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

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.

An electromagnetic wave travelling along z-axis is given as: E = E₀ cos (kz – ωt.). Choose the correct options from the following;

1. The associated magnetic field is given as `B = 1/c hatk xx E = 1/ω (hatk xx E)`.
2. The electromagnetic field can be written in terms of the associated magnetic field as `E = c(B xx hatk)`.
3. `hatk.E = 0, hatk.B` = 0.
4. `hatk xx E = 0, hatk xx B` = 0.

The charge on a parallel plate capacitor varies as q = q₀ cos 2πνt. The plates are very large and close together (area = A, separation = d). Neglecting the edge effects, find the displacement current through the capacitor?

A variable frequency a.c source is connected to a capacitor. How will the displacement current change with decrease in frequency?

Show that the magnetic field B at a point in between the plates of a parallel-plate capacitor during charging is `(ε_0mu_r)/2 (dE)/(dt)` (symbols having usual meaning).

Show that average value of radiant flux density ‘S’ over a single period ‘T’ is given by S = `1/(2cmu_0) E_0^2`.

You are given a 2 µF parallel plate capacitor. How would you establish an instantaneous displacement current of 1 mA in the space between its plates?

Sea water at frequency ν = 4 × 10⁸ Hz has permittivity ε ≈ 80 εo, permeability µ ≈ µo and resistivity ρ = 0.25 Ω–m. Imagine a parallel plate capacitor immersed in seawater and driven by an alternating voltage source V(t) = V_o sin (2πνt). What fraction of the conduction current density is the displacement current density?

A long straight cable of length `l` is placed symmetrically along z-axis and has radius a(<< l). The cable consists of a thin wire and a co-axial conducting tube. An alternating current I(t) = I₀ sin (2πνt) flows down the central thin wire and returns along the co-axial conducting tube. The induced electric field at a distance s from the wire inside the cable is E(s,t) = µ₀I₀ν cos (2πνt) In `(s/a)hatk`.

1. Calculate the displacement current density inside the cable.
2. Integrate the displacement current density across the cross-section of the cable to find the total displacement current I^d.
3. Compare the conduction current I0 with the displacement current `I_0^d`.

Do all the electrons that absorb a photon come out as photoelectrons?

Consider a metal exposed to light of wavelength 600 nm. The maximum energy of the electron doubles when light of wavelength 400 nm is used. Find the work function in eV.

Two monochromatic beams A and B of equal intensity I, hit a screen. The number of photons hitting the screen by beam A is twice that by beam B. Then what inference can you make about their frequencies?

Consider a thin target (10^–2 cm square, 10^–3 m thickness) of sodium, which produces a photocurrent of 100 µA when a light of intensity 100W/m² (λ = 660 nm) falls on it. Find the probability that a photoelectron is produced when a photons strikes a sodium atom. [Take density of Na = 0.97 kg/m³].

Consider a 20 W bulb emitting light of wavelength 5000 Å and shining on a metal surface kept at a distance 2 m. Assume that the metal surface has work function of 2 eV and that each atom on the metal surface can be treated as a circular disk of radius 1.5 Å.

1. Estimate no. of photons emitted by the bulb per second. [Assume no other losses]
2. Will there be photoelectric emission?
3. How much time would be required by the atomic disk to receive energy equal to work function (2 eV)?
4. How many photons would atomic disk receive within time duration calculated in (iii) above?
5. Can you explain how photoelectric effect was observed instantaneously?

Two H atoms in the ground state collide inelastically. The maximum amount by which their combined kinetic energy is reduced is ______.

A 100 m long antenna is mounted on a 500 m tall building. The complex can become a transmission tower for waves with λ.

A speech signal of 3 kHz is used to modulate a carrier signal of frequency 1 MHz, using amplitude modulation. The frequencies of the sidebands will be ______.

I-V characteristics of four devices are shown in Figures.

(i)	(ii)	(iii)	(iv)

Identify devices that can be used for modulation.