Punjab Physics 2023 Paper 1 — Solved MCQs

Question 1

The concept of an electric field was introduced by:

Accepted Answer

B. Faraday — Michael Faraday, a British scientist, introduced the concept of an electric field in the early 19th century. He used the concept to describe the influence that a charged object exerts on its surroundings, even in the absence of other charges.

Question 2

Electric field intensity due to an infinite sheet of charge is:

Accepted Answer

C. Option C — For an infinite sheet of charge, the electric field intensity at a distance r from the sheet is given by E = σ/2ε, where σ is the surface charge density and ε is the permittivity of the medium.

Question 3

The value of drift velocity of electrons is of the order of:

Accepted Answer

C. 10-3ms-1 — The drift velocity of electrons in a conductor, under normal conditions, is on the order of 10^-3 m/s. This means that electrons move very slowly through a conductor in the direction opposite to the applied electric field.

Question 4

Formula for shunt resistance Rs﻿ is:

Accepted Answer

A. Option A — In terms of current, the shunt resistor is used to provide a path for a known fraction of the total current to bypass the main circuit element, such as an ammeter. The total current $I_t$ in the circuit is the sum of the current flowing through the main circuit element (let's call it $I$) and the current flowing through the shunt resistor ($I_s$). The relationship between the total current, the current through the main circuit element, and the current through the shunt resistor can be expressed as:It = I + IsThe current through the shunt resistor can be calculated using Ohm's Law:Is = V/Rswhere $V$ is the voltage across the shunt resistor and $R_s$ is the resistance of the shunt resistor. The voltage across the shunt resistor can be calculated using the total current and the total resistance in the circuit:V = Rt.ItSubstituting the expressions for $I_s$ and $V$ into the equation for total current, we get:It = I + It.Rt/RsSolving for $I$, which is the current through the main circuit element, we find:I = It - It.Rt/RsThis equation gives the current through the main circuit element in terms of the total current, total resistance, and shunt resistance

Question 5

Voltmeter is connected in the circuit in:

Accepted Answer

B. Parallel — A voltmeter is connected in parallel to the component or portion of the circuit across which the voltage is to be measured. This is because a voltmeter is designed to measure the potential difference between two points in a circuit, which requires it to be connected in parallel to those points.

Question 6

The principle of an A.C. generator is based on:

Accepted Answer

D. Faraday’s law of electromagnetic induction — Faraday's law states that a change in magnetic field through a coil of wire induces an emf in the wire. This principle is the fundamental basis for how AC generators produce electricity. As the coil rotates in a magnetic field, the magnetic flux through the coil changes, inducing an emf and generating an alternating current.

Question 7

When the motor is just started back emf always:

Accepted Answer

A. Becomes zero — When a motor is not rotating or is just starting to rotate, there is no relative motion between the coils and the magnetic field. As a result, no emf is induced in the coils, and the back emf is zero.

Question 8

Root mean square value of an alternating voltage is:

Accepted Answer

B. Option B — The rms value of an alternating voltage is given by Vo/√2, where Vo is the peak voltage. This relationship holds for a sinusoidal voltage waveform.

Question 9

Power dissipated in a pure inductor is:

Accepted Answer

A. Zero — In a pure inductor, the voltage and current are out of phase by 90 degrees. This means that when the current is at its maximum (or minimum), the voltage across the inductor is zero, and when the voltage is at its maximum (or minimum), the current through the inductor is zero. As a result, the average power dissipated over a complete cycle is zero.

Question 10

The value of potential barrier for silicon at room temperature is:

Accepted Answer

C. 0.7V — This is the correct value for the potential barrier of silicon at room temperature. Silicon is a semiconductor with a bandgap energy that corresponds to a potential barrier of approximately 0.7V.

Question 11

The ratio of impurity addition in n intrinsic semiconductor is:

Accepted Answer

D. 1 to 106 — Explanation will be added soon

Question 12

SI unit of current gain of transistor is:

Accepted Answer

C. No unit — Current gain is a dimensionless quantity, representing the ratio of output current to input current in a transistor. It is expressed without any specific unit.

Question 13

When platinum wire is heated it appears cherry red at temperature:

Accepted Answer

B. 900°C — This is the correct temperature. At around 900°C, platinum wire begins to emit a red glow, known as cherry red, due to its high melting point and emissivity.

Question 14

A photocell is based on:

Accepted Answer

A. Photoelectric effect — The photoelectric effect is the phenomenon where electrons are emitted from a material when it is exposed to light or electromagnetic radiation of sufficient frequency. Photocells, also known as photoelectric cells, utilize this effect to convert light energy into electrical energy.

Question 15

Normally an electron can reside in excited state for about:

Accepted Answer

D. 10-8s — This is the correct duration. Electrons in excited states typically have lifetimes on the order of nanoseconds (10^-9 seconds) to picoseconds (10^-12 seconds), depending on the specific electronic transition involved.

Question 16

Dead time of the counter is:

Accepted Answer

C. Option C — The dead time of a counter refers to the minimum time interval required between two consecutive detections or events to ensure that each event is counted separately. Its value is 10-4.

Question 17

The building blocks of protons and neutrons are called:

Accepted Answer

A. Quarks — Quarks are elementary particles that combine to form composite particles called hadrons. Protons and neutrons are examples of hadrons, each composed of three quarks.

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