Some Natural Phenomena

📚 Static Electricity: The charges produced by rubbing are called static charges because they do not move — they stay on the surface of the object. When these charges move, they produce an electric current.

⚠️ Remember: When you rub a glass rod with silk, the glass gets a positive charge and the silk gets an equal negative charge. Charges are always produced in equal and opposite pairs.

💡 Key Point: If the foil strips spread apart, the object is charged. If they stay together, the object is not charged. The greater the divergence, the greater the charge.

🏡 At Home: The third round pin in a three-pin plug is connected to the earthing wire. This is why refrigerators, geysers, and washing machines have three-pin plugs — to protect us from electric shocks.

🚨 Danger: Lightning can kill people, burn houses, and destroy trees. It always follows the shortest and easiest path to the ground — usually through the tallest object in the area.

📡 Measuring Earthquakes: The power of an earthquake is measured on the Richter scale and recorded by an instrument called a seismograph. Each number on the scale represents 10 times more shaking than the previous number.

🏗️ Earthquake-Resistant Buildings: In earthquake-prone areas, buildings should be designed to withstand tremors. Use mud or timber instead of heavy roof materials, and place heavy objects on lower shelves. Consult a structural engineer for construction.

Q1. What is a charged object?

✔ A charged object is one that has acquired the property of attracting other light objects, usually by being rubbed with another material.

Q2. What kind of charge is produced on a glass rod rubbed with silk?

✔ A positive charge is produced on the glass rod when it is rubbed with a silk cloth.

Q3. What is lightning?

✔ Lightning is a huge electric spark that occurs between charged clouds, or between clouds and the earth, during a thunderstorm.

Q4. What is an electroscope used for?

✔ An electroscope is used to detect whether an object is electrically charged or not.

Q5. What is earthing?

✔ Earthing is the process of safely transferring electric charge from a charged body to the earth through a conductor.

Q6. Name the scale used to measure the power of an earthquake.

✔ The Richter scale is used to measure the power (magnitude) of an earthquake.

Q7. What is the focus of an earthquake?

✔ The focus is the point inside the earth where the earthquake actually originates.

Q8. What is the epicentre?

✔ The epicentre is the point on the earth’s surface directly above the focus of an earthquake, where damage is usually greatest.

Q9. Name the instrument used to record earthquake vibrations.

✔ A seismograph is the instrument used to record vibrations produced by an earthquake.

Q10. What is a lightning conductor?

✔ A lightning conductor is a metal rod fixed on tall buildings that safely conducts lightning charges to the ground.

Q1. Why does a plastic comb rubbed with dry hair attract small pieces of paper?

✔ When a plastic comb is rubbed with dry hair, it acquires a negative electric charge due to friction. This charged comb attracts uncharged light objects like small paper pieces because of electrostatic attraction.

Q2. State the rule about attraction and repulsion of charges.

✔ Like charges (both positive or both negative) repel each other. Unlike charges (one positive and one negative) attract each other.

Q3. Why do we see lightning before we hear thunder?

✔ Light travels much faster than sound — about 3,00,000 km/s compared to only about 344 m/s for sound. So the light from lightning reaches us almost instantly, while the sound of thunder takes longer to arrive.

Q4. What happens to the aluminium strips of a charged electroscope?

✔ When a charged object is touched to the top of the electroscope, both foil strips receive the same kind of charge. Since like charges repel each other, the strips move apart (diverge), indicating that the object is charged.

Q5. Why is earthing important for electrical appliances?

✔ Earthing safely transfers any accidental electric charge from the body of an appliance to the ground through a thick copper wire. This prevents electric shocks to people who might touch the appliance and protects the appliance from damage.

Q6. How is lightning produced in the sky?

✔ During thunderstorms, rapid movement of water droplets and ice particles inside clouds causes them to rub against each other, producing large amounts of positive and negative charges. When these charges become very large, they jump between clouds or between clouds and the ground as a huge electric spark called lightning.

Q7. Why should you not use an umbrella during a thunderstorm?

✔ Umbrellas with metal rods can attract lightning because lightning tends to strike the tallest and most conducting object in the area. Using an umbrella outdoors during a thunderstorm increases the risk of being struck by lightning.

Q8. What causes an earthquake?

✔ Earthquakes are caused by the sudden movement of tectonic plates that make up the earth’s crust. When these plates collide, slide past, or move apart along fault lines, they release enormous amounts of energy, causing the ground to shake.

Q9. How does a lightning conductor protect a building?

✔ A lightning conductor is a pointed metal rod fixed on the top of a building and connected to a metal plate buried deep in the ground through a thick copper wire. When lightning strikes, the electric charge passes safely through the wire into the earth instead of damaging the building.

Q10. Which parts of India are highly earthquake-prone?

✔ The most earthquake-prone regions of India are Kashmir, the Western and Central Himalayas, the North-East, the Rann of Kutch in Gujarat, and the Delhi region. These areas lie near active fault zones between tectonic plates.

Q1. Can lightning strike a building without a lightning conductor?

✔ Yes. Without a lightning conductor, lightning can strike the building directly and cause fire, structural damage, and injury to people inside.

Q2. Is it safe to stand under a tall isolated tree during a thunderstorm?

✔ No. Lightning tends to strike the tallest object in the area, so standing under a tall isolated tree greatly increases the risk of being struck.

Q3. Do two positively charged objects attract each other?

✔ No. Two positively charged objects are like charges, and like charges repel each other according to the rule of charges.

Q4. Can we predict earthquakes accurately in advance?

✔ No. Even today, science cannot predict when or where an earthquake will strike with accuracy — we can only identify high-risk zones.

Q5. Is it safe to take a bath during a thunderstorm?

✔ No. Water and metal pipes can conduct lightning charges from outside, so taking a bath or shower during a thunderstorm is dangerous.

Q6. Does a plastic scale rubbed with dry hair get charged?

✔ Yes. Rubbing a plastic scale with dry hair produces static charges by friction, which is why it can attract small paper bits.

Q7. Is a closed car a safe place during a thunderstorm?

✔ Yes. The metal body of a closed car acts like a cage — any lightning charge flows around the outside of the car and safely to the ground.

Q8. Do all earthquakes cause massive damage?

✔ No. Most earthquakes are very mild and are only detected by instruments; only earthquakes above magnitude 5 on the Richter scale usually cause significant damage.

Q9. Do glass rod and silk cloth develop opposite charges when rubbed?

✔ Yes. Glass rod acquires a positive charge and silk cloth acquires an equal negative charge — charges are always produced in equal and opposite pairs.

Q10. Is lightning the same phenomenon as sparks from a comb?

✔ Yes. Benjamin Franklin proved that lightning is a huge electric spark — the same phenomenon as the tiny spark from a rubbed comb, only on a massive scale.

Q1. Like charges attract each other.

✘ False. Like charges repel each other. Unlike charges attract each other.

Q2. The Richter scale is used to measure the intensity of lightning.

✘ False. The Richter scale is used to measure the power (magnitude) of earthquakes, not lightning.

Q3. Benjamin Franklin proved that lightning is an electric spark.

✔ True. In 1752, Benjamin Franklin’s kite experiment proved that lightning is a huge electric discharge.

Q4. It is safe to use a corded telephone during a thunderstorm.

✘ False. Corded telephones can conduct lightning charges from wires and should not be used during a thunderstorm; only cordless or mobile phones (away from windows) are safe.

Q5. Earthquakes occur most often at the boundaries of tectonic plates.

✔ True. Most earthquakes occur in fault zones, which are the boundaries where tectonic plates meet.

Q6. Rubbing produces charges only on plastic and rubber.

✘ False. Rubbing can produce charges on many materials including glass, silk, fur, wool, ebonite, and plastic — not just plastic and rubber.

Q7. The focus of an earthquake is on the earth’s surface.

✘ False. The focus is the point inside the earth where the earthquake originates. The point on the surface directly above it is called the epicentre.

Q8. A thunderstorm sound (thunder) is caused by rapidly heated air expanding.

✔ True. Lightning heats the surrounding air to extremely high temperatures, causing it to expand rapidly and produce the loud sound called thunder.

Q9. The leaves of an electroscope come together when a charged object touches it.

✘ False. The leaves of an electroscope move apart (diverge) because they receive the same charge and repel each other.

Q10. Earthing wires are usually connected to a metal plate deep in the ground.

✔ True. The earthing wire runs from the appliance or lightning conductor to a metal plate buried deep in the ground for safe discharge of charge.

Q1. A glass rod rubbed with silk acquires a __________ charge.

✔ positive

Q2. Like charges __________ each other.

✔ repel

Q3. Unlike charges __________ each other.

✔ attract

Q4. The instrument used to detect whether an object is charged is called an __________.

✔ electroscope

Q5. The process of transferring electric charge safely to the earth is called __________.

✔ earthing

Q6. A huge electric spark in the sky is called __________.

✔ lightning

Q7. The loud sound accompanying lightning is called __________.

✔ thunder

Q8. The power of an earthquake is measured on the __________ scale.

✔ Richter

Q9. Earthquakes are caused by movement of __________ plates.

✔ tectonic

Q10. Benjamin __________ proved that lightning is an electric discharge.

✔ Franklin

Q1. Describe how you can make a simple electroscope at home and how it detects charge.

✔ Take a clean glass jar and a cork or cardboard lid. Push a straightened metal paper clip through the lid so one end is inside the jar and the other end is on top. Attach two thin strips of aluminium foil to the bottom end of the clip inside the jar so they hang side by side. When a charged object is touched to the top of the clip, the two foil strips receive the same charge, repel each other, and move apart — showing that the object is charged.

Q2. Explain the formation of lightning step by step.

✔ During a thunderstorm, air currents inside the clouds move upward while water droplets and ice particles move downward. This vigorous movement causes them to rub against one another, producing large positive and negative charges — positive charges collect at the top of the cloud and negative charges at the bottom. Positive charges also build up on the ground below the clouds by induction. When the charge difference becomes very large, air can no longer resist the flow, and a huge electric spark jumps between the cloud and the ground — this is lightning.

Q3. What safety measures should be taken during a thunderstorm if you are outdoors?

✔ If you are caught outdoors during a thunderstorm, do not stand under a tall isolated tree, on a hilltop, or in an open field — these places attract lightning. Avoid holding umbrellas with metal rods and stay far away from electric wires, poles, and metal fences. If shelter is not available, squat low on the ground with your feet close together, place your hands on your knees, and tuck your head between your knees to minimise your body’s contact with the ground. Get out of ponds, pools, and other open water immediately, and if you are in a car, stay inside with the windows closed.

Q4. Describe the construction and working of a lightning conductor.

✔ A lightning conductor is a device fixed on the top of tall buildings to protect them from lightning damage. It consists of a pointed metal rod fixed at the highest point of the building, connected by a thick copper wire that runs down the outer wall to a metal plate buried deep in the ground. When lightning strikes the building, the electric charge is immediately captured by the pointed rod and passes safely through the copper wire into the earth. This prevents the lightning from passing through the walls or interior of the building, thereby protecting people and property from harm.

Q5. What are tectonic plates and how are they related to earthquakes?

✔ The outermost layer of the earth — the crust — is not a single continuous piece but is broken into several large fragments called tectonic plates. These plates float on the molten material below and are constantly moving very slowly. Where two plates meet, they may collide, slide past each other, or move apart, and the boundaries where these movements happen are called fault zones. When the plates suddenly slip or lock and release stored energy, the ground shakes strongly — this is an earthquake, and it is why most earthquakes happen along fault zones.

Q6. What safety measures should be taken during an earthquake if you are inside a building?

✔ If you are inside a building during an earthquake, immediately take shelter under a strong table, desk, or bed to protect yourself from falling objects. Stay away from heavy furniture, glass windows, mirrors, and objects that may fall and injure you. If you are in bed, cover your head with a pillow. Do not use lifts — use stairs to evacuate only after the shaking has stopped, and turn off the gas supply and main electricity switch on your way out to prevent fire.

Q7. Explain what the Richter scale measures and what different values represent.

✔ The Richter scale is used to measure the magnitude or power of an earthquake. Values below 3 are usually not felt by people and are only detected by instruments called seismographs. Values between 3 and 5 are felt by people but cause little damage, while values between 5 and 7 can damage weak buildings. Earthquakes above magnitude 7 on the Richter scale are highly destructive — buildings collapse and there can be widespread damage and loss of life.

Q8. Why is a closed car considered safe during a thunderstorm?

✔ A closed car with a metal body is safe during a thunderstorm because the metal shell acts like a protective cage around the passengers. If lightning strikes the car, the electric charge flows around the outside of the metal body and safely into the ground through the tyres or the wet road surface. The charge does not enter the inside of the car because electric charges always flow along the outer surface of a conductor. However, you should keep the windows closed and avoid touching any metal parts inside the car until the storm passes.

Q9. Distinguish between the focus and the epicentre of an earthquake with examples.

✔ The focus of an earthquake is the point deep inside the earth’s crust where the earthquake actually originates due to the sudden slipping of tectonic plates. The epicentre is the point on the earth’s surface that lies directly above the focus. For example, if the focus is 30 km below the ground under a certain city, then the point on the surface of the city right above the focus is the epicentre. Damage from an earthquake is usually greatest at the epicentre and decreases as we move away from it.

Q10. What features should an earthquake-resistant building have?

✔ Earthquake-resistant buildings are designed to withstand tremors without collapsing. They should be built with lighter roof materials such as mud or timber instead of heavy concrete, and heavy objects should be placed on lower shelves so they don’t fall on people during shaking. The structural framework should be strong and flexible — using reinforced steel and concrete that can bend slightly rather than break. A qualified structural engineer or architect should be consulted before construction, especially in high-risk zones like Kashmir, Kutch, and North-East India, to ensure the building can safely handle earthquake forces.