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Some Natural Phenomena - Lightning and Transfer of Electric Charges

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Charging by Rubbing: When two objects are rubbed together, they can acquire an electric charge. For example, a plastic comb rubbed with dry hair acquires a small electric charge.

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Types of Charges: There are two types of charges: positive (++) and negative (−-). By convention, the charge acquired by a glass rod when rubbed with silk is called positive, and that on a plastic straw rubbed with polythene is negative.

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Interaction of Charges: Like charges repel each other, while unlike (opposite) charges attract each other.

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The Electroscope: A device used to detect whether an object is carrying a charge or not. It consists of two thin metal foils (usually gold or aluminum) that diverge when a charged object touches the metal disc due to the transfer of like charges.

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Transfer of Charge: Electric charge can be transferred from a charged object to another through a metal conductor. The process of transferring charge from a charged object to the Earth is called 'Earthing' (⊥⊥).

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Lightning: During a thunderstorm, air currents move upward while water droplets move downward. These vigorous movements cause separation of charges. Positive charges accumulate near the upper edges of the clouds and negative charges accumulate near the lower edges. When the magnitude of accumulated charges becomes very large, the air (an insulator) can no longer resist their flow, resulting in an electric discharge seen as lightning.

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Lightning Conductor: A device used to protect buildings from the effects of lightning. It consists of a metallic rod, taller than the building, installed in the walls of the building during construction. One end of the rod is kept out in the air and the other is buried deep in the ground to provide a direct path for the discharge of electric charge to the Earth.

📐Formulae

Q=neQ = ne (Where QQ is the total charge, nn is the number of electrons, and e≈1.6×10−19 Ce ≈ 1.6 \times 10^{-19} \text{ C} is the elementary charge)

I=QtI = \frac{Q}{t} (Relationship between current II, charge QQ, and time tt during a discharge)

F=kq1q2r2F = k \frac{q_1 q_2}{r^2} (Coulomb's Law representing the force FF between two charges q1q_1 and q2q_2 separated by distance rr)

💡Examples

Problem 1:

If an object has a net positive charge of 3.2×10−18 C3.2 \times 10^{-18} \text{ C}, calculate the number of electrons it has lost.

Solution:

Using the formula Q=neQ = ne, we rearrange it to find n=Qen = \frac{Q}{e}. Given Q=3.2×10−18 CQ = 3.2 \times 10^{-18} \text{ C} and e=1.6×10−19 Ce = 1.6 \times 10^{-19} \text{ C}, then n=3.2×10−181.6×10−19=20n = \frac{3.2 \times 10^{-18}}{1.6 \times 10^{-19}} = 20.

Explanation:

Since the object is positively charged, it has lost electrons. The calculation shows it lost 2020 electrons.

Problem 2:

Explain why a glass rod rubbed with silk attracts pieces of paper, but a copper rod held in hand and rubbed with silk does not.

Solution:

The glass rod is an insulator; charges produced by rubbing remain static on its surface. The copper rod is a conductor; the charges produced flow through the human body to the Earth (Earthing).

Explanation:

Static electricity requires the material to be an insulator or to be insulated from the ground. Since the human body is a conductor, it prevents charge accumulation on the copper rod.