Science (English Medium) कक्षा १२ - CBSE Question Bank Solutions

A capacitor has capacitance C and reactance X, if capacitance and frequency become double, then reactance will be ______.

A sinusoidal voltage of peak value 283 V and frequency 50 Hz is applied to a series LCR circuit in which R = 3 Ω, L = 25.48 mH, and C = 796 µF, then the power dissipated at the resonant condition will be ______.

In a pure semiconductor crystal of Si, if antimony is added then what type of extrinsic semiconductor is obtained. Draw the energy band diagram of this extrinsic semiconductor so formed.

Is the momentum conserved when charge crosses a junction in an electric circuit? Why or why not?

The relaxation time τ is nearly independent of applied E field whereas it changes significantly with temperature T. First fact is (in part) responsible for Ohm’s law whereas the second fact leads to variation of ρ with temperature. Elaborate why?

1. Consider circuit in figure. How much energy is absorbed by electrons from the initial state of no current (ignore thermal motion) to the state of drift velocity?
2. Electrons give up energy at the rate of RI² per second to the thermal energy. What time scale would one associate with energy in problem (a)? n = no of electron/volume = 10²⁹/m³, length of circuit = 10 cm, cross-section = A = (1mm)²

When an AC voltage of 220 V is applied to the capacitor C ______.

1. the maximum voltage between plates is 220 V.
2. the current is in phase with the applied voltage.
3. the charge on the plates is in phase with the applied voltage.
4. power delivered to the capacitor is zero.

A device ‘X’ is connected to an a.c source. The variation of voltage, current and power in one complete cycle is shown in figure.

1. Which curve shows power consumption over a full cycle?
2. What is the average power consumption over a cycle?
3. Identify the device ‘X’.

Explain why the reactance provided by a capacitor to an alternating current decreases with increasing frequency.

In the LCR circuit shown in figure, the ac driving voltage is v = v_m sin ωt.

1. Write down the equation of motion for q (t).
2. At t = t₀, the voltage source stops and R is short circuited. Now write down how much energy is stored in each of L and C.
3. Describe subsequent motion of charges.

The conductivity of a semiconductor increases with increase in temperature because ______.

Why are elemental dopants for Silicon or Germanium usually chosen from group XIII or group XV?

Sn, C, and Si, Ge are all group XIV elements. Yet, Sn is a conductor, C is an insulator while Si and Ge are semiconductors. Why?

Suppose a ‘n’-type wafer is created by doping Si crystal having 5 × 10²⁸ atoms/m3 with 1 ppm concentration of As. On the surface 200 ppm Boron is added to create ‘P’ region in this wafer. Considering n i = 1.5 × 10¹⁶ m^–3, (i) Calculate the densities of the charge carriers in the n and p regions. (ii) Comment which charge carriers would contribute largely for the reverse saturation current when diode is reverse biased.

An iron cored coil is connected in series with an electric bulb with an AC source as shown in figure. When iron piece is taken out of the coil, the brightness of the bulb will ______.

The figure shows a piece of pure semiconductor S in series with a variable resistor R and a source of constant voltage V. Should the value of R be increased or decreased to keep the reading of the ammeter constant, when semiconductor S is heated? Justify your answer

An a.c. source generating a voltage ε = ε₀ sin ωt is connected to a capacitor of capacitance C. Find the expression for the current I flowing through it. Plot a graph of ε and I versus ωt to show that the current is ahead of the voltage by π/2.