Grade 7 Q&A: Chapter 8: Static Electricity

Concept Questions

Q1: What is electric charge? Name the two types of electric charge.

Answer: Electric charge is an intrinsic property of matter that causes it to experience a force when placed in an electromagnetic field. The two types of electric charge are positive charge (+) and negative charge (-).

Q2: How do like charges interact? How do unlike charges interact?

Answer: Like charges repel each other (positive repels positive, and negative repels negative). Unlike charges attract each other (positive attracts negative).

Q3: Explain how an object becomes positively charged and negatively charged.

Answer: An object becomes positively charged when it loses electrons. An object becomes negatively charged when it gains electrons.

Q4: Describe the process of charging by friction with an example.

Answer: Charging by friction occurs when two different neutral objects are rubbed against each other. This rubbing causes electrons to transfer from one object to the other. The object that gains electrons becomes negatively charged, and the object that loses electrons becomes positively charged. Example: Rubbing a plastic comb through dry hair makes the comb negatively charged and the hair positively charged.

Q5: What is electric discharge?

Answer: Electric discharge is the sudden flow of electric charge through a medium, such as air, that is normally an insulator. It occurs when a large amount of static charge builds up, overcoming the insulating capacity of the medium.

Q6: How is lightning formed during a thunderstorm?

Answer: During thunderstorms, vigorous air currents move upwards, and water droplets move downwards. This movement causes friction between water particles and ice crystals in the clouds, leading to the accumulation and separation of electric charges. Positive charges gather at the top of the clouds, and negative charges accumulate at the bottom. When these accumulated charges become very large, the air can no longer act as an insulator, leading to a sudden, massive flow of charge, which is lightning.

Q7: What is a lightning conductor, and what is its purpose?

Answer: A lightning conductor (or lightning rod) is a metal rod or strip installed on the highest point of a building, connected to a thick copper wire that runs down the side of the building and is buried deep into the ground. Its purpose is to provide a safe and easy path for the electric charge from lightning to flow directly into the Earth, thereby protecting the building from damage.

Q8: List three safety precautions one should take during a thunderstorm.

Answer: Three safety precautions during a thunderstorm are: 1. Do not stand under isolated trees or in open fields. 2. Avoid contact with metal objects and electrical appliances. 3. Seek shelter in a sturdy building or a car (metal body acts as a Faraday cage).

Q9: Why does a glass rod become positively charged when rubbed with silk cloth?

Answer: When a glass rod is rubbed with silk cloth, the glass rod loses electrons to the silk cloth. Since electrons are negatively charged, the loss of electrons leaves the glass rod with an excess of positive charges (protons), thus making it positively charged.

Q10: What is the difference between charging by friction and charging by induction?

Answer:

• Charging by Friction: Involves direct contact and rubbing of two objects, leading to the physical transfer of electrons from one object to another. Both objects become oppositely charged.
• Charging by Induction: Involves charging an object without direct contact with a charged object. It causes a redistribution of charges within the neutral object due to the proximity of a charged object, often requiring grounding to achieve a net charge.

Q11: Why is it dangerous to stand under an isolated tree during a thunderstorm?

Answer: It is dangerous to stand under an isolated tree during a thunderstorm because an isolated tree is often the tallest object in an open area, making it a likely target for a lightning strike. If lightning strikes the tree, the charge can easily transfer to a person standing nearby, causing severe injury or death.

Q12: Explain why a car provides relatively safe shelter during a lightning storm.

Answer: A car provides relatively safe shelter during a lightning storm because its metal body acts as a Faraday cage. If lightning strikes the car, the electric charge travels through the outer metal frame and into the ground, bypassing the occupants inside, who are insulated by the rubber tires.

Q13: What are protons, electrons, and neutrons? What are their charges?

Answer: Protons, electrons, and neutrons are subatomic particles that make up atoms.

• Protons: Positively charged particles (+).
• Electrons: Negatively charged particles (-).
• Neutrons: Particles with no electric charge (neutral).

Q14: How does a lightning conductor protect a building?

Answer: A lightning conductor protects a building by providing a low-resistance path for the lightning's immense electric charge to flow directly into the Earth. When lightning strikes the conductor, the charge is safely channeled away from the building's structure, preventing fires, structural damage, and electrical surges within the building.

Q15: Why should you avoid touching metal objects during a thunderstorm?

Answer: You should avoid touching metal objects during a thunderstorm because metals are good conductors of electricity. If lightning strikes a nearby metal object (like a fence, pole, or plumbing), the electric charge can travel through it and cause a severe electric shock if you are in contact with it.

Textbook Exercise Solutions

A. Fill in the blanks:

1. Particles with positive charge are called protons.
2. Like charges repel each other.
3. The process of charging an object by rubbing is called charging by friction.
4. Lightning is an example of electric discharge.
5. A device used to protect buildings from lightning is a lightning conductor.

B. Match the following:

Column A

1. Positive charge
2. Negative charge
3. Like charges
4. Unlike charges
5. Lightning conductor

Column B

1. Repel
2. Attract
3. Loss of electrons
4. Gain of electrons
5. Protects buildings

Answers:

1. Positive charge - Loss of electrons
2. Negative charge - Gain of electrons
3. Like charges - Repel
4. Unlike charges - Attract
5. Lightning conductor - Protects buildings

C. Answer the following questions:

Q1: What is static electricity? Explain with an example.

Answer: Static electricity is the build-up of electric charge on the surface of an object. It deals with electric charges at rest. An example is when you rub a plastic comb through dry hair; the comb becomes charged and can attract small pieces of paper, or your hair might stand on end.

Q2: How are objects charged by friction?

Answer: Objects are charged by friction when two different neutral objects are rubbed against each other. During this rubbing, electrons are transferred from the surface of one object to the surface of the other. The object that loses electrons becomes positively charged, and the object that gains electrons becomes negatively charged. The total charge remains conserved.

Q3: Describe how lightning is formed.

Answer: Lightning is formed during thunderstorms due to the vigorous movement of air currents and water droplets/ice crystals within the clouds. This constant friction causes a separation of electric charges: positive charges accumulate near the top of the clouds, and negative charges accumulate near the bottom. When the amount of accumulated charge becomes very large, the insulating capacity of the air breaks down, leading to a sudden, massive flow of electric charge between clouds or between a cloud and the ground. This discharge is seen as a bright flash (lightning) and heard as thunder.

Q4: What is a lightning conductor? How does it work?

Answer: A lightning conductor is a metal rod or strip installed on the highest point of a building, connected to a thick copper wire that runs down the side of the building and is buried deep into the ground. It works by providing a low-resistance path for the electric charge from a lightning strike to flow safely into the Earth. If lightning strikes the building, it hits the conductor first, and the charge is immediately channeled through the wire into the ground, preventing the building from being damaged by the massive electrical energy.

Q5: List safety precautions to be taken during a thunderstorm.

Answer: Safety precautions to be taken during a thunderstorm include:

• Seek shelter in a sturdy building or a car (metal body acts as a Faraday cage).
• Avoid open fields, isolated trees, and high ground.
• Stay away from metal objects, electrical appliances, and water sources.
• Do not use landline phones; mobile phones are generally safer if not charging.
• If outdoors and no shelter is available, squat low to the ground with your head between your knees, minimizing contact with the ground.

D. Give reasons:

Q1: A plastic comb rubbed through dry hair attracts small pieces of paper.

Answer: A plastic comb rubbed through dry hair attracts small pieces of paper because it becomes electrically charged due to friction. When rubbed, the comb gains electrons from the hair, becoming negatively charged. This charged comb then induces an opposite charge on the closer side of the neutral paper pieces, leading to an attractive force between the comb and the paper.

Q2: Lightning conductors are installed on tall buildings.

Answer: Lightning conductors are installed on tall buildings to protect them from the destructive effects of lightning strikes. Tall buildings are more likely to be struck by lightning. The conductor provides a safe, low-resistance path for the immense electric charge from lightning to flow directly into the Earth, preventing fires, structural damage, and electrical surges within the building.

Q3: It is dangerous to stand under an isolated tree during a thunderstorm.

Answer: It is dangerous to stand under an isolated tree during a thunderstorm because an isolated tree is often the tallest object in an open area, making it a preferential target for a lightning strike. If lightning strikes the tree, the massive electric current can easily jump from the tree to a person standing nearby, causing severe injury or death due.

Q4: We should not use landline phones during a thunderstorm.

Answer: We should not use landline phones during a thunderstorm because lightning can strike telephone poles or lines, and the electric current can travel through the phone wires into the house. Using a landline phone during such an event can result in a dangerous electric shock.

Q5: A car provides relatively safe shelter during a lightning storm.

Answer: A car provides relatively safe shelter during a lightning storm because its metal body acts like a Faraday cage. If lightning strikes the car, the electric charge travels along the outer metal frame of the vehicle and into the ground, rather than passing through the occupants inside. The rubber tires also provide some insulation from the ground.

References

1. Maharashtra State Board 7th Standard Science Syllabus (Based on current curriculum for static electricity)
2. Screenshot_2025_0528_000942.jpg (Provided content and exercise questions)