Physics: CUET Mock Test - 7 - CUET MCQ

The process in which an electron escapes from the metal surface is known as electron emission. Electrons are loosely bound to the nucleus, and therefore, a little energy push sets these electrons flying out of their respective orbits.

Concept:

Photodiode:

• A photo diode is a semiconductor device that converts light into electrical current.
• It is commonly used in remote controls to detect signals from an infrared (IR) .

Zener Diode:

• A Zener diode is a type of diode that is designed to operate in the reverse breakdown region of its voltage-current characteristic curve. This makes it useful as a voltage regulator, maintaining a constant voltage across a load even as the input voltage varies.

LED:

• A Light Emitting Diode (LED) is a semiconductor device that emits light when a current is passed through it. It is commonly used as an indicator light in electronic devices.

Transistor:

• A transistor is a semiconductor device that can be used as an amplifier or a switch. It is commonly used as an amplifier to amplify weak signals or as a switch to control the flow of current in electronic circuits.

A - IV
B - III
C - I
D - II

The correct answer is option (2)

Concept:

A transistor:

• A transistor can be used as a switch or as an amplifier, but not as a rectifier.
• A rectifier is a device that converts alternating current (AC) to direct current (DC) by allowing current to flow in only one direction.
• A transistor can be used as an amplifier, amplifying a small signal into a larger one, or as a switch, allowing or blocking the flow of current.

Zener diode:

• A Zener diode is a type of diode that is designed to operate in the reverse breakdown region of its voltage-current characteristic curve.
• This makes it useful as a voltage regulator, maintaining a constant voltage across a load even as the input voltage varies.

NOT Gate:

• A NOT gate is not a universal gate, as it cannot be used to implement all possible Boolean functions. In contrast, NAND and NOR gates are universal gates, as they can be used to implement any Boolean function.

A photodiode:

• A photodiode is a type of semiconductor device that converts light into electrical current.
• It is not typically used as an oscillator, which is a device that generates a repetitive signal without the need for an external input signal.

From the above, it is clear that statements (B) and (D) are correct.

So, the correct answer is option (2).

Concept:

Solar Cell:

• A solar cell is a semiconductor device that converts sunlight into electricity. The I-V (current-voltage) characteristics of a solar cell show how the current generated by the cell varies with the voltage across it.
• The solar cell operates in the photovoltaic mode, where it generates a voltage when light falls on it. When the solar cell is connected to an external load, the voltage generated by the cell pushes the electrons through the load, which generates a current.
• The I-V characteristics of a solar cell show that the current generated by the cell is directly proportional to the incident light intensity, and the voltage generated by the cell is proportional to the number of cells connected in series.

LED:

• A light-emitting diode (LED) is a semiconductor device that emits light when a current is passed through it.
• The I-V characteristics of an LED show that it is a non-linear device, where the current through the LED increases exponentially with the applied voltage.
• Once the voltage reaches a certain threshold value, the LED starts emitting light.
• The I-V characteristics of an LED also show that the current through the LED is very sensitive to changes in the applied voltage.
• This means that even small variations in the applied voltage can cause large changes in the current through the LED.

Zener Diode:

• A Zener diode is a specially designed diode that operates in the reverse breakdown region.
• The I-V characteristics of a Zener diode show that it has a very low breakdown voltage, typically in the range of a few volts.
• Once the breakdown voltage is reached, the Zener diode starts conducting in the reverse direction, and the current through the diode increases rapidly.
• The I-V characteristics of a Zener diode are very sharp, which makes it useful as a voltage reference or voltage regulator.

Photo Diode:

• A Photodiode is again a special-purpose p-n junction diode fabricated with a transparent window to allow light to fall on the diode.
• It is operated under reverse bias. When the photodiode is illuminated with light (photons) with energy (hν) greater than the energy gap (Eg ) of the semiconductor, then electron-hole pairs are generated due to the absorption of photons.
• The diode is fabricated such that the generation of e-h pairs takes place in or near the depletion region of the diode.
• Due to the electric field of the junction, electrons and holes are separated before they recombine.
• The direction of the electric field is such that electrons reach the n-side and holes reach the p-side. Electrons are collected on the n-side and holes are collected on p-side giving rise to an emf. When an external load is connected, current flows.
• The magnitude of the photocurrent depends on the intensity of incident light (photocurrent is proportional to incident light intensity)

The correct answer is option (1)

Concept:

NOR Gate:

• It is a digital circuit having two or more inputs but only one output.
• It gives a high output if both the inputs are low (0) otherwise it gives a Low output (0).
• It is described by the Boolean expression:
• The above logic gate is the NOR gate.

The truth table for NOR gate:

Explanation:

We know that

This can be compared with two inputs AND Gate.

The correct answer is option (2)

Concept:

• The process of superimposing a message signal with a carrier wave is known as modulation.
• In modulation, the message signal is used to modify the characteristics of the carrier wave, such as its amplitude, frequency or phase, in order to carry information.
• The resulting modulated wave can then be transmitted through a communication channel, such as a radio frequency transmission system, where it can be received and demodulated to recover the original message signal.
• Modulation is a key technique used in many forms of communication, including radio, television, and digital communications.

Additional Information Need for Modulation:
1. It is needed in order to match the characteristics of the input signal to that of channel characteristics.
2. Multiplexing is made possible due to modulation.
3. Wireless transmission of low-pass signal is made possible due to modulation.
4. Practically realization antenna size can be achieved
5. Some modulation technique reduces the effect of noise on the signal.

Important PointsThe AM, FM, and PM output waveforms are as shown:

The correct answer is option 4):

Concept:

Faraday's first law of electromagnetic induction:

Whenever a conductor is placed in a varying magnetic field, an electromotive force is induced. If the conductor circuit is closed, a current is induced which is called an induced current

​Faraday's second law of electromagnetic induction:

The induced emf in a coil is equal to the rate of change of flux linked with the coil

Where dΦ = change in magnetic flux and e = induced e.m.f.

The negative sign says that it opposes the change in magnetic flux which is explained by Lenz law.

Concept:

Amplitude modulation (AM):

The process of changing the amplitude of a carrier wave in accordance with the amplitude of the audio frequency (AF) signal is known as amplitude modulation (AM).

In this kind of modulation, the amplitude of the carrier signal is modified according to the instantaneous amplitude of the base-band signal. In AM the frequency and the phase of the carrier signal remain constant. Amplitude modulation is used in broadcasting (radio and TV).

Sideband frequencies:

The AM wave contains three frequencies f_c, (f_c + f_m), and (f_c - f_m)

where f_c is called carrier frequency, (f_c + f_m) and (f_c - f_m) are called sideband frequencies. Sideband frequencies are close to the carrier frequency.

(f_c + f_m): Upper sideband (USB) frequency

(f_c - f_m): Lower sideband (LSB) frequency

Explanation:

Given:

Frequency of Audio sine waves, f_m = 3 kHz

Frequency of carrier signal, f_c = 1.5 MHz = 1500 kHz

Sideband frequencies are-(f_c + f_m) and (f_c - f_m)

= (1500+3) and (1500-3)

= 1503 kHz and 1497 kHz

Hence, option (4) is the correct answer.

For an inverting amplifier, as shown above, the voltage gain is given by:

The ideal opamp will be having virtual ground and the current entering the op-amp will be 0.

Therefore, by applying KCL at the inverting terminal of opamp we get:

The correct answer is (A) is false, but (R) is true.

Concept:

Scalar quantity

• A scalar quantity is a physical quantity that only has magnitude.
• It has no direction.
• A scalar quantity is represented by a number only.
• For example – Energy, volume, mass, electric current, distance, speed, temperature, area.

Vector quantity

• A vector quantity is a physical quantity that has both a magnitude and a direction.
• For example – Torque, weight, acceleration, displacement, force, momentum, angular velocity.

Explanation:

• Electric current does have a direction: It flows from a point of high potential to a point of low potential. However, it is still considered a scalar quantity and not a vector quantity. Because vector quantities behave and are added together.
• The addition of vector quantities follows specific rules called vector addition.
• When two vector quantities are added, the result depends not just on their magnitudes but also on their directions.
• For instance, if you're traveling north and then east, your resulting direction is different than if you traveled east and then north.
• In contrast, the magnitude of the current at any point in a circuit is the sum of the currents through that point, regardless of their directions.
• This is the main rule for adding scalar quantities, not vectors.
• Thus, despite having a direction, electric current does not abide by the laws of vector addition and is therefore considered a scalar quantity.

This is why statements, (A) Electric current is a vector quantity is false, and (R) Electric current is a quantity having magnitude as well as direction is true.

Concept:

• Biot-Savarts Law: The law that gives the magnetic field generated by a constant electric current is Biot-savarts law.
• Let us take a current-carrying wire of current I and we need to find the magnetic field at a distance r from the wire then it is given by:
  

Where μ0= 4π × 10-7 T.m/A is the permeability of free space/vacuum, dl = small element of wire and r ̂ is the unit position vector of the point where we need to find the magnetic field.

Explanation:

From the above expression of the Biot-savart law, the magnetic field is:

• Directly proportional to the length of the wire. So statement (i) is correct.
• Directly proportional to the electric current. So statement (iii) is correct.
• Biot-savart law gives the magnetic field, not the electric field. So statement (ii) is wrong.

There are four types of electron emissions, namely, thermionic emission, photoelectric emission, secondary emission, and field emission. These are the different methods of producing electron emissions.

Only transverse waves show the property of polarization. Their vibrations can occur in all directions perpendicular to the direction of travel. The longitudinal, infrared, and microwaves do not show the property of polarization.

tan i_p = μ = √3.
i_p = tan^-1(√3)
i_p = 60^o
Therefore, the polarization angle of a medium of refractive index √3 is 60^o.

An electric field vector is used to represent the polarization of an em wave. It is perpendicular to the plane of propagation of the light wave. Therefore, a vertically polarized electromagnetic wave of wavelength λ has its electric field vector oscillating in the vertical direction.

Plane of vibration, the plane of polarization, and direction of propagation of the light wave are mutually perpendicular to the polarized light wave in a plane. The other factors are not mutually perpendicular to the polarized light wave in a plane.

The plane containing the direction of propagation of light and perpendicular to the plane of vibration is called the plane of polarization. It contains no vibrations. Polarized light waves are light waves in which the vibrations occur in a single plane.

Polaroids absorb only that part of the light which produces a dazzling effect in the eye. But colored glasses absorb more light incident on them and so the image appears dim. Therefore, sunglasses are made of Polaroids.

As i_p + r_p = 90^o
r_p = 90^o – 63.8^o
r_p = 26.2^o
Therefore, the angle of refraction in the glass is 26.2^o.

Coherent light is a form of light whose photons share the same frequency and whose wavelengths are in phase with one another. The phase difference between the waves should be constant in case of coherent sources.

LED is the odd one out. LED is short for light-emitting diode. LED is not a coherent source, whereas, others are examples of coherent sources. The light emitted from an LED is neither spectrally coherent nor even highly monochromatic.

Coherent waves are the waves with the same frequency and the wavelength of the waves are in phase as well. Therefore, the phase difference is constant. But the coherent waves do not have the same amplitude. Since the amplitude is different, there will be no complete constructive interference where they meet, so they will contribute poorly to an interference pattern.

Coherence length is defined as the propagation distance over which a coherent wave maintains a specified degree of coherence. Wave interference is strong when the paths taken by all of the interfering waves are lesser than the coherence length.

The formula for calculating coherence time is given as:

Where Τ_c is the coherence time, λ is the wavelength, ∆λ is the spectral width of the source, and c is the speed of light in a vacuum (i.e. 3 × 10⁸ m/s).

LASER is the short form for Laser Amplification by Stimulated Emission Radiation. The amplified light beam coming out of a LASER is essentially due to the emission of electrons stimulated by incident radiation consisting of photons. As a result, this causes the coherent behavior of the LASER beam.

Light of shorter wavelengths is scattered much more than light of longer wavelengths. The amount of scattering is inversely proportional to the fourth power of the wavelength. This is known as Rayleigh scattering.

Generally, there are two kinds of rainbows, namely primary rainbow and secondary rainbow. The primary rainbow occurs due to one internal reflection and two refractions, whereas secondary rainbow occurs due to two internal reflections and two refractions, from the water drops suspended in the air.

During sunrise and sunset, the rays have to travel a large part of the atmosphere since the rays are close to the horizon at that time. That is why, red being the least scattered enters our eyes, as all the other colors get scattered away.

A rainbow is an example of a dispersion of sunlight by the water drops in the atmosphere. This is a phenomenon due to the combined effect of dispersion, refraction, and reflection of sunlight by spherical water droplets of rain. Others all are common illustrations of scattering of light.

A prism is a homogenous, transparent medium enclosed two plane surfaces inclined at an angle. These surfaces are called the refracting surfaces and the angle between the incident ray and emergent ray is known as the angle of deviation.