines of force about a positive point charge are

radially outwards

circular clockwise

radially inwards

parallel straight lines

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25.

An electric dipole of moment µ of 400 µC m is placed in a transverse electric field (E) of 50 Vm^{-1} at an angle of 30° to E. Then, a torque of

10

^{-2}Nm acts along the direction E10

^{-3}Nm acts along the direction µ10

^{-2}Nm acts normal to both E and µ10

^{-3}Nm acts along the direction E

26.

The velocity acquired by a charged particle of mass m and charge Q accelerated from rest by a potential of V is

$\frac{\mathrm{QV}}{\mathrm{m}}$

$\sqrt{\frac{\mathrm{m}}{\mathrm{Qv}}}$

$\sqrt{\frac{2\mathrm{QV}}{\mathrm{m}}}$

mQV

27.

An electron moving with a constant velocity v along X-axis enters a uniform electric field applied along Y-axis. Then, the electron moves

with uniform acceleration along Y-axis

without any acceleration along Y-axis

in a trajectory represented as y = ax

^{2}in a trajectory represented as y = ax

28.

A plane square sheet of charge of side 0.5 m has uniform surface charge density. An electron at 1 cm from the centre of the sheet experiences a force of 1.6 x 10^{-12} N directed away from the sheet. The total charge on the plane square sheet is (ε_{0} = 8.854 × 10^{-12} C^{2} m^{-2} N^{-1})

16.25 µC

− 22.15 µC

− 44.27 µC

144.27 µC

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29.

The time period of revolution of a charge q_{1} and of mass m moving in a circular path of radius r due to Coulomb force of attraction with another charge q_{2} at its centre is

$\sqrt{\frac{16{\mathrm{\pi \epsilon}}_{0}{\mathrm{mr}}^{3}}{{\mathrm{q}}_{1}{\mathrm{q}}_{2}}}$

$\sqrt{\frac{8{\mathrm{\pi}}^{2}{\mathrm{\epsilon}}_{0}{\mathrm{mr}}^{3}}{{\mathrm{q}}_{1}{\mathrm{q}}_{2}}}$

$\sqrt{\frac{{\mathrm{\epsilon}}_{0}{\mathrm{mr}}^{3}}{16{\mathrm{q}}_{1}{\mathrm{q}}_{2}}}$

$\sqrt{\frac{16{\mathrm{\pi}}^{3}{\mathrm{\epsilon}}_{0}{\mathrm{mr}}^{3}}{{\mathrm{q}}_{1}{\mathrm{q}}_{2}}}$

30.

When a comb rubbed with dry hair attracts pieces of paper. This is because the

comb polarizes the piece of paper

comb induces a net dipole moment opposite to the direction of field

electric field due to the comb is uniform

comb induces a net dipole moment perpendicular to the direction of field

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