List of top Physics Questions asked in CBSE CLASS XII

A closely wound solenoid of 800 turns and area of cross section 2.5 × 10-4 m2 carries a current of 3.0 A. Explain the sense in which the solenoid acts like a bar magnet. What is its associated magnetic moment?

If the bar magnet in exercise 5.13 is turned around by 180º, where will the new null points be located?

(a) For the telescope described in Exercise 9.27 (a), what is the separation between the objective lens and the eyepiece? 

(b) If this telescope is used to view a 100 m tall tower 3 km away, what is the height of the image of the tower formed by the objective lens? 

(c) What is the height of the final image of the tower if it is formed at 25cm?

A short bar magnet has a magnetic moment of 0.48 J T-1. Give the direction and magnitude of the magnetic field produced by the magnet at a distance of 10 cm from the centre of the magnet on ( a ) the axis, ( b) the equatorial lines (normal bisector) of the magnet.

A small telescope has an objective lens of focal length 140cm and an eyepiece of focal length 5.0cm. What is the magnifying power of the telescope for viewing distant objects when? 

(a) the telescope is in normal adjustment (i.e., when the final image is at infinity)? 

(b) the final image is formed at the least distance of distinct vision (25cm)?

An angular magnification (magnifying power) of 30X is desired using an objective of focal length 1.25cm and an eyepiece of focal length 5cm. How will you set up the compound microscope?

A rectangular wire loop of sides 8 cm and 2 cm with a small cut is moving out of a region of uniform magnetic field of magnitude 0.3 T directed normal to the loop. What is the emf developed across the cut if the velocity of the loop is 1 cm s-1 in a direction normal to the (a) longer side, (b) shorter side of the loop? For how long does the induced voltage last in each case?

A long solenoid with 15 turns per cm has a small loop of area 2.0 cm2 placed inside the solenoid normal to its axis. If the current carried by the solenoid changes steadily from 2.0 A to 4.0 A in 0.1 s, what is the induced emf in the loop while the current is changing?

Predict the direction of induced current in the situations described by the following Figs. 6.18(a) to (f ).
6.18(a) to (f )

A pair of adjacent coils has a mutual inductance of 1.5 H. If the current in one coil changes from 0 to 20 A in 0.5 s, what is the change of flux linkage with the other coil?
Current in a circuit falls from 5.0 A to 0.0 A in 0.1 s. If an average emf of 200 V induced, give an estimate of the self-inductance of the circuit.

A horizontal straight wire 10 m long extending from east to west is falling with a speed of 5.0 m s-1, at right angles to the horizontal component of the earth’s magnetic field, 0.30 \(\times\)10-4 Wb m-2 .

  1. What is the instantaneous value of the emf induced in the wire? 
  2. What is the direction of the emf?
  3. Which end of the wire is at the higher electrical potential?
Answer the following questions: (a) The angle subtended at the eye by an object is equal to the angle subtended at the eye by the virtual image produced by a magnifying glass. In what sense then does a magnifying glass provide angular magnification? (b) In viewing through a magnifying glass, one usually positions one’s eyes very close to the lens. Does angular magnification change if the eye is moved back? (c) Magnifying power of a simple microscope is inversely proportional to the focal length of the lens. What then stops us from using a convex lens of smaller and smaller focal length and achieving greater and greater magnifying power? (d) Why must both the objective and the eyepiece of a compound microscope have short focal lengths? (e) When viewing through a compound microscope, our eyes should be positioned not on the eyepiece but a short distance away from it for best viewing. Why? How much should be that short distance between the eye and eyepiece?

A 1.0 m long metallic rod is rotated with an angular frequency of 400 rad s-1 about an axis normal to the rod passing through its one end. The other end of the rod is in contact with a circular metallic ring. A constant and uniform magnetic field of 0.5 T parallel to the axis exists everywhere. Calculate the emf developed between the centre and the ring.

Figure shows a capacitor made of two circular plates each of radius 12 cm, and separated by 5.0 cm. The capacitor is being charged by an external source (not shown in the figure). The charging current is constant and equal to 0.15 A.

  1. Calculate the capacitance and the rate of charge of potential difference between the plates.
  2. Obtain the displacement current across the plates.
  3. Is Kirchhoff’s first rule (junction rule) valid at each plate of the capacitor? Explain.
capacitor made of two circular plates
What should be the distance between the object in Exercise 9.23 and the magnifying glass if the virtual image of each square in the figure is to have an area of 6.25 mm2 . Would you be able to see the squares distinctly with your eyes very close to the magnifier? [Note: Exercises 9.22 to 9.24 will help you clearly understand the difference between magnification in absolute size and the angular magnification (or magnifying power) of an instrument.]
Two moving coil meters, \(M_1\) and \(M_2\) have the following particulars:
\(R_1\) =\(10\,\Omega\) , \(N_1\) = 30,
\(A_1\) = \(3.6 × 10^{–3} m^2\), \(B_1\) = \(0.25\, T\) 
\(R_2\) = \(14 \,\Omega\), \(N_2\) = 42,
\(A_2\) = \(1.8 × 10^{–3 }m^2 \), \(B_2\) = \(0.50 T \)
(The spring constants are identical for the two meters).
Determine the ratio of (a) current sensitivity and (b) voltage sensitivity of \(M_2\) and \(M_1\).

A p-n photodiode is fabricated from a semiconductor with band gap of 2.8 eV. Can it detect a wavelength of 6000 nm?

Suppose that the electric field amplitude of an electromagnetic wave is \(E_0 = 120 \ N/C\) and that its frequency is \(ν = 50.0 \ MHz\). 

  1. Determine, B0, ω, k, and λ. 
  2. Find expressions for E and B.

Two amplifiers are connected one after the other in series (cascaded). The first amplifier has a voltage gain of 10 and the second has a voltage gain of 20. If the input signal is 0.01 volt, calculate the output ac signal.

For a CE-transistor amplifier, the audio signal voltage across the collected resistance of 2 kΩ is 2 V. Suppose the current amplification factor of the transistor is 100, find the input signal voltage and base current, if the base resistance is 1 kΩ.

In a chamber, a uniform magnetic field of \(6.5\, G \,(1 G = 10 ^{–4} T)\) is maintained. An electron is shot into the field with a speed of \(4.8 × 10^6 m s^{–1 }\) normal to the field. Explain why the path of the electron is a circle. Determine the radius of the circular orbit. \((e = 1.5 × 10^{–19} C, m_e = 9.1×10^{–31} kg)\)

Determine the current in each branch of the network shown in figure:

In a plane electromagnetic wave, the electric field oscillates sinusoidally at a frequency of \(2.0 × 10^10 \ Hz\) and amplitude \(48\  V m^{−1}\). 

  1. What is the wavelength of the wave?
  2. What is the amplitude of the oscillating magnetic field?
  3. Show that the average energy density of the E field equals the average energy density of the B field. \([c = 3 × 10^8 m s^{−1}]\)
A square coil of side 10 cm consists of 20 turns and carries a current of 12 A. The coil is suspended vertically and the normal to the plane of the coil makes an angle of \(30\degree\) with the direction of a uniform horizontal magnetic field of magnitude 0.80 T. What is the magnitude of torque experienced by the coil?