Science (English Medium) Class 12 - CBSE Question Bank Solutions for Physics

The diagram shows the four energy levels of an electron in the Bohr model of the hydrogen atom. Identify the transition in which the emitted photon will have the highest energy.

An increase in the intensity of the radiation causing photo-electric emission from a surface does not affect the maximum K.E. of the photoelectrons. Explain.

In an interference experiment, a third bright fringe is obtained at a point on the screen with a light of 700 nm. What should be the wavelength of the light source in order to obtain the fifth bright fringe at the same point?

• Assertion (A): Propagation of light through an optical fibre is due to total internal reflection taking place at the core-cladding interface.
• Reason (R): Refractive index of the material of the cladding of the optical fibre is greater than that of the core.

(i) Explain with the help of a diagram the formation of depletion region and barrier potential in a pn junction.

Explain the meaning of the term mutual inductance.

What happens when a forward bias is applied to a p-n junction?

What causes the setting up of high electric field even for small reverse bias voltage across the diode?

Two infinitely long straight parallel wires, '1' and '2', carrying steady currents I₁ and I₂ in the same direction are separated by a distance d. Obtain the expression for the magnetic field `vecB`due to the wire '1' acting on wire '2'. Hence find out, with the help of a suitable diagram, the magnitude and direction of this force per unit length on wire '2' due to wire '1'. How does the nature of this force changes if the currents are in opposite direction? Use this expression to define the S.I. unit of current.

Draw a necessary arrangement for winding of primary and secondary coils in a step-up transformer. State its underlying principle and derive the relation between the primary and secondary voltages in terms of number of primary and secondary turns. Mention the two basic assumptions used in obtaining the above relation.

Define mutual inductance.

In an experiment, two coils c₁ and c₂ are placed close to each other. Find out the expression for the emf induced in the coil c₁ due to a change in the current through the coil c₂.

The current flowing through an inductor of self inductance L is continuously increasing. Plot a graph showing the variation of Magnetic potential energy stored versus the current.

Using the concept of force between two infinitely long parallel current carrying conductors, define one ampere of current.

How does one understand this motional emf by invoking the Lorentz force acting on the free charge carriers of the conductor? Explain.

Two point charges q_A = 3 μC and q_B = −3 μC are located 20 cm apart in vacuum.

1. What is the electric field at the midpoint O of the line AB joining the two charges?
2. If a negative test charge of magnitude 1.5 × 10⁻⁹ C is placed at this point, what is the force experienced by the test charge?

An oil drop of 12 excess electrons is held stationary under a constant electric field of 2.55 × 10⁴ N C⁻¹ (Millikan’s oil drop experiment). The density of the oil is 1.26 g cm⁻³. Estimate the radius of the drop. (g = 9.81 m s⁻²; e = 1.60 × 10⁻¹⁹ C).

A cube of side b has a charge q at each of its vertices. Determine the potential and electric field due to this charge array at the centre of the cube.

If one of the two electrons of a H₂ molecule is removed, we get a hydrogen molecular ion `"H"_2^+`. In the ground state of an `"H"_2^+`, the two protons are separated by roughly 1.5 Å, and the electron is roughly 1 Å from each proton. Determine the potential energy of the system. Specify your choice of zero potential energy.

At room temperature (27.0°C) the resistance of a heating element is 100 Ω. What is the temperature of the element if the resistance is found to be 117 Ω, given that the temperature coefficient of the material of the resistor is 1.70 × 10⁻⁴ °C⁻¹.