Punjab Physics 2017 Paper 3 — Solved MCQs

Question 1

The force experienced by a unit +ve charge placed at a point in an electric field is called as:

Accepted Answer

D. Electric field intensity — The electric field intensity, often denoted by E, is the force experienced by a unit positive charge placed at a point in an electric field. It represents the strength of the electric field at that point and is directed in the direction of the field lines. Mathematically, it is expressed as E = F / q, where E is the electric field intensity, F is the force experienced by the charge, and q is the magnitude of the charge.

Question 2

The unit of RC is:

Accepted Answer

B. Second — In the context of RC circuits, RC represents the time constant, which is the product of the resistance (R) and the capacitance (C). The unit of resistance is ohms (Ω), and the unit of capacitance is farads (F), so the unit of RC is ohm-farads (Ω·F). However, the time constant RC itself is in seconds (s), as it represents the time taken for the voltage across the capacitor in an RC circuit to reach approximately 63.2% of its final value during charging or discharging.

Question 3

Kirchhoff's first rule is manifestation of law of conservation of:

Accepted Answer

D. Charge — Kirchhoff's first rule, or Kirchhoff's current law, is a manifestation of the law of conservation of electric charge. This law states that the total electric charge in an isolated system remains constant over time, meaning that charge is neither created nor destroyed. Kirchhoff's first rule specifically applies to electrical circuits, where it states that the algebraic sum of currents entering and exiting a junction (or node) in a circuit is zero, based on the conservation of charge.

Question 4

Electrical energy is measured in:

Accepted Answer

C. Kilowatt hour — A kilowatt hour (kWh) is a unit of energy. It is equal to the amount of energy used by a one-kilowatt electrical device in one hour. Kilowatt hours are commonly used to measure electrical energy consumption in homes and businesses.

Question 5

A charge particle moving in magnetic field experiences a force given by:

Accepted Answer

D. D — Explanation will be added soon!

Question 6

The SI unit of induced emf is:

Accepted Answer

C. Volt — The volt (V) is the SI unit of electromotive force (emf) or electric potential difference. It measures the amount of work required to move a unit charge between two points in an electric circuit. Induced emf is also measured in volts.

Question 7

If we make the magnetic field stronger, the value of induced current is:

Accepted Answer

B. Increased — An increase in the magnetic field strength leads to a higher rate of change of magnetic flux through a conductor. This increased rate of change of magnetic flux induces a greater emf in the conductor, resulting in a larger induced current. Therefore, the induced current is increased when the magnetic field strength is increased.

Question 8

Power dissipation in a pure inductive circuit is:

Accepted Answer

B. Zero — In a pure inductive circuit, the power dissipation is indeed zero. This is because in an ideal inductor, the voltage and current are out of phase by 90 degrees. This means that the power dissipated as heat, which is given by P = I2 * R, where I is the current and R is the resistance, is zero since R is zero in an ideal inductor.

Question 9

A device through which direct current cannot flow is:

Accepted Answer

B. Capacitor — A capacitor is a passive electronic component that stores energy in an electric field. It consists of two conductive plates separated by an insulating material (dielectric). A capacitor blocks DC due to its ability to store charge. When a DC voltage is applied, the capacitor charges up and blocks any further flow of DC, behaving like an open circuit to DC. However, capacitors allow alternating current (AC) to pass through, as they charge and discharge with the changing polarity of the AC signal.

Question 10

The magnetism produced by electrons within an atom is due to their:

Accepted Answer

C. Both spin and orbital motion — The total magnetic moment of an atom is the vector sum of the magnetic moments due to the spin and orbital motion of its electrons. Therefore, the magnetism produced by electrons within an atom is due to both their spin and orbital motion.

Question 11

Conversion of AC into DC is called as:

Accepted Answer

A. Rectification — Rectification is the process of converting alternating current (AC) into direct current (DC). This is typically done using a device called a rectifier, which allows current to flow in only one direction.

Question 12

The potential barrier for silicon is:

Accepted Answer

C. 0.7 V — The potential barrier for silicon is often considered to be around 0.7 V. This value represents the energy barrier that must be overcome for a charge carrier (such as an electron or hole) to move across the junction between a p-type and an n-type semiconductor.

Question 13

Aging process of the human body is slowed down by motion at very high speed is predicted by?

Accepted Answer

B. Einstein — Albert Einstein's theory of special relativity predicts that time dilation occurs as objects move at speeds approaching the speed of light. This means that time appears to pass more slowly for an object in motion relative to an observer at rest. This prediction has been experimentally confirmed and is known as the "twin paradox," where one twin ages more slowly than the other due to differences in their motion.

Question 14

Due to the annihilation of electron and positron, the number of photons produced is:

Accepted Answer

B. 2 — The annihilation of an electron and a positron produces two photons. This is because the total energy and momentum of the initial particles must be conserved in the process. The two photons produced move in opposite directions to conserve momentum, and each carries energy equivalent to the rest mass energy of the electron or positron.

Question 15

Helium Neon laser discharge tube contains Neon?

Accepted Answer

A. 15% — This is the correct percentage of neon in a helium-neon laser discharge tube. The majority of the gas in the tube is helium, which is used as the primary gas to create the discharge. Neon is added in a smaller percentage (around 18%) to help stabilize the discharge and enhance the laser's efficiency and output.

Question 16

The number of protons in any atom are equal to the number of:

Accepted Answer

A. Electrons — In an atom, the number of protons is equal to the number of electrons. This equality ensures that the atom is electrically neutral, as the positive charge of the protons is balanced by the negative charge of the electrons.

Question 17

Geiger Muller counter can be used to detect:

Accepted Answer

D. Nuclear radiations — A Geiger-Muller counter is a type of radiation detector commonly used to detect and measure ionizing radiation, including alpha and beta particles, as well as gamma rays. It operates by detecting the ionization produced by the radiation in a gas-filled tube, which triggers a measurable electrical pulse.

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