Chapter 1: Electric Charges and Fields Short Questions | physics Class 12 CBSE

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Chapter 1: Electric Charges and Fields Short Questions | physics Class 12 CBSE | ByteNotes.in

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Question 1

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What is meant by the term 'electrostatics'? State two everyday phenomena that are explained by it.

Electrostatics deals with the study of forces, fields, and potentials arising from electric charges at rest; the term 'static' refers to the fact that the charges do not move.
Sparks or crackling sounds when removing synthetic clothes in dry weather are due to static charge accumulated by friction being discharged through the body.
Lightning during thunderstorms is a large-scale electric discharge between charged clouds or between clouds and the earth, caused by accumulated static charge.

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Question 2

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State the two types of electric charges. What happens when two like charges and two unlike charges are brought near each other?

There are exactly two types of electric charge: positive charge (e.g., on a glass rod rubbed with silk) and negative charge (e.g., on a plastic rod rubbed with cat's fur).
Like charges (both positive or both negative) repel each other; when two glass rods each rubbed with silk are brought close, they push each other away.
Unlike charges (one positive and one negative) attract each other; when a glass rod rubbed with silk is brought near a plastic rod rubbed with fur, they attract.

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Question 3

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Explain how charge is transferred when a glass rod is rubbed with silk, and identify what type of charge each acquires.

When a glass rod is rubbed with silk, electrons from the glass rod are transferred to the silk cloth because electrons in the glass are less tightly bound.
The glass rod loses electrons and therefore acquires a positive charge, while the silk cloth gains electrons and therefore acquires a negative charge.
No new charge is created in this process; only a transfer of existing electrons occurs, consistent with the law of conservation of charge.

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Question 4

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Distinguish between conductors and insulators with one example each. How does charge behave differently on them?

Conductors are materials with free electrons or ions that allow easy passage of electricity; examples include metals, the human body, and the earth. Insulators like glass, plastic, nylon, and wood have no free charge carriers and resist electricity.
When charge is transferred to a conductor, it spreads instantaneously over the entire surface of the conductor due to the mobility of free charges.
When charge is placed on an insulator, it remains localised at the point where it was placed and does not spread, because there are no free charge carriers to transport it.

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Question 5

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State the property of additivity of electric charge. How does it differ from the additivity of mass?

Additivity of charge means that the total charge of a system is the algebraic sum of all individual charges in the system, i.e., Q = q₁ + q₂ + q₃ + ... + qₙ, with proper signs taken into account.
Both charge and mass are scalars (have magnitude but no direction), but mass is always positive, whereas charge can be positive, negative, or zero.
Because charge can be negative, the total charge of a system can be less than, equal to, or greater than any individual charge — unlike mass where the total is always the sum of positive values.

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Question 6

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State the law of conservation of electric charge. Give one example from nature where charge is conserved during particle creation.

The law of conservation of electric charge states that the total electric charge of an isolated system always remains constant; charge can neither be created nor destroyed, only transferred from one body to another.
When a neutron decays into a proton and an electron, the total charge before (zero, for the neutron) equals the total charge after (+e for the proton and −e for the electron), which sums to zero.
This shows that even when new charged particles are created in nuclear processes, the algebraic sum of all charges is conserved, confirming that charge creation always happens in equal and opposite pairs.

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Question 7

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What is quantisation of electric charge? What is the value of the basic unit of charge and who demonstrated it experimentally?

Quantisation of charge means that the electric charge on any body is always an integral multiple of a basic unit of charge e, expressed as q = ne, where n is any positive or negative integer.
The basic unit of charge is e = 1.602192 × 10⁻¹⁹ C, which is the magnitude of charge carried by an electron (−e) or a proton (+e).
Quantisation of charge was first suggested by Faraday's laws of electrolysis and was experimentally demonstrated by Millikan in 1912 through his famous oil drop experiment.

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Question 8

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At macroscopic scales, why can the quantisation of electric charge be ignored? When does it become significant?

At the macroscopic level, we deal with charges of a few microcoulombs, which correspond to something like 10¹³ times the elementary charge e; at this scale, charge appears to vary continuously rather than in discrete steps of e.
The step size e = 1.6 × 10⁻¹⁹ C is so small compared to macroscopic charges that increasing or decreasing by one unit of e produces no perceptible difference, so quantisation can be practically ignored.
Quantisation becomes significant at the microscopic level, where the charges involved are of the order of a few tens or hundreds of e, i.e., when individual electrons or protons are being considered and counted.

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Question 9

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State Coulomb's law for the force between two point charges. Write the expression in both scalar and vector form.

Coulomb's law states that the electrostatic force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them, and acts along the line joining the charges.
In scalar form: F = k|q₁q₂|/r² = (1/4πε₀) × (|q₁q₂|/r²), where k ≈ 9 × 10⁹ N m² C⁻² and ε₀ = 8.854 × 10⁻¹² C² N⁻¹ m⁻².
In vector form: F₂₁ = (1/4πε₀) × (q₁q₂/r²₂₁) r̂₂₁, where r̂₂₁ is the unit vector pointing from charge q₁ to charge q₂; this single equation handles both attraction and repulsion correctly.

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Question 10

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Coulomb's law and Newton's law of gravitation both show an inverse-square dependence on distance. State two fundamental differences between these forces.

Gravitational force is always attractive, while the Coulomb (electrostatic) force can be both attractive (unlike charges) and repulsive (like charges).
The Coulomb force is enormously stronger than the gravitational force; the ratio of electric to gravitational force between an electron and a proton is approximately 2.4 × 10³⁹, showing the dominance of electrostatics at the atomic scale.
Gravitational force depends on mass (always positive), whereas Coulomb force depends on electric charge, which can be positive, negative, or zero, allowing for complete cancellation of electric forces in neutral matter.

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Question 11

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State the principle of superposition of electric forces. Why is this not considered an 'obvious' result?

The superposition principle states that the total force on any charge due to a number of other charges is the vector sum of all the individual Coulomb forces on that charge, with each individual force calculated as if the other charges were absent.
The principle states two distinct things: first, the force between two charges is unaffected by the presence of other nearby charges; second, there are no additional three-body or four-body forces that arise only when more than two charges are present simultaneously.
It is not obvious because in principle, a third charge could modify the interaction between two charges (as occurs in some systems), but experiments confirm that this does not happen for electric charges — the individual Coulomb forces are strictly pairwise and independent.

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Question 12

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Define electric field at a point. State its SI unit and write the expression for the electric field due to a point charge Q at distance r.

The electric field E at a point in space is defined as the force experienced by a unit positive test charge placed at that point, without disturbing the original source charges: E = lim(q→0) F/q.
The SI unit of electric field is newton per coulomb (N/C), also written as volt per metre (V/m).
The electric field due to a point charge Q at a distance r is E = (1/4πε₀) × (Q/r²) r̂, directed radially outward if Q > 0 and radially inward if Q < 0.

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Question 13

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What is a test charge? Why must it be made negligibly small in the definition of electric field?

A test charge is a small positive charge q placed at a point in space to measure the electric field due to a source charge configuration without significantly disturbing it.
If the test charge q is not negligibly small, it exerts a significant force on the source charge Q, causing it to move from its original position, thereby altering the very field that is being measured.
By taking q → 0, the force F also becomes negligible, but the ratio F/q remains finite and well-defined, giving the true electric field of the source charge at that point independent of the test charge used.

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Chapter 1: Electric Charges and Fields | Short Questions

What is meant by the term 'electrostatics'? State two everyday phenomena that are explained by it.

State the two types of electric charges. What happens when two like charges and two unlike charges are brought near each other?

Explain how charge is transferred when a glass rod is rubbed with silk, and identify what type of charge each acquires.

Distinguish between conductors and insulators with one example each. How does charge behave differently on them?

State the property of additivity of electric charge. How does it differ from the additivity of mass?

State the law of conservation of electric charge. Give one example from nature where charge is conserved during particle creation.

What is quantisation of electric charge? What is the value of the basic unit of charge and who demonstrated it experimentally?

At macroscopic scales, why can the quantisation of electric charge be ignored? When does it become significant?

State Coulomb's law for the force between two point charges. Write the expression in both scalar and vector form.

Coulomb's law and Newton's law of gravitation both show an inverse-square dependence on distance. State two fundamental differences between these forces.

State the principle of superposition of electric forces. Why is this not considered an 'obvious' result?

Define electric field at a point. State its SI unit and write the expression for the electric field due to a point charge Q at distance r.

What is a test charge? Why must it be made negligibly small in the definition of electric field?

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