A. IN-TEXT ACTIVITIES
- Activity 12.1
Question (text from the book):
“Take a used ballpen refill and rub it vigorously with a piece of polythene. Bring it close to small pieces of paper. Take care not to touch the rubbed end of the refill with your hand or with a metallic object. Repeat the activity with small pieces of dry leaf, husk and mustard seeds. Record your observations.”
Answer / What to do and observe:
- Rub the plastic refill with polythene.
- Bring the rubbed (charged) end near the small bits of paper (or the dry leaf, husk, mustard seeds).
- Observation: You will notice that these bits move toward the refill or get attracted.
- Conclusion: Rubbing gives the refill a static charge, which then attracts these small, light particles.
- Activity 12.2
Question (text from the book):
“Collect the objects and the materials listed in Table 12.1. Try to charge each by rubbing with the materials mentioned in the Table. Record your findings. You can add more items to the Table.”
Answer / What to do and observe:
- Table 12.1 might have items like: plastic refill, balloon, eraser, steel spoon (each to be rubbed with polythene, woolen cloth, etc.).
- Rub each item with the suggested material.
- Bring each rubbed item near small bits of paper and note if it attracts the paper.
- Observation: Usually, plastic objects (like a refill or a balloon) will become charged and attract pieces of paper. A metal spoon does not hold charge easily when you rub it in a normal way because charges pass quickly through the metal to your hand (and then to the earth).
- Activity 12.3
Part (a) Question:
“Inflate two balloons. Hang them in such a way that they do not touch each other. Rub both the balloons with a woolen cloth and release them. What do you observe?”
Part (a) Answer:
- After rubbing, both balloons get the same type of charge.
- Like charges repel. So the balloons push away from each other.
Part (b) Question:
“Rub a refill and place it gently in a glass tumbler so it can rotate freely. Rub another refill with polythene. Bring it close to the refill in the tumbler. Be careful not to touch the charged end with your hand. Do the two attract or repel each other?”
Part (b) Answer:
- Both refills are rubbed with the same material, so they have the same type of charge.
- You will see that they repel each other (move apart).
Part (c) Question:
“Rub a refill and place it gently in the glass tumbler as before. Now bring an inflated charged balloon near it. Observe what happens.”
Part (c) Answer:
- The balloon and the refill may have different charges (because the balloon was rubbed with wool, while the refill was rubbed with polythene).
- Different charges attract each other, so they pull toward each other.
Overall conclusion for Activity 12.3:
- Objects with like charges repel each other.
- Objects with unlike charges attract each other.
- Activity 12.4
Question (text from the book):
“Take an empty jam bottle. Take a piece of cardboard slightly bigger than the mouth of the bottle. Pierce a hole in it so that a metal paper clip can be inserted. Open out the paper clip as shown in the figure. Cut two strips of aluminium foil about 4 cm × 1 cm each. Hang them on the paper clip. Insert the paper clip in the cardboard lid so that it is perpendicular to it. Charge a refill and touch it with the end of the paper clip. Observe what happens. Now, touch other charged bodies with the end of the paper clip. Do foil strips behave in the same way in all cases? Can this apparatus be used to detect whether a body is charged or not? Can you explain why the foil strips repel each other?”
Answer:
- When you charge the refill and touch the paper clip, the charge travels down to the foil strips.
- Because the two strips get the same charge, they move apart or spread out (repel each other).
- If you bring any other charged object near the metal clip, the foil strips again get charged and repel.
- If you touch the clip with your hand, the charge goes to the earth through your body, and the strips fall back to normal.
- This setup is called an electroscope. It is used to test if an object is charged.
- Activity 12.5
Question (text from the book):
“Ask your parents about the huge damage to life and property caused by the earthquakes in Uri and Tangdhar towns of North Kashmir (8 October 2005) and in Bhuj district of Gujarat (26 January 2001). Collect a few pictures showing the damage caused by these earthquakes from newspapers and magazines of those days. Prepare a short report on the suffering of the people during and after the earthquakes.”
Answer / Guidance:
- You can talk to elders, look at old newspapers or magazines, and collect pictures of the destruction.
- In your short report, mention how people lost their homes, the rescue operations, how people got food and water, and how houses and roads were repaired.
- Activity 12.6
Question (text from the book):
“Take an outline map of the world. Locate the eastern coast and Andaman and Nicobar Islands in India. Mark other countries around the Indian Ocean which could have suffered damage from the tsunami on 26 December 2004. Collect accounts of the devastation caused by the tsunami in India from your parents, or other elders in the family or in the neighbourhood.”
Answer / Guidance:
- On the world map, mark India’s eastern coastline and the Andaman & Nicobar Islands.
- Mark nearby countries like Sri Lanka, Indonesia, Thailand, Maldives, etc.
- Ask elders for stories or look up old news about how people in coastal areas lost their belongings and how relief work was done.
B. EXERCISE QUESTIONS
Question 1:
“Which of the following cannot be charged easily by friction?
(a) A plastic scale
(b) A copper rod
(c) An inflated balloon
(d) A woollen cloth.”
Answer: (b) A copper rod
Because metal lets the charges pass through to your hand, so it does not easily hold static charge when rubbed in normal conditions.
Question 2:
“When a glass rod is rubbed with a piece of silk cloth the rod
(a) and the cloth both acquire positive charge.
(b) becomes positively charged while the cloth has a negative charge.
(c) and the cloth both acquire negative charge.
(d) becomes negatively charged while the cloth has a positive charge.”
Answer: (b) The glass rod becomes positively charged while the silk cloth becomes negatively charged.
Question 3:
“Write T against true and F against false in the following statements.
(a) Like charges attract each other (T/F)
(b) A charged glass rod attracts a charged plastic straw (T/F)
(c) Lightning conductor cannot protect a building from lightning (T/F)
(d) Earthquakes can be predicted in advance (T/F)”
Answer:
(a) F (False) because like charges repel each other.
(b) T (True) because they have unlike charges, so they attract.
(c) F (False) because a lightning conductor can protect a building.
(d) F (False) because earthquakes cannot be predicted in advance.
Question 4:
“Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain.”
Answer:
In dry winter air, your sweater can become charged by friction against your body or clothes. The crackling sound happens due to tiny electric sparks when charges move through the air.
Question 5:
“Explain why a charged body loses its charge if we touch it with our hand.”
Answer:
Our body is a conductor connected to the earth, so the extra charge flows from the charged object through our body into the earth (earthing). Hence the object becomes neutral.
Question 6:
“Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?”
Answer:
- Earthquakes are measured on the Richter scale.
- A magnitude 3 earthquake would still be recorded by a seismograph, but it usually causes very little or no damage.
Question 7:
“Suggest three measures to protect ourselves from lightning.”
Answer:
- Stay indoors in a building or car; close doors and windows.
- Do not stand under tall trees or near electric poles in an open field.
- Avoid using wired phones or touching electrical appliances; you can use mobile or cordless phones instead.
Question 8:
“Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?”
Answer:
- Two charged balloons have the same type of charge, so they repel each other.
- An uncharged balloon can be induced to have an opposite charge on its near side when a charged balloon approaches, resulting in attraction.
Question 9:
“Describe with the help of a diagram an instrument which can be used to detect a charged body.”
Answer:
- The instrument is an electroscope.
- It consists of a metal rod passing through a cardboard. Two strips of aluminium foil hang from the bottom end of the rod inside a jar.
- When a charged object touches the metal rod, the same charge travels to the foil strips, and they spread apart (repel each other).
- That is how we detect charge.
Question 10:
“List three states in India where earthquakes are more likely to strike.”
Answer:
- Jammu & Kashmir (especially areas like Uri and Tangdhar).
- Gujarat (like the Bhuj region and Rann of Kutch).
- The entire North-East region, including Assam.
(Also parts of the Himalayas, Rajasthan, and the Indo-Gangetic plain can be added.)
Question 11:
“Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?”
Answer:
- Move to an open area away from buildings, trees, and electric poles.
- If you are in a vehicle, ask the driver to move slowly to a safe area, and remain inside until tremors stop.
Question 12:
“The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day. Would you carry an umbrella? Explain.”
Answer:
- No, do not carry an umbrella during a thunderstorm.
- The metal rod of the umbrella can attract lightning and is unsafe in open areas.
C. EXTENDED LEARNING — ACTIVITIES & PROJECTS
- Open a water tap. Adjust the flow so that it forms a thin stream. Charge a refill. Bring it near the water stream. Observe what happens. Write a short report on the activity.
- When the charged refill is brought near the thin stream of water, the water bends toward it due to electrostatic attraction.
- Make your own charge detector. Take a paper strip roughly 10 cm × 3 cm. Give it a shape as shown in the figure. Balance it on a needle. Bring a charged body near it. Observe what happens.
- The paper will move (get attracted or repelled) if the body is charged. This shows that static electricity can move a light object.
- Do this at night. Switch off the fluorescent tube light so that the room is dark. Charge a balloon. Bring it near the tube. You might see a faint glow.
- The charged balloon can excite the gas in the tube slightly, causing a faint glow.
- Find out if there is an organisation in your area which provides relief to those suffering from natural disasters. Enquire about the type of help they render. Prepare a brief report on the problems of earthquake victims.
- Talk to local NGOs, Red Cross, or disaster management groups.
- Write a short report describing how they supply food, water, medicine, shelter, etc., to victims.