CUET UG Physics Booster Test – 2 Fundamentals of Electric Charge

Thales of Miletus is credited with the earliest recorded observation of frictional electricity when he noticed that rubbed amber attracted light objects. Benjamin Franklin later introduced the convention of positive and negative charges. Amber is the material that exhibited this attraction effect, while "elektron" is the Greek word for amber from which the word electricity originated. Therefore, the correct matching is 1-d, 2-c, 3-a, and 4-b. This historical sequence is discussed in NCERT while introducing electric charges and their discovery.

NCERT explains that the word electricity originates from the Greek word "elektron," meaning amber. Around 600 BC, Thales observed that amber attracted light objects after rubbing. However, amber was not the only material capable of showing this effect. Many material pairs such as glass and silk or plastic and fur can become charged through friction and attract light objects. Therefore, Option C is incorrect because it wrongly states that only amber exhibited this phenomenon. The remaining options are historically accurate and consistent with NCERT.

According to the law of conservation of charge, charge can neither be created nor destroyed; it can only be transferred from one body to another. Initially, the total charge of the system is zero. During rubbing, charges are redistributed among X, Y, and Z. Although individual bodies acquire positive or negative charges, the algebraic sum of all charges remains unchanged. Therefore, the net charge of the entire system remains zero. This principle is fundamental to electrostatics and is repeatedly emphasized in NCERT.

Electrostatics is the branch of physics dealing with charges at rest and the fields, forces, and potentials produced by them. The term static refers to quantities that do not change with time. Electric shocks from metal bars or vehicles occur because accumulated charges suddenly discharge through the body. However, the crackling sound heard while removing synthetic clothes is actually caused by electrical discharge and not unrelated to it. Therefore, Statements 1, 2, and 4 are correct, while Statement 3 is incorrect. Hence, Option A is correct.

Benjamin Franklin proposed the convention of positive and negative charges. When a glass rod is rubbed with silk, the glass rod loses electrons and becomes positively charged, making Statement 2 incorrect. When plastic is rubbed with fur, the plastic gains electrons and becomes negatively charged, while the fur becomes positively charged. Therefore, Statements 3 and 4 are correct. Together with Statement 1, they form the correct set. Hence, Option D is the correct answer according to NCERT conventions.

The concept of polarity refers to the existence of two distinct kinds of electric charges—positive and negative. This distinction is necessary to explain why some charged bodies attract while others repel. NCERT explains that like charges repel each other whereas unlike charges attract each other. Such different interactions are possible only because charge possesses polarity. Polarity does not create magnetic poles, nor does it imply that all matter contains only one type of charge. Neutral bodies contain equal amounts of positive and negative charges and therefore do not possess a single uniform polarity. Hence, Option B correctly describes the significance of charge polarity.

When glass is rubbed with silk, the glass becomes positively charged while the silk becomes negatively charged. Cat's fur rubbed with plastic becomes positively charged. Therefore, glass and cat's fur carry like positive charges and repel each other. Similarly, silk pieces acquire similar charges and repel one another. Wool strands involved in similar charging processes acquire like charges and show repulsion. However, a plastic rod and a glass rod possess opposite charges and therefore attract rather than repel. Thus, Statements 1, 2, and 4 are correct, while Statement 3 is incorrect. Therefore, Option A is the correct answer.

When a glass rod is rubbed with silk, electrons move from the glass rod to the silk. Consequently, the glass rod becomes positively charged and the silk becomes negatively charged. Similarly, a plastic rod rubbed with fur acquires a negative charge. Since opposite charges attract, the positively charged glass rod attracts both the negatively charged plastic rod and the negatively charged silk. Therefore, both interactions mentioned in the question are attractive in nature. Hence, Option C correctly describes the force behavior between the given pairs of objects.

When a glass rod is rubbed with silk, electrons are transferred from the glass rod to the silk. Since the glass rod loses electrons, it becomes positively charged. The magnitude of charge acquired is calculated using: Q = ne = (5 × 10¹⁰)(1.6 × 10⁻¹⁹) = 8.0 × 10⁻⁹ C Because electrons are lost, the charge on the glass rod is positive. Therefore, the net charge acquired by the glass rod is +8.0 × 10⁻⁹ C. Hence, Option B is the correct answer. This calculation is based on the quantisation of charge discussed in NCERT.

During frictional charging, only loosely bound electrons move from one body to another. Protons remain tightly bound within atomic nuclei and do not participate in charge transfer. The number of electrons transferred is extremely small compared to the enormous number of electrons present in a material. When a body loses electrons, it develops a deficiency of negative charge and becomes positively charged. Therefore, Statements 1, 3, and 4 are correct. Statement 2 is incorrect because protons are never transferred during ordinary charging processes. Hence, Option A is the correct answer according to NCERT.

An electrically neutral object contains both positive and negative charges in equal amounts. Neutrality arises because the total positive charge exactly balances the total negative charge. Therefore, Statements 2 and 4 are correct. Statement 1 is incorrect because all ordinary matter contains protons and electrons. Statement 3 is also incorrect because losing all free electrons would make the object highly positively charged rather than neutral. NCERT explains that matter is normally electrically neutral due to the balance between positive and negative charges. Hence, Option B is the correct answer.

When two objects carrying equal and opposite charges are brought into contact, the charges neutralize each other. As a result, the combined system becomes electrically neutral and the strong electrostatic effects disappear. Therefore, Statements A, B, and C correctly describe charge nullification. Statement D is incorrect because once the charged glass rod and silk are brought into contact, the opposite charges cancel each other and the objects no longer remain strongly electrified. Hence, they will not continue attracting light objects powerfully. Therefore, Option D is the incorrect statement and thus the correct answer.

Object A has charge = (y − x)e. Object B has charge = (0 − 2x)e = −2xe. Object C has charge = (2y − 0)e = +2ye. Therefore, total charge: Q = (y − x)e + (−2x)e + (2y)e Q = (3y − 3x)e Thus, the net charge of the system is (3y − 3x)e. This result follows directly from the fact that protons contribute positive charge and electrons contribute negative charge. Therefore, Option A is correct.

Charge is quantized and exists in integral multiples of the elementary charge e. At macroscopic scales involving microcoulombs, the discreteness of charge is usually not noticeable and charge appears continuous. However, when dealing with only a few tens or hundreds of elementary charges, quantization becomes significant and cannot be ignored. Fractional transfer of electrons does not occur in ordinary electrostatic phenomena. Therefore, Option C correctly describes the importance of charge quantization at microscopic scales as discussed in NCERT.

A gold-leaf electroscope consists of a metal rod passing vertically through an insulating stopper into a box. Two thin gold leaves are attached to the lower end of the rod. When a charged object touches the metal knob, charge flows through the rod to the leaves, causing them to diverge due to repulsion. The electroscope is a simple device used to detect the presence of charge. Therefore, Statements 2, 3, and 4 are correct. Statement 1 is incorrect because the metal rod is vertical rather than horizontal. Hence, Option B is the correct answer.

In a gold-leaf electroscope, charge supplied to the knob spreads through the conducting rod and reaches the gold leaves. The leaves acquire charges of the same sign and repel each other. The amount of divergence provides an indication of the amount of charge present on the electroscope. Therefore, Statements 1, 2, and 3 are correct. However, the electroscope is not designed to calculate the exact gravitational force acting on the leaves. Its purpose is charge detection and qualitative estimation of charge magnitude. Hence, Statement 4 is incorrect. Therefore, Option A is the correct answer.

One mole of water contains 6.02 × 10²³ molecules. Each H₂O molecule contains 10 electrons (2 from hydrogen atoms and 8 from oxygen). Therefore, total electrons: N = 10 × 6.02 × 10²³ = 6.02 × 10²⁴ The total negative charge is: Q = Ne = (6.02 × 10²⁴)(1.6 × 10⁻¹⁹) ≈ 9.6 × 10⁵ C Thus, the magnitude of negative charge present in 18 g of water is approximately 9.6 × 10⁵ C. Therefore, Option C is correct. This calculation highlights the enormous amount of charge present in ordinary matter despite its overall electrical neutrality.

NCERT emphasizes that many familiar forces such as adhesion, cohesion, elasticity, and surface tension ultimately arise from electrical interactions between charged particles within atoms and molecules. Glue adheres because of intermolecular electrical forces rather than interactions between bodies devoid of charges. Therefore, Statement B is incorrect. Statements A, B, and C correctly describe the pervasive role of electrical forces in nature. The electric force governs atomic and molecular interactions and contributes significantly to the behavior of matter. Hence, Option D is the correct answer.

Electric charge is fundamentally quantized and exists in integral multiples of the elementary charge e. However, at the macroscopic scale, the number of charges involved is so enormous that charge appears continuous. At the microscopic scale, where only a few elementary charges are involved, the discrete nature of charge becomes apparent. Therefore, macroscopic charging is effectively continuous, while microscopic charging is discrete. This distinction is highlighted in NCERT while discussing quantization of charge. Hence, Option C is the correct answer.

According to NCERT, electric charge arises due to the transfer of electrons. When an atom or body loses electrons, it is left with more protons than electrons and becomes positively charged; this process is called positive ionization. When a body gains electrons, it acquires an excess of negative charge and becomes negatively charged. A body having excess electrons is therefore negatively charged, whereas a body having a deficit of electrons is positively charged. Thus, charging by losing electrons corresponds to positive ionization, charging by gaining electrons corresponds to negative charging, excess of electrons corresponds to a negatively charged body, and deficit of electrons corresponds to a positively charged body. Therefore, the correct matching is 1-c, 2-d, 3-a, 4-b.