An Overview of Ncert Books Class 12 Physics Chapter 15 Free Download
FAQs on Ncert Books Class 12 Physics Chapter 15 Free Download
1. What types of questions are commonly considered important from the Communication Systems chapter for the CBSE Class 12 Physics exam (2025-26)?
For the CBSE Class 12 Physics board exam, important questions from Communication Systems typically include:
Definitions (1-mark): Questions asking to define terms like transducer, attenuation, bandwidth, or modulation.
Block Diagrams (2-3 marks): Drawing and labelling the block diagram of a basic communication system is a frequently asked question.
Conceptual Questions (2-3 marks): Questions probing the 'why' and 'how,' such as the necessity of modulation.
Derivations (3-marks): Deriving the expression for an amplitude-modulated wave.
Distinctions (2-marks): Differentiating between concepts like sky wave and space wave propagation.
2. What are the functions of a transmitter and a repeater in a communication system?
In a communication system, the transmitter processes the incoming message signal to make it suitable for transmission through a channel. It modulates the message signal onto a high-frequency carrier wave. A repeater is a combination of a receiver and a transmitter. It is used to extend the range of communication by picking up the attenuated signal, amplifying it, and then re-transmitting it towards the receiver.
3. Why is modulation essential for transmitting low-frequency audio signals over long distances?
Modulation is essential for several key reasons:
Practical Antenna Size: For effective transmission, the antenna size should be comparable to the wavelength (λ) of the signal. Low-frequency signals have very large wavelengths, requiring impractically large antennas. Modulation shifts the signal to a higher frequency (lower wavelength), allowing for smaller, practical antennas.
Avoids Signal Mixing: Without modulation, signals from different transmitters would use the same frequency range, leading to interference and mixing. Modulation allows different signals to be assigned unique carrier frequency bands, preventing overlap.
Effective Power Radiation: The power radiated by an antenna is very low for low-frequency signals. By modulating to a higher frequency, the signal can be radiated with much greater power and efficiency.
4. Draw a labelled block diagram showing the essential elements of a basic communication system.
A basic communication system consists of three essential elements: a Transmitter, a Channel, and a Receiver. The transmitter converts the message from the information source into a signal suitable for the channel. The channel is the physical medium (like free space, wires, or optical fibre) that carries the signal. The receiver reconstructs a recognisable form of the original message from the received signal. Noise is an unwanted signal that can corrupt the message anywhere along the channel.
5. What is Amplitude Modulation (AM)? Write the expression for an amplitude-modulated wave.
Amplitude Modulation (AM) is a process where the amplitude of a high-frequency carrier wave is varied in accordance with the instantaneous amplitude of the message signal, while the frequency and phase of the carrier wave remain constant.
If the carrier wave is c(t) = A_c sin(ω_c t) and the message signal is m(t) = A_m sin(ω_m t), the expression for the amplitude-modulated wave C_m(t) is given by:
C_m(t) = (A_c + A_m sin(ω_m t)) sin(ω_c t).
6. How does sky wave propagation fundamentally differ from space wave propagation?
The fundamental difference lies in their mechanism and frequency range:
Sky Wave Propagation: This mode is used for radio waves in the 2 MHz to 30 MHz frequency range. The waves are transmitted towards the sky and are reflected back to Earth by the ionosphere. This allows for long-distance, over-the-horizon communication.
Space Wave Propagation: This mode is used for frequencies above 40 MHz (e.g., TV broadcast, satellite communication). The waves travel in a straight line from the transmitter to the receiver, known as line-of-sight (LOS) communication. They are not reflected by the ionosphere and can penetrate it.
7. A carrier wave of frequency 1.2 MHz is amplitude-modulated by a message signal of frequency 12 kHz. What will be the frequencies of the sidebands and the required bandwidth for this wave?
For an amplitude-modulated wave, two sidebands are produced:
The Upper Sideband (USB) frequency is the sum of the carrier and message signal frequencies: f_USB = f_c + f_m = 1.2 MHz + 12 kHz = 1200 kHz + 12 kHz = 1212 kHz.
The Lower Sideband (LSB) frequency is the difference between the carrier and message signal frequencies: f_LSB = f_c - f_m = 1200 kHz - 12 kHz = 1188 kHz.
The bandwidth required is the difference between the highest and lowest frequencies, which is twice the message signal frequency: Bandwidth = 2 × f_m = 2 × 12 kHz = 24 kHz.
8. Why can't ground waves be used for high-frequency communication over long distances?
Ground waves are unsuitable for high-frequency, long-distance communication due to a phenomenon called attenuation. As ground waves propagate along the surface of the Earth, they induce currents in the ground and lose energy through absorption. This energy loss increases rapidly with an increase in the frequency of the wave. For frequencies above a few megahertz, the absorption is so high that the wave gets attenuated within a very short distance from the transmitter, making it impractical for long-range use.
9. How is an amplitude-modulated wave detected?
An amplitude-modulated wave is detected using a process called demodulation. A simple detector circuit consists of a rectifier and an envelope detector. The AM signal is first passed through a rectifier (like a diode), which allows only the positive half of the wave to pass through. This rectified signal is then fed to an envelope detector, typically an RC circuit. The capacitor charges and discharges in a way that it follows the 'envelope' of the rectified wave, which corresponds to the original message signal. This successfully extracts the low-frequency message from the high-frequency carrier.
