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

A transformer is essentially an a.c. device. It cannot work on d.c. It changes alternating voltages or currents. It does not affect the frequency of a.c. It is based on the phenomenon of mutual induction. A transformer essentially consists of two coils of insulated copper wire having different numbers of turns and wound on the same soft iron core.

The number of turns in the primary and secondary coils of an ideal transformer is 2000 and 50 respectively. The primary coil is connected to a main supply of 120 V and secondary coil is connected to a bulb of resistance 0.6 Ω.

The value of current in the bulb is ______.

A transformer is essentially an a.c. device. It cannot work on d.c. It changes alternating voltages or currents. It does not affect the frequency of a.c. It is based on the phenomenon of mutual induction. A transformer essentially consists of two coils of insulated copper wire having different numbers of turns and wound on the same soft iron core.

The number of turns in the primary and secondary coils of an ideal transformer is 2000 and 50 respectively. The primary coil is connected to a main supply of 120 V and secondary coil is connected to a bulb of resistance 0.6 Ω.

The value of current in primary coil is ______.

A transformer is essentially an a.c. device. It cannot work on d.c. It changes alternating voltages or currents. It does not affect the frequency of a.c. It is based on the phenomenon of mutual induction. A transformer essentially consists of two coils of insulated copper wire having different numbers of turns and wound on the same soft iron core.

The number of turns in the primary and secondary coils of an ideal transformer is 2000 and 50 respectively. The primary coil is connected to a main supply of 120 V and secondary coil is connected to a bulb of resistance 0.6 Ω.

Power in primary coil is ______.

A rectangular conducting loop consists of two wires on two opposite sides of length l joined together by rods of length d. The wires are each of the same material but with cross-sections differing by a factor of 2. The thicker wire has a resistance R and the rods are of low resistance, which in turn are connected to a constant voltage source V₀. The loop is placed in uniform a magnetic field B at 45° to its plane. Find τ, the torque exerted by the magnetic field on the loop about an axis through the centres of rods.

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.

The self inductance L of a solenoid of length l and area of cross-section A, with a fixed number of turns N increases as ______.

Consider an infinitely long wire carrying a current I(t), with `(dI)/(dt) = λ` = constant. Find the current produced in the rectangular loop of wire ABCD if its resistance is R (Figure).

Electrical energy is transmitted over large distances at high alternating voltages. Which of the following statements is (are) correct?

1. For a given power level, there is a lower current.
2. Lower current implies less power loss.
3. Transmission lines can be made thinner.
4. It is easy to reduce the voltage at the receiving end using step-down transformers.

For an LCR circuit, the power transferred from the driving source to the driven oscillator is P = I²Z cos φ.

1. Here, the power factor cos φ ≥ 0, P ≥ 0.
2. The driving force can give no energy to the oscillator (P = 0) in some cases.
3. The driving force cannot syphon out (P < 0) the energy out of oscillator.
4. The driving force can take away energy out of the oscillator.

The line that draws power supply to your house from street has ______.

1. zero average current.
2. 220 V average voltage.
3. voltage and current out of phase by 90°.
4. voltage and current possibly differing in phase `phi` such that `|phi| < pi/2`.

Draw the effective equivalent circuit of the circuit shown in figure, at very high frequencies and find the effective impedance.

A 60 W load is connected to the secondary of a transformer whose primary draws line voltage. If a current of 0.54 A flows in the load, what is the current in the primary coil? Comment on the type of transformer being used.

1 MW power is to be delivered from a power station to a town 10 km away. One uses a pair of Cu wires of radius 0.5 cm for this purpose. Calculate the fraction of ohmic losses to power transmitted if

1. power is transmitted at 220 V. Comment on the feasibility of doing this.
2. a step-up transformer is used to boost the voltage to 11000 V, power transmitted, then a step-down transfomer is used to bring voltage to 220 V. (ρ_Cu = 1.7 × 10^–8 SI unit)

Photons absorbed in matter are converted to heat. A source emitting n photon/sec of frequency ν is used to convert 1 kg of ice at 0°C to water at 0°C. Then, the time T taken for the conversion ______.

1. decreases with increasing n, with ν fixed.
2. decreases with n fixed, ν increasing.
3. remains constant with n and ν changing such that n ν = constant.
4. increases when the product n ν increases.