21.

The electric field due to a uniformly charged sphere of radius R as a function of the distance from its centre is represented graphically by

• 

• 

• 

• 

Equipotential surfaces associated with an electric field which is increasing in magnitude along the x-direction are

• planes parallel to yz - plane

• planes parallel to xz-plane

• planes parallel to xz-plane

• coaxial cylinders of increasing radii around  the x-axis

A circular coil of radius is R carries an electric current. The magnetic field due to the coil at a point on the axis of the coil located at a point on the axis of the coil located at a distance r from the centre of the coil, such that r >> R, varies as 

• $\frac{1}{\mathrm{r}}$

• $\frac{1}{{\mathrm{r}}^{{\displaystyle \raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}}$

• $\frac{1}{{\mathrm{r}}^{ 2}}$

• $\frac{1}{{\mathrm{r}}^3}$

The magnetic field due to straight conductor of uniform cross-section of radius  ' a '  and carrying a steady current is represented by

A 40F capacitor in a defibrillator is charged to 3000 V. The energy stored in the capacitor is sent through the patient during a pulse of duration 2 ms. The power delivered to the patient is

• 45 kW

• 90 kW

• 180 kW

• 360 kW

Eels are able to generate current with biological cells called electroplaques. The electroplaques in an eel are arranged in I 00 rows, each row stretching horizontally along the body of the fish containing 5000 electroplaques. The arrangement is suggestively shown below. Each electroplaques has an emf of 0.15 V and internal resistance of 0.25 Ω

The water surrounding the eel completes a circuit between the head and its tail. If the water surrounding it has a resistance of 500 Ω, the current an eel can produce in water is about

• 1.5 A

• 3.0 A

• 15 A

• 30 A

Liquid oxygen remains suspended between two pole faces of a magnet because it is

• diamagnetic

• paramagnetic

• ferromagnetic

• antiferromagnetic

In the basic CsCl crystal structure, Cs⁺ and Cl^- ions are arranged in a bcc configuration as shown in the figure. The net electrostatic force exerted by the eight Cs^- ions on the Cl^- ion is

• $\frac{1}{4{\mathrm{\pi \epsilon }}_{\mathrm{o}}} \frac{4{\mathrm{e}}^2}{3{\mathrm{a}}^2}$

• $\frac{1}{4{\mathrm{\pi \epsilon }}_{\mathrm{o}}} \frac{16{\mathrm{e}}^2}{3{\mathrm{a}}^2}$

• $\frac{1}{4{\mathrm{\pi \epsilon }}_{\mathrm{o}}} \frac{32{\mathrm{e}}^2}{3{\mathrm{a}}^2}$

• zero

The magnetic moment of a current (I) carrying circular coil of radius (r) and number of turns (n) varies as

• $\frac{1}{{\mathrm{r}}^2}$

• $\frac{1}{\mathrm{r}}$

• r

• r²

The cyclotron frequency of an electron  gyrating in a magnetic field of  1 T approximately

• 28 MHz

• 280 MHz

• 2.8 GHz

• 28 GHz