Science (English Medium) इयत्ता १२ - CBSE Question Bank Solutions

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.

There are two sources of light, each emitting with a power of 100 W. One emits X-rays of wavelength 1 nm and the other visible light at 500 nm. Find the ratio of number of photons of X-rays to the photons of visible light of the given wavelength?

The simple Bohr model is not applicable to He 4 atom because ______.

1. He⁴ is an inert gas.
2. He⁴ has neutrons in the nucleus.
3. He⁴ has one more electron.
4. electrons are not subject to central forces.

Imagine removing one electron from He⁴ and He³. Their energy levels, as worked out on the basis of Bohr model will be very close. Explain why.

If the magnetizing field on a ferromagnetic material is increased, its permeability ______.

A uniform magnetic field gets modified as shown in figure when two specimens A and B are placed in it.

(a)	(b)

1. Identify the specimen A and B.
2. How is the magnetic susceptibility of specimen A different from that of specimen B?

The curve of binding energy per nucleon as a function of atomic mass number has a sharp peak for helium nucleus. This implies that helium nucleus is ______.

• Assertion (A): Diamagnetic substances exhibit magnetism.
• Reason (R): Diamagnetic materials do not have a permanent magnetic dipole moment.

Define the critical angle for a given pair of media and total internal reflection. Obtain the relation between the critical angle and refractive index of the medium.

Calculate the self-inductance of a coil using the following data obtained when an AC source of frequency `(200/pi)` Hz and a DC source are applied across the coil.

A point object in the air is placed symmetrically at a distance of 60 cm in front of a concave spherical surface with a refractive index of 1.5. If the radius of curvature of the surface is 20 cm, find the position of the image formed.

The energy of a photon of wavelength 663 nm is ______.

Which of the following has a permeability less than that of free space?

The energy of a photon of wavelength λ is ______.

• Assertion (A): The deflecting torque acting on a current-carrying loop is zero when its plane is perpendicular to the direction of the magnetic field.
• Reason (R): The deflecting torque acting on a loop of the magnetic moment `vecm` in a magnetic field `vecB` is given by the dot product of `vecm` and `vecB`.

A current of 1A flows through a coil when it is connected across a DC battery of 100V. If the DC battery is replaced by an AC source of 100 V and angular frequency of 100 rad s^-1, the current reduces to 0.5 A. Find

1. the impedance of the circuit.
2. self-inductance of coil.
3. Phase difference between the voltage and the current.

The primary coil having N_P turns of an ideal transformer is supplied with an alternating voltage V_P. Obtain an expression for the voltage V_S induced in its secondary coil having N_S turns.