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

The relaxation time τ is nearly independent of applied E field whereas it changes significantly with temperature T. First fact is (in part) responsible for Ohm’s law whereas the second fact leads to variation of ρ with temperature. Elaborate why?

1. Consider circuit in figure. How much energy is absorbed by electrons from the initial state of no current (ignore thermal motion) to the state of drift velocity?
2. Electrons give up energy at the rate of RI² per second to the thermal energy. What time scale would one associate with energy in problem (a)? n = no of electron/volume = 10²⁹/m³, length of circuit = 10 cm, cross-section = A = (1mm)²

A paramagnetic sample shows a net magnetisation of 8 Am^–1 when placed in an external magnetic field of 0.6 T at a temperature of 4 K. When the same sample is placed in an external magnetic field of 0.2 T at a temperature of 16 K, the magnetisation will be ______.

S is the surface of a lump of magnetic material.

1. Lines of B are necessarily continuous across S.
2. Some lines of B must be discontinuous across S.
3. Lines of H are necessarily continuous across S.
4. Lines of H cannot all be continuous across S.

A long solenoid has 1000 turns per metre and carries a current of 1 A. It has a soft iron core of µ_r = 1000. The core is heated beyond the Curie temperature, T_c.

1. The H field in the solenoid is (nearly) unchanged but the B field decreases drastically.
2. The H and B fields in the solenoid are nearly unchanged.
3. The magnetisation in the core reverses direction.
4. The magnetisation in the core diminishes by a factor of about 10⁸.

Essential difference between electrostatic shielding by a conducting shell and magnetostatic shielding is due to ______.

1. electrostatic field lines can end on charges and conductors have free charges.
2. lines of B can also end but conductors cannot end them.
3. lines of B cannot end on any material and perfect shielding is not possible.
4. shells of high permeability materials can be used to divert lines of B from the interior region.

A permanent magnet in the shape of a thin cylinder of length 10 cm has M = 10⁶ A/m. Calculate the magnetisation current I_M.

Explain quantitatively the order of magnitude difference between the diamagnetic susceptibility of N₂ (~5 × 10^–9) (at STP) and Cu (~10^–5).

From molecular view point, discuss the temperature dependence of susceptibility for diamagnetism, paramagnetism and ferromagnetism.

There are two coils A and B as shown in figure. A current starts flowing in B as shown, when A is moved towards B and stops when A stops moving. The current in A is counterclockwise. B is kept stationary when A moves. We can infer that ______.

Same as problem 4 except the coil A is made to rotate about a vertical axis (figure). No current flows in B if A is at rest. The current in coil A, when the current in B (at t = 0) is counterclockwise and the coil A is as shown at this instant, t = 0, is ______.

Consider a magnet surrounded by a wire with an on/off switch S (Figure). If the switch is thrown from the off position (open circuit) to the on position (closed circuit), will a current flow in the circuit? Explain.

A wire in the form of a tightly wound solenoid is connected to a DC source, and carries a current. If the coil is stretched so that there are gaps between successive elements of the spiral coil, will the current increase or decrease? Explain.

A solenoid is connected to a battery so that a steady current flows through it. If an iron core is inserted into the solenoid, will the current increase or decrease? Explain.

Consider a metal ring kept on top of a fixed solenoid (say on a carboard) (Figure). The centre of the ring coincides with the axis of the solenoid. If the current is suddenly switched on, the metal ring jumps up. Explain

Consider a metal ring kept (supported by a cardboard) on top of a fixed solenoid carrying a current I (Figure). The centre of the ring coincides with the axis of the solenoid. If the current in the solenoid is switched off, what will happen to the ring?

A conducting wire XY of mass m and neglibile resistance slides smoothly on two parallel conducting wires as shown in figure. The closed circuit has a resistance R due to AC. AB and CD are perfect conductors. There is a ˆ. magnetic field `B = B(t)hatk`.

1. Write down equation for the acceleration of the wire XY.
2. If B is independent of time, obtain v(t) , assuming v(0) = u₀.
3. For (b), show that the decrease in kinetic energy of XY equals the heat lost in R.

A metallic ring of mass m and radius `l` (ring being horizontal) is falling under gravity in a region having a magnetic field. If z is the vertical direction, the z-component of magnetic field is B_z = B_o (1 + λz). If R is the resistance of the ring and if the ring falls with a velocity v, find the energy lost in the resistance. If the ring has reached a constant velocity, use the conservation of energy to determine v in terms of m, B, λ and acceleration due to gravity g.

A long solenoid ‘S’ has ‘n’ turns per meter, with diameter ‘a’. At the centre of this coil we place a smaller coil of ‘N’ turns and diameter ‘b’ (where b < a). If the current in the solenoid increases linearly, with time, what is the induced emf appearing in the smaller coil. Plot graph showing nature of variation in emf, if current varies as a function of mt² + C.

When an AC voltage of 220 V is applied to the capacitor C ______.

1. the maximum voltage between plates is 220 V.
2. the current is in phase with the applied voltage.
3. the charge on the plates is in phase with the applied voltage.
4. power delivered to the capacitor is zero.