Punjab Pseb Class 12 Physics — Real Previous-Year Papers

In the Bohr model of hydrogen atom, the lowest orbit corresponds to : (a) maximum energy. (b) minimum energy. (c) zero energy. (d) infinite energy.

On doping the conductivity of semi-conductor : (a) increases. (b) remains same. (c) decreases. (d) none of these.

Applying forward bias in p-n junction, the potential barrier : (a) decreases. (b) increases. (c) remains unchanged. (d) becomes zero.

When the distance between the charges is halved, the force between the charges become : (a) half (b) twice (c) four times (d) none of these

If a dielectric is kept between two plates of capacitor, its capacitance : (a) increases (b) decreases (c) First increases then decreases (d) none of these

Wheatstone bridge is used to measure : (a) e.m.f. (b) potential (c) resistance (d) current

The ultimate individual unit of magnetism in any magnet is : (a) north pole (b) south pole (c) magnetic dipole (d) quadrupole

The laws of electromagnetic induction have been used in the construction of : (a) voltmeter (b) ammeter (c) electric motor (d) generator

Diffraction of light is the phenomenon of bending of light around the corners of an aperture in its path. (T/F)

The strength of bar magnet is maximum at its center. (T/F)

Semi-conductor is a device that obeys Ohm's law. (T/F)

The density of nuclear matter is independent of the size of the nucleus. (T/F)

The size of the mirror does not affect the nature of image. (T/F)

Define threshold wavelength and frequency. **OR** Calculate the de-Broglie wavelength for electrons moving with speed of 10⁵ m/s.

Write an expression for mass defect.

Why can't two equipotential surfaces intersect each other ? **OR** Calculate the Coulomb's force between proton and electron separated by distance of 0.8 x 10^-15 m.

What is resistance ? Define SI unit of resistance. **OR** Calculate the resistivity of material of conductor of length 3 m, area of cross section 0.2 mm² having a resistance of 2 ohm.

Derive an expression for magnetic force. How the direction of magnetic force is determined ?

What are the factors which reduce the efficiency of a transformer ?

What is displacement current ?

Find the focal length of combination of two thin lenses in contact. **OR** Two lenses, one diverging of power 2D and the other converging of power 6D are combined together. Calculate the focal length and power of combination.

Write down at least three properties of nuclear force.

With the help of circuit diagram explain V-I characteristics of p-n junction in reverse biasing.

Derive an expression for potential energy of a charged capacitor.

Find the total resistance of a circuit, in which three resistances are connected in series.

Explain Biot Savrat's law for a small current carrying conductor. **OR** A proton enters into a magnetic field of intensity 5 x 10^-2 Tesla with velocity of 10⁵ m/s at an angle of 30 degrees with the field. Calculate the magnitude of force acting on proton due to this field.

Define coefficient of self-induction and find an expression for it for a solenoid. **OR** A wire is cut across a flux of 0.2 x 10^-2 weber in 0.12 seconds. What is the e.m.f. induced in the wire ?

Read the following paragraph and answer the questions given below. Ferromagnetic substances are those which get strongly magnetized when placed in an external magnetic field. They have strong tendency to move from a region of weak magnetic field to strong magnetic field, that is, they get strongly attracted to a magnet. The individual atoms (or ions or molecules) in a ferromagnetic material possess a dipole moment as in a paramagnetic material. However, they interact with one another in such a way that these spontaneously align themselves in a common direction over a macroscopic volume called domain. Each domain has a net magnetization. Typical domain size is 1 mm and the domain contains about 10¹1 atoms. In the first instant the magnetization varies randomly from domain to domain and there is no bulk magnetization. When we apply an external magnetic field B0 the domains orient themselves in the direction of B0 and simultaneously that domain oriented in the direction of B0 grows in size. We may wonder as to what happens when the external field is removed, in some ferromagnetic materials the magnetization persists. Such materials are called hard magnetic materials of hard magnets. Alnico, an alloy of iron, aluminium, nickel, cobalt and copper is one such material. There is a class of magnetic materials in which the magnetization disappears on removal of the external field. Soft iron is one such material. Such materials are called soft ferromagnetic materials. There are number of elements which are ferromagnetic. Iron, cobalt, Nickel, gadolinium etc. The ferromagnetic property depends on temperature. At high enough temperature a ferromagnet becomes a paramagnet. That domain structure disintegrates with temperature. This disappearance of magnetization with the temperature is gradual. Questions : (i) What will be the effect of temperature on ferromagnets ? (ii) What are ferromagnetic substances ? (iii) What is the domain size of ferromagnetic substances ? (iv) How many numbers of atoms are there in one domain ? (v) Write any three examples of ferromagnets.

Using Gauss theorem derive an expression for electric field intensity due to uniformly charged hollow sphere (shell) at a point outside the shell and at a point inside the shell. **OR** Derive an expression for capacitance of parallel plate capacitor with a dielectric slab between the plates.

Write the assumptions, sign conventions and then derive the mirror formula for concave mirror. **OR** What is diffraction of light ? Explain diffraction at single slit and deduce expression for width of its central maxima.