21.

A wave travelling along positive x-axis is given by y=A sin (ωt -kx). If it is reflected from rigid boundary such that 80% amplitude is reflected then equation of reflected wave is

• y = A sin(ωt - kx)

• y = - 0.8 A sin(ωt + kx)

• y = 0.8 A sin(ωt + kx)

• y=A sin(ωt + 0.8 kx)

Standing waves are formed on a string when interference occurs between two waves  having

• the same amplitude travelling in the same direction with no phase difference between them

• the same amplitude, travelling in the opposite direction with no phase difference between them

• different amplitudes travelling in the same direction

• different amplitudes travelling in the opposite direction

Two capacitors, one 4 pF and the other 6 pF, connected in parallel, are charged by a 100 V battery. The energy stored in the capacitors is

• 1.2 × 10⁻⁸ J

• 2.4 × 10⁻⁸ J

• 5.0 × 10⁻⁸ J

• 1.2 × 10⁻⁶ J 

The electrical resistivity of a sample

• is proportional to its length

• is proportional to the area of cross-section

• is inversely proportional to the length

• neither depends on the length nor on the area of cross-section

A voltmeter of resistance 998 $\mathrm{\Omega }$ is connected across a cell of emf 2V and internal resistance 2$\mathrm{\Omega }$. The potential difference across the voltmeter is

• 1.99 V

• 3.5 V

• 5 V

• 6 V

A solenoid of length 5O cm and a radius of cross-section 1 cm has 1000 turns of wire wound over it. If the current carried is 5 A, the magnetic field on its axis, near the centre of the solenoid is approximately (permeability of free space ${\mathrm{\mu }}_0$= $4\mathrm{\pi } \times {10}^{-7}$ T-m/A)

• 0.63 × 10⁻² T

• 1.26 × 10⁻² T

• 2.51 × 10⁻² T

• 6.3 T

A conducting square loop of side L and resistance R moves in its plane with a uniform velocity v, perpendicular to one of its sides. A magnetic field of induction B, constant in space and time and pointing perpendicularly into the plane of the square, exists  everywhere in space. The current induced in the loop is

• BLv/R in the clockwise direction

• BLv/R in the anticlockwise direction

• 2 BLv/R in the clockwise direction

• zero

A sinusoidal voltage of peak value 300 V and an angular frequency $\mathrm{\omega }$= 400 rad/s is applied to series L-C-R circuit, in which R = 3 $\mathrm{\Omega }$, L = 20 mH and C = 625 µF. The peak current in the circuit is

• 30$\sqrt{2}$ A

• 60 A

• 100 A

• 60$\sqrt{2}$ A

A circuit contains a capacitor and inductance each with negligible resistance. The capacitor is initially charged and the charging battery is disconnected. At subsequent time, the charge on the capacitor will

• increase exponentially

• decrease exponentially

• decrease linearly

• remain constant

A transformer of 100% efficiency has 200 turns in the primary and 40000 turns in secondary. It is connected to a 220 V main supply and secondary feeds to a 100 k$\mathrm{\Omega }$resistance. The potential difference per turn is

• 1.1 V

• 25 V

• 18 V

• 11 V