List of top Questions asked in CBSE CLASS XII

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. \((e = 1.5 × 10^{–19} C, m_e = 9.1×10^{–31} kg)\)

The rate constant for the first order decomposition of \(H_2O _2\) is given by the following equation:
\(log\  k = 14.34 - 1.25 \times  10^4\  K/T \)
Calculate \(E_a\) for this reaction and at what temperature will its half-period be 256 minutes?

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?
A closely wound solenoid 80 cm long has 5 layers of windings of 400 turns each. The diameter of the solenoid is 1.8 cm. If the current carried is 8.0 A, estimate the magnitude of B inside the solenoid near its centre.

A card sheet divided into squares each of size 1 mm2 is being viewed at a distance of 9 cm through a magnifying glass (a converging lens of focal length 9 cm) held close to the eye. 
(a) What is the magnification produced by the lens? How much is the area of each square in the virtual image? 
(b) What is the angular magnification (magnifying power) of the lens? (c) Is the magnification in (a) equal to the magnifying power in (b)? Explain

The number density of free electrons in a copper conductor estimated in Example 3.1 is \(8.5 × 10^{28} m^{−3}\). How long does an electron take to drift from one end of a wire 3.0 m long to its other end? The area of cross-section of the wire is \(2.0 × 10^{−6} m^2\) and it is carrying a current of 3.0 A.

Two long and parallel straight wires A and B carrying currents of 8.0 A and 5.0 A in the same direction are separated by a distance of 4.0 cm. Estimate the force on a 10 cm section of wire A.
At what angle should a ray of light be incident on the face of a prism of refracting angle 60° so that it just suffers total internal reflection at the other face? The refractive index of the material of the prism is 1.524.

(a)Determine the ‘effective focal length’ of the combination of the two lenses if they are placed 8.0cm apart with their principal axes coincident. Does the answer depend on which side of the combination a beam of parallel light is incident? Is the notion of effective focal length of this system useful at all? (b)An object 1.5cm in size is placed on the side of the convex lens in the arrangement(a)above. The distance between the object and the convex lens is 40cm. Determine the magnification produced by the two-lens system and the size of the image.

A storage battery of emf 8.0 V and internal resistance 0.5 Ω is being charged by a 120 V dc supply using a series resistor of 15.5 Ω. What is the terminal voltage of the battery during charging? What is the purpose of having a series resistor in the charging circuit?

The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is \(B_0 = 510\  nT\). What is the amplitude of the electric field part of the wave?

A screen is placed 90cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20cm. Determine the focal length of the lens.

The image of a small electric bulb fixed on the wall of a room is to be obtained on the opposite wall 3m away by means of a large convex lens. What is the maximum possible focal length of the lens required for the purpose?

(a) Figure 9.28 shows a cross-section of a light pipe made of a glass fiber of a refractive index of 1.68. The outer covering of the pipe is made of a material with a refractive index 1.44. What is the range of the angles of the incident rays with the axis of the pipe for which total reflections inside the pipe take place, as shown in the figure? (b)What is the answer if there is no outer covering of the pipe?

A silver wire has a resistance of 2.1 Ω at 27.5 °C, and a resistance of 2.7 Ω at 100 °C. Determine the temperature coefficient of resistivity of silver.

A charged particle oscillates about its mean equilibrium position with a frequency of \(10^9 Hz\). What is the frequency of the electromagnetic waves produced by the oscillator?

A radio can tune in to any station in the 7.5 MHz to 12 MHz band. What is the corresponding wavelength band?

A small pin fixed on a tabletop is viewed from above from a distance of 50cm. By what distance would the pin appear to be raised if it is viewed from the same point through a 15cm thick glass slab held parallel to the table? The refractive index of glass=1.5.Does the answer depend on the location of the slab?

Use the mirror equation to deduce that:(a)an object placed between f and 2f of a concave mirror produces a real image beyond 2f.(b)a convex mirror always produces a virtual image independent of the location of the object.(c)the virtual image produced by a convex mirror is always diminished in size and is located between the focus and the pole.(d)an object placed between the pole and focus of a concave mirror produces a virtual and enlarged image.[Note:This exercise helps you deduce algebraically properties of images that one obtains from explicit ray diagrams.]

A plane electromagnetic wave travels in vacuum along z-direction. What can you say about the directions of its electric and magnetic field vectors? If the frequency of the wave is 30 MHz, what is its wavelength?