Electric Charges and Fields MCQs with Answers – Part 1 (Class 12 Physics)

Explanation: Electric charge is the property because of which bodies can exert electrostatic attraction or repulsion. A rubbed comb attracting small paper pieces is an everyday sign that charge has appeared or charge separation has occurred. Mass and temperature are physical quantities too, but they are not the basic cause of electrostatic attraction and repulsion. The colour of a material does not decide whether it is charged. Charge is the central property, while force and field describe how charged bodies influence one another.

Explanation: Rubbing can transfer charge between two materials, so the glass rod may become electrically charged. The attraction of light paper bits is a common visible effect of electrostatic interaction. This does not mean the rod has lost all its mass or that gravity has changed its nature. Magnetism is a different interaction and is not needed to explain this ordinary rubbing example. The movement of paper pieces is an introductory sign of electric charge or charge separation, not proof of a new gravitational effect.

Explanation: Like charges repel each other, while unlike charges attract each other. Repulsion is therefore a stronger sign that both bodies are charged and have the same kind of charge. A charged body can attract a neutral body due to charge separation, so attraction alone is not always enough to prove unlike charges. Neutral bodies do not normally repel each other electrostatically. The key idea is that repulsion between two small charged bodies identifies like charges.

Explanation: Electric charge has two kinds, conventionally called positive and negative. The signs \(+\) and \(-\) attached to charge do not act like arrows in space. Direction is needed for vector quantities such as force \(\vec{F}\) and electric field \(\vec{E}\), not for charge itself. Both positive and negative charges are measured in the same SI unit, the coulomb \(\text{C}\). A symbol such as \(-q\) tells the kind of charge relative to \(+q\), not a left, right, upward, or downward direction.

Explanation: The SI unit of electric charge is the coulomb, written as \(\text{C}\). The unit \(\text{N}\) belongs to force, not charge. The unit \(\text{N C}^{-1}\) is used for electric field, because field is force per unit charge. The unit \(\text{V m}^{-1}\) is also a unit of electric field, but it is not the unit of charge. Keeping \(\text{C}\) for charge separate from \(\text{N}\) for force avoids mixing the cause with one of its effects.

Explanation: Rubbing can make the balloon electrically charged. The charged balloon can interact with charges in the wall, causing an attractive electrostatic effect. Thermal expansion would involve a change in size due to temperature, which is not the main observation here. Circular motion and radioactive decay do not explain why a rubbed balloon sticks to a wall. The example belongs to the opening idea that charged bodies can attract nearby objects.

Explanation: Lightning is associated with large charge separation in clouds and sudden electric discharge. It is a natural and dramatic example showing that electric charge can build up and then move rapidly. Sound is produced after lightning as thunder, but sound reflection is not the cause of lightning. Magnetism between raindrops is not the ordinary explanation used in electrostatics. At the introductory level, lightning gives a broad picture of charge separation and discharge rather than a detailed weather model.

Explanation: Electric charge is the property associated with electrostatic attraction and repulsion. The coulomb \(\text{C}\) is the SI unit in which charge is measured. Like charges mean charges of the same kind, and they repel each other. Unlike charges mean opposite kinds of charge, and they attract each other. These four links form the basic language needed before studying Coulomb’s law, electric field, and flux.

Explanation: A charged comb can attract small neutral objects because charges inside the neutral object may shift slightly. This produces a nearer region of opposite charge effect and a farther region of like charge effect, giving net attraction. The observation does not prove that the neutral object has an opposite net charge from the beginning. It also does not show that both objects remain completely unaffected electrically. Attraction is less conclusive than repulsion because attraction can occur between unlike charged bodies as well as between a charged body and a neutral body.

Explanation: Charge \(q\) is scalar because it has magnitude and sign but no spatial direction. Force \(\vec{F}\) is a vector because it has both magnitude and direction. The sign of charge helps decide whether the force is attractive or repulsive, but the force direction is decided by the line of interaction and the arrangement of charges. A unitless ratio cannot describe force because force is measured in newton \(\text{N}\). Separating scalar charge from vector force is essential before using vector addition in electrostatics.

Explanation: Electric charge \(q\) is a scalar quantity, while electrostatic force \(\vec{F}\) is a vector quantity. The distance \(r\) and area magnitude \(A\) are usually treated as scalar magnitudes in the basic formulas. The constants \(k\) and \(\varepsilon_0\) are not vectors in elementary electrostatics. The symbols \(Q\) and \(q\) both represent charges, so both are scalar quantities with possible signs. The arrow over \(\vec{F}\) is not decoration; it signals that direction must be considered.

Explanation: Electric field is defined as force per unit positive test charge, so its unit is \(\text{N C}^{-1}\). The unit of force is \(\text{N}\), and the unit of charge is \(\text{C}\). Distance is measured in metre \(\text{m}\), while area is measured in \(\text{m}^2\). The unit \(\text{N C}^{-1}\) already shows the idea of field as force divided by charge. It should not be confused with \(\text{C}\), which measures charge itself.

Explanation: In electrostatics, \(r\) commonly represents the separation distance between charges or the distance from a source charge. The symbol \(A\) represents area, especially when electric flux through a surface is introduced later. The units help identify the quantities: \(\text{m}\) is the unit of length, while \(\text{m}^2\) is the unit of area. Force would have unit \(\text{N}\), and charge would have unit \(\text{C}\). Reading symbols with their units prevents confusing geometry-related quantities with electric quantities.

Explanation: \( \textbf{Given relation:} \) \(F\propto \frac{1}{r^2}\). \( \textbf{Change made:} \) The new separation is \(2r\). \( \textbf{New force comparison:} \) \[ \frac{F'}{F}=\frac{\frac{1}{(2r)^2}}{\frac{1}{r^2}} \] \( \textbf{Simplification:} \) \[ \frac{F'}{F}=\frac{1}{4} \] \( \textbf{Result:} \) \[ F'=\frac{F}{4} \] \( \textbf{Interpretation:} \) Doubling distance does not merely halve the force because the distance appears squared in the denominator. \( \textbf{Final answer:} \) The force becomes \(\frac{F}{4}\).

Explanation: The Coulomb constant \(k\) is connected to the vacuum permittivity \(\varepsilon_0\) by \(k=\frac{1}{4\pi\varepsilon_0}\). This relation appears in Coulomb’s law and later connects naturally with Gauss’s law. The symbol \(k\) is a constant used in the force and field formulas, while \(\varepsilon_0\) describes the permittivity of free space. They are not the same physical quantity and do not have the same units. Placing \(\varepsilon_0\) in the denominator is part of the standard SI form of electrostatic laws.

Explanation: Distance \(r\) is measured in metre \(\text{m}\), not in \(\text{C m}^{-1}\). The unit \(\text{C m}^{-1}\) belongs to linear charge density, a later idea involving charge per unit length. Charge \(q\) has unit \(\text{C}\), force \(\vec{F}\) has unit \(\text{N}\), and electric field \(\vec{E}\) can have unit \(\text{N C}^{-1}\). The mismatched row is therefore the one that assigns a charge-density unit to a distance. Unit patterns are often enough to identify the physical meaning of a symbol.

Explanation: Force is a vector quantity, so direction matters during addition. Two equal forces in exactly opposite directions cancel each other. The result is not twice one force because doubling would occur only when the forces point in the same direction. There is no northward force in the arrangement, so a northward resultant cannot appear. This simple vector-addition idea is needed later when forces or fields from more than one charge are combined.

Explanation: The arrow notation marks a vector quantity. Electrostatic force \(\vec{F}\) has a direction along which it acts, and electric field \(\vec{E}\) has a direction defined by the force on a positive test charge. These quantities also have units, such as \(\text{N}\) for force and \(\text{N C}^{-1}\) for electric field. They do not represent charge itself, because charge is usually written as \(q\) or \(Q\) without a vector arrow. The notation prepares the learner to add electric forces and fields vectorially rather than as plain numbers.

Explanation: The symbol \(r\) generally represents distance or separation, especially in inverse-square relations. Area is represented by \(A\), and in flux ideas it may also be associated with an area vector \(\vec{A}\). The entries \(q\), \(\vec{E}\), and \(\vec{F}\) match charge, electric field, and force respectively. Confusing \(r\) with area would lead to wrong use of formulas involving distance and surface. The distinction matters because distance appears in laws such as inverse-square relations, while area appears in electric flux.

Explanation: \( \textbf{Graph information:} \) The vertical quantity is \(F\), and the horizontal quantity is \(\frac{1}{r^2}\). \( \textbf{Straight-line meaning:} \) A straight line through the origin means the vertical quantity is directly proportional to the horizontal quantity. \( \textbf{Therefore:} \) \[ F\propto \frac{1}{r^2} \] \( \textbf{Condition:} \) This interpretation assumes the charges and medium are kept unchanged. \( \textbf{Why other options fail:} \) If \(F\) were independent of distance, the graph against \(\frac{1}{r^2}\) would not rise through the origin. \( \textbf{Physical meaning:} \) Increasing distance reduces force rapidly because the separation appears as \(r^2\) in the denominator. \( \textbf{Final answer:} \) The graph supports \(F\propto \frac{1}{r^2}\).