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

A 1 KW signal is transmitted using a communication channel which provides attenuation at the rate of – 2dB per km. If the communication channel has a total length of 5 km, the power of the signal received is `["gain in dB" = 10 log ("P"_0/"P"_"i")]`

Hole is ______.

A basic communication system consists of ______.

1. transmitter.
2. information source.
3. user of information.
4. channel.
5. receiver.

Choose the correct sequence in which these are arranged in a basic communication system:

Audio sine waves of 3 kHz frequency are used to amplitude modulate a carrier signal of 1.5 MHz. Which of the following statements are true?

1. The sideband frequencies are 1506 kHz and 1494 kHz.
2. The bandwidth required for amplitude modulation is 6 kHz.
3. The bandwidth required for amplitude modulation is 3 MHz.
4. The side band frequencies are 1503 kHz and 1497 kHz. 

Which of the following would produce analog signals and which would produce digital signals?

1. A vibrating tuning fork.
2. Musical sound due to a vibrating sitar string.
3. Light pulse.
4. Output of NAND gate.

Figure shows a communication system. What is the output power when input signal is of 1.01 mW? (gain in dB = 10 log10 (P_o/P_i).

An audio signal is modulated by a carrier wave of 20 MHz such that the bandwidth required for modulation is 3 kHz. Could this wave be demodulated by a diode detector which has the values of R and C as

1. R = 1 kΩ, C = 0.01 µF.
2. R = 10 kΩ, C = 0.01 µF.
3. R = 10 kΩ, C = 0.1 µF.

An ac voltage V = V₀ sin ωt is applied across a pure inductor of inductance L. Find an expression for the current i, flowing in the circuit and show mathematically that the current flowing through it lags behind the applied voltage by a phase angle of `π/2`. Also draw graphs of V and i versus ωt for the circuit.

Two crystals C₁ and C₂, made of pure silicon, are doped with arsenic and aluminium respectively.

Why is doping of intrinsic semiconductors necessary?

What is meant by doping of an intrinsic semiconductor?

Name the two types of atoms used for doping of Ge/Si.

A current of 0.8 A flows in a conductor of 40 Ω for 1 minute. The heat produced in the conductor will be ______.

Answer the following giving reasons:

Impurities are added to intrinsic semiconductors.

• Assertion (A): The resistance of an intrinsic semiconductor decreases with an increase in its temperature.
• Reason (R): The number of conduction electrons as well as hole increase in an intrinsic semiconductor with the rise in its temperature.

An ideal inductor is connected across an AC source of voltage. The current in the circuit ______.

What is the ratio of inductive and capacitive reactance in an ac circuit?

In a coil of resistance 100 Ω a current is induced by changing the magnetic flux through it. The variation of current with time is shown in the figure. The magnitude of change in flux through the coil is ______.

Using Huygens’s construction of secondary wavelets draw a diagram showing the passage of a plane wavefront from a denser to a rarer medium. Using it verifies Snell’s law.

Define the term drift velocity.

Derive an expression for  drift velocity of free electrons.