Frank Brothers Solutions for ICSE Class 9 Physics Chapter 8.1 Electricity And Magnetism Static Electricity

Page No: 300

Question 1. Define electrostatics.
Answer: Electrostatics is defined as the study of charges at rest.
In simple words: It is the branch of physics where we learn about electricity that stays in one place on an object instead of flowing through wires.

📝 Teacher's Note: Use the example of a comb attracting bits of paper after being rubbed through hair to show that these charges aren't moving like current in a circuit.

🎯 Exam Tip: The keyword "at rest" is essential for this definition. Without it, the definition describes general electricity.

Question 2. What is another name for frictional electricity?
Answer: Frictional electricity is also known as static electricity.
In simple words: Static electricity is the same thing as frictional electricity because it is usually created when two things rub against each other.

📝 Teacher's Note: Explain that "Static" means stationary. Frictional electricity is the process (rubbing), and static electricity is the result (stored charge).

🎯 Exam Tip: Remember that both terms are interchangeable in the context of school-level physics.

Question 3. Name the scientist who observed the electric properties of amber.
Answer: Thales of Miletus was the scientist who observed the electric properties of amber.
In simple words: Long ago in Greece, a man named Thales noticed that rubbing amber (fossilized tree sap) made it pick up light things like feathers.

📝 Teacher's Note: Mention that the word "electron" actually comes from the Greek word for amber, "elektron."

🎯 Exam Tip: This is a factual history question; keep the name "Thales of Miletus" in mind for one-mark answers.

Question 4. How many kinds of charges are there?
Answer: There are two kind of charges – positive and negative charge.
In simple words: Just like there are two poles on a magnet, electricity has two types of "flavors": plus (positive) and minus (negative).

📝 Teacher's Note: Use a simple diagram of a plus sign and a minus sign to represent these two fundamental types of charges.

🎯 Exam Tip: Always specify "positive" and "negative" when asked for the types of charges.

Question 5. What does a positively charged body signify?
Answer: A positively charged body signifies that its nucleus contains more number of protons than electrons.
In simple words: An object becomes positive if it loses some of its electrons. Since protons are plus and electrons are minus, having more protons makes the whole object positive.

📝 Teacher's Note: Emphasize that protons do not move between objects; only electrons are transferred. A "positive" object is actually one that is "electron-poor."

🎯 Exam Tip: Focus your answer on the "imbalance" or "deficiency of electrons."

Question 6. What does a negatively charged body signify?
Answer: A negatively charged body signifies that it contains number of electrons more than number of protons.
In simple words: If an object gains extra electrons from something else, it becomes negative because it now has more minus-charges than plus-charges.

📝 Teacher's Note: Relate this to a math problem: if you start at zero and add more "negative" numbers, the total becomes negative.

🎯 Exam Tip: Use the term "excess of electrons" to describe a negatively charged body for full marks.

Question 7. Who assigned the algebraic signs to charges?
Answer: Benjamin Franklin was the scientist who first assigned the algebraic signs to charges.
In simple words: The famous American scientist Benjamin Franklin decided to call the two types of charges "Positive" and "Negative."

📝 Teacher's Note: It's interesting to note that his choice was arbitrary. If he had swapped them, all our electronics math would still work, just with flipped signs!

🎯 Exam Tip: This is a common historical science fact; associate "Franklin" with "+" and "-" signs.

Question 8. What charge does a glass rod acquire when rubbed with silk?
Answer: When a glass rod is rubbed with silk, the glass rod acquires the positive charge.
In simple words: Rubbing glass against silk makes the glass lose electrons to the silk, so the glass becomes positive.

📝 Teacher's Note: Use the "Glass-Silk-Positive" (GSP) mnemonic to help students remember this specific pair.

🎯 Exam Tip: In this pair, always remember that the silk becomes negative because it gains what the glass loses.

Question 9. What charge does an ebonite rod acquire when rubbed with cat’s fur?
Answer: When ebonite rod is rubbed with cat’s fur, the ebonite rod acquires the negative charge.
In simple words: Ebonite (a hard rubber) loves electrons. When rubbed with fur, it steals them from the fur and becomes negative.

📝 Teacher's Note: This is the opposite of the glass rod experiment. Here, the rod is the one gaining electrons.

🎯 Exam Tip: Be careful not to confuse glass-silk (positive rod) with ebonite-fur (negative rod).

Question 10. Define the quantization of charge.
Answer: The quantization of charge is the property by virtue of which every charge exists only in discrete lumps or packets of some minimum charge.
In simple words: You can only have electricity in "sets." Just like you can buy one egg or two eggs but never half an egg, you can have one electron's worth of charge or two, but never 1.5.

📝 Teacher's Note: Use the analogy of a staircase versus a ramp. You can stand on step 1 or step 2, but not "between" them. That is what "quantized" means.

🎯 Exam Tip: The formula \( Q = ne \) is the mathematical way to express this. Mentioning it shows good understanding.

Question 11. Name a quantity other than electric charge that is quantized.
Answer: Energy other than electric charge is quantized.
In simple words: Scientists have discovered that even energy itself travels in tiny packets rather than a smooth, continuous flow.

📝 Teacher's Note: This refers to "Photons" or "Quanta" in quantum physics. It shows that quantization is a fundamental rule of nature.

🎯 Exam Tip: "Energy" or "Light energy" are acceptable answers for this general question.

Question 12. Calculate the number of electrons in 1 Coulomb of charge.
Answer: \( Q = Ne \)
\( \implies N = Q/e = 1\text{C} / (1.6 \times 10^{-19}) = 6.25 \times 10^{18} \)
In simple words: To get just one "unit" (Coulomb) of electricity, you need a massive pile of more than 6 quintillion electrons!

📝 Teacher's Note: This calculation shows how tiny a single electron is. One Coulomb is a very large amount of static charge.

🎯 Exam Tip: Memorize the value \( 6.25 \times 10^{18} \) as it is a very common numerical constant in physics exams.

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Question 13. Which is bigger: 1 Coulomb of charge or the charge on an electron?
Answer: 1C is bigger than charge on an electron.
In simple words: A Coulomb is like a whole bucket of water, while the charge of an electron is just a single tiny drop.

📝 Teacher's Note: Remind students of the previous calculation. It takes billions of billions of electrons to make just 1 Coulomb.

🎯 Exam Tip: 1C is vastly larger; the charge on an electron is only \( 1.6 \times 10^{-19}\text{ C} \).

Question 14. What is the total charge on 100 electrons?
Answer: \( Q = Ne \)
The charge on an electron \( = -1.6 \times 10^{-19}\text{ C} \)
\( \implies Q = 100 \times 1.6 \times 10^{-19} = 1.6 \times 10^{-17}\text{ C} \)
In simple words: If you bundle 100 electrons together, you just multiply the charge of one by 100.

📝 Teacher's Note: Ensure students understand how to move the decimal point when multiplying by 100 (which is \( 10^2 \)).

🎯 Exam Tip: Always include the negative sign for electron charge unless only the magnitude is asked for.

Question 15. Is a charge of \( -3.6 \times 10^{-18}\text{ C} \) possible? Explain.
Answer: \( Q = Ne \)
\( \implies N = Q/e = -3.6 \times 10^{-18} / (1.6 \times 10^{-19}) = 22.5 \)
Since, n is not an integral value so this value of charge is not possible because charge is quantized.
In simple words: No, because you can't have 22 and a half electrons. Electrons only come in whole numbers, so this specific amount of electricity can't exist.

📝 Teacher's Note: This is the most practical application of quantization. If "n" isn't a whole number (like 1, 2, 3...), the charge is impossible.

🎯 Exam Tip: To prove a charge is impossible, show that "n" results in a fraction or decimal.

Question 16. What is the nature of frictional forces?
Answer: The origin of frictional forces is electrical in nature.
In simple words: When things rub together and feel "sticky," it's actually the tiny atoms and their electrical charges pulling on each other.

📝 Teacher's Note: Friction is one of the four fundamental forces that is actually explained by electromagnetism at the atomic level.

🎯 Exam Tip: Use the phrase "electrical in nature" or "electromagnetic" to describe friction.

Question 17. What is the cause of charging by friction?
Answer: The cause of charging is the frictional forces between the two bodied when they are rubbed against each other.
In simple words: Rubbing two objects together forces electrons to jump from one to the other because of the friction between them.

📝 Teacher's Note: Explain that work is being done during rubbing, and this energy helps electrons overcome their "bond" to the atom and move to the other surface.

🎯 Exam Tip: Mention the "transfer of electrons" as the physical result of rubbing.

Question 18. Name the scientists involved in identifying and naming the two types of charges.
Answer: The Sir Williams Gilbert was the scientist who showed two charges and Benjamin Franklin was the scientist who gave name to charges.
In simple words: Gilbert found out there were two types, and Franklin is the one who chose the names "plus" and "minus."

📝 Teacher's Note: William Gilbert was the court physician to Queen Elizabeth I and was one of the first to distinguish between magnetism and static electricity.

🎯 Exam Tip: Remember Gilbert for "identifying" and Franklin for "naming."

Question 19. State two differences between electric charge and mass.
Answer: Two differences between charge and mass are:

• Electric charge can be positive, negative or zero while mass of the body is strictly positive.
• Electric charge is quantized while the quantization of mass is not yet established.

In simple words: You can have "minus" electricity, but you can't have "minus" weight. Also, electricity only comes in specific tiny packets, but we aren't sure if mass does too.

📝 Teacher's Note: This is a high-level conceptual comparison. Use the example of a neutral atom to show that charge can be zero, but it still has mass.

🎯 Exam Tip: The "positive/negative" vs "strictly positive" is the easiest difference to remember for marks.

Question 20. Does the mass of a body get affected when it is charged?
Answer: Yes, mass of the body get affected on charging.
In simple words: Yes. Since charging means adding or removing electrons (which have a tiny bit of weight), a negative object gets slightly heavier and a positive one gets slightly lighter.

📝 Teacher's Note: Though the mass of an electron is extremely small (\( 9.1 \times 10^{-31}\text{ kg} \)), it is still a physical change that can be measured with ultra-precise tools.

🎯 Exam Tip: State "Yes" and explain that adding electrons increases mass, while removing them decreases it.

Question 21. What force will exist between two bodies B and C carrying opposite charges?
Answer: There will be attractive force between the B and C because both carry opposite charges.
In simple words: Just like the saying "opposites attract," a positive object and a negative object will pull toward each other.

📝 Teacher's Note: Use the analogy of a boy and girl dancing together—they are different but pulled toward each other. This helps younger students remember the rule.

🎯 Exam Tip: Always use the word "attractive" for unlike charges.

Question 22. State the basic law of electrostatics regarding attraction and repulsion.
Answer: Unlike charges attract each other and like charges repel each other.
In simple words: Charges that are different (plus and minus) want to be together. Charges that are the same (plus and plus, or minus and minus) push each other away.

📝 Teacher's Note: This is the "Fundamental Law of Electrostatics." Demonstrate this with two charged balloons—they will push apart if both are charged the same way.

🎯 Exam Tip: Make sure to mention both "attract" and "repel" for a complete statement of the law.

Question 23. Does the motion of an object affect the charge on it?
Answer: No, the motion of the object does not affect the charge on the body.
In simple words: Whether an object is sitting still or zooming at the speed of light, its electrical charge stays exactly the same.

📝 Teacher's Note: This is a unique property. Unlike mass or length (which can change at extremely high speeds in relativity), charge is "invariant."

🎯 Exam Tip: The answer is a definitive "No." Charge is independent of velocity.

Question 24. List the properties of an electric charge.
Answer: The properties of an electric charge are:

• Electric charges are quantized.
• Like charges repel each other while unlike charges attract each other.

In simple words: Electricity comes in small, specific amounts, and charges either pull or push each other depending on if they are different or the same.

📝 Teacher's Note: You can also add that charge is conserved—it can't be created or destroyed, only moved from one object to another.

🎯 Exam Tip: If asked for three properties, include "Conservation of Charge" as the third point.

Question 25. Can photons have an electric charge? Why?
Answer: Photons can never have charge because charges never exist without rest mass.
In simple words: Since light (photons) has no weight when it's still, it can't "carry" electricity. You need a physical particle like an electron to have a charge.

📝 Teacher's Note: This explains why light beams don't bend when you put a magnet or a battery near them. They are electrically neutral.

🎯 Exam Tip: The reason is specifically the lack of "rest mass."

Question 26. If a glass rod acquires a charge of \( +1.6 \times 10^{-19}\text{ C} \), what charge will the silk cloth acquire?
Answer: The silk cloth will acquire the charge opposite to the charge of glass rod. So the charge acquired by silk cloth will be \( = -1.6 \times 10^{-19}\text{ C} \).
In simple words: Electricity is like a trade. If the rod loses one electron's worth of charge, the silk must gain exactly that much.

📝 Teacher's Note: This is a perfect example of the "Conservation of Charge." The total charge before and after rubbing remains zero.

🎯 Exam Tip: The values are always the same, just flip the plus sign to a minus sign (or vice versa).

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Question 27. What is the S.I. unit of charge?
Answer: The S.I unit of charge is coulomb.
In simple words: Just like we measure weight in kilograms, we measure electricity in "Coulombs."

📝 Teacher's Note: It is named after Charles-Augustin de Coulomb. Abbreviate it with a capital "C".

🎯 Exam Tip: Always use the unit "C" or "Coulomb" when writing numerical answers for charge.

Question 28. What is the value of charge on an electron? Is a smaller charge possible?
Answer: The value of charge on an electron \( = -1.6 \times 10^{-19}\text{ C} \). No, the charge less than charge of an electron is not possible.
In simple words: An electron's charge is the smallest "crumb" of electricity. You can't break it down into anything smaller.

📝 Teacher's Note: (Advanced note: While "Quarks" have fractional charges, they never exist alone in nature, so for school physics, the electron is the smallest unit).

🎯 Exam Tip: The electron charge is considered the "elementary charge" (\( e \)).

Question 29. What is the net charge on a normal atom?
Answer: The net charge on an atom is zero.
In simple words: A normal atom is balanced. It has the same number of "plus" protons and "minus" electrons, so they cancel each other out.

📝 Teacher's Note: Use the term "electrically neutral" to describe an atom with zero net charge.

🎯 Exam Tip: An atom only becomes charged (an ion) when it loses or gains electrons.

Question 30. What are free electrons?
Answer: The electrons of the outermost orbit of an atom are the free electrons and they easily leave their respective atoms and become free to move inside the solid.
In simple words: These are the "wanderer" electrons on the outside of an atom that can easily jump to other atoms. They are the ones that actually move when you rub things.

📝 Teacher's Note: These are also the same electrons that allow metals to conduct electricity so well.

🎯 Exam Tip: Mention "outermost orbit" and "easy to move" in your definition.

Question 31. Define an ion and explain how positive and negative ions are formed.
Answer: An ion is an atom which has either gained one or more electrons or which has lost one or more electrons. When an atom’s valence electrons leave their atom and become free to move inside the solid and move to another atom then positive ion is formed. And, the other atom which gain electrons become the negative ion.
In simple words: An ion is just an atom that isn't balanced anymore. If it gives away electrons, it's a positive ion. If it takes extra ones, it's a negative ion.

📝 Teacher's Note: Think of it as a see-saw. Protons and electrons are balanced until some electrons jump off or jump on.

🎯 Exam Tip: Remember: Losing electrons = Positive ion; Gaining electrons = Negative ion.

Question 32. What type of charge exists on positive and negative ions?
Answer:

• On positive ion, there exist a positive charge.
• On negative ion, there exists a negative charge.

In simple words: The names say it all! Positive ions are positive, and negative ions are negative.

📝 Teacher's Note: This seems obvious, but it helps students link the terminology to the physical state of the atom.

🎯 Exam Tip: This is often part of a larger question about atom structure.

Question 33. Describe an observation showing attraction and repulsion between charged objects.
Answer: If the electrified silk cloth or the ebonite rod are brought near to each other then they will repel each other but when electrified silk cloth and glass rod are brought together then they attract each other. The charges on the electrified silk cloth and ebonite rod is negatives while on glass rod, its positive so we can say that like charges repel each other while unlike charges attract each other.
In simple words: If you have two things that are both negative, they push each other away. If one is plus and one is minus, they pull together.

📝 Teacher's Note: This experiment proves that there are exactly two types of charges. If there were a third, we'd see a different behavior.

🎯 Exam Tip: Use the words "electrified" or "charged" to describe the rods in your answer.

Question 34. Why does hair stand on end when rubbed with a nylon cloth?
Answer: When the nylon cloth get rubbed with hairs then it acquires the negative charge because few free electrons get transferred from hairs to nylon and they get attracted towards nylon cloth so the hair stands on their end.
In simple words: The nylon steals electrons from your hair. Your hair and the cloth now have opposite charges, so they pull toward each other, making your hair lift up.

📝 Teacher's Note: Also, since each individual hair becomes positively charged, the hairs try to repel *each other*, which makes them spread out fan-like.

🎯 Exam Tip: Attribute the phenomenon to the "transfer of electrons" during friction.

Question 35. Is a charge of \( -1.8\text{ e} \) possible? Why?
Answer: The charge is quantized and \( Q = ne \) where n should be only integer. Since \( Q = -1.8\text{ e} \) where n is not an integer. So, this charge is not possible.
In simple words: No. Since you can't have "point-eight" of an electron, you can never have this amount of charge. It must always be a whole number of electrons.

📝 Teacher's Note: This is a variation of Question 15. It tests the same "whole number rule" of quantization.

🎯 Exam Tip: Always state the principle: "n must be an integer."

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Question 36. Provide a labeled diagram of a gold leaf electroscope.
Answer: In simple words: This is a tool with two tiny gold flaps inside a jar. When they get electricity, they move apart, telling us if something is charged.

📝 Teacher's Note: Gold is used because it can be beaten into extremely thin, light sheets that can move with even the tiniest electrical force.

🎯 Exam Tip: Label the "Metal Cap," "Insulator/Plug," and "Gold Leaves" clearly. These are the most important parts.

Question 37. What is the purpose of an electroscope?
Answer: The purpose of an electroscope is to detect the presence of charge on the body and nature of charge (whether its positively charged or negatively charged) on the body.
In simple words: An electroscope is like a "charge detector." It tells you if an object has electricity and if that electricity is plus or minus.

📝 Teacher's Note: It is a qualitative instrument—it tells you *what* kind of charge is there, not exactly *how many* Coulombs.

🎯 Exam Tip: Mention both "presence" and "nature" of charge in your answer.

Question 38. Identify parts P, Q, and R in a negatively charged gold leaf electroscope.
Answer: P is a Cap, Q is plug and R is the bottle in the negatively charged gold leaf electroscope. P is a conductor and Q, R is an insulator.
In simple words: The top part (P) is metal so electricity can move through it. The plug and jar (Q, R) are made of materials that stop electricity so it doesn't leak away.

📝 Teacher's Note: The bottle (R) is glass to prevent outside air from moving the delicate leaves and to keep the charge inside.

🎯 Exam Tip: Clearly distinguish which parts are "conductors" and which are "insulators."

Question 39. What happens in an electroscope when a charged body is brought near?
Answer:
(i) P will have the positive charge.
(ii) Q will have no charge.
(iii) Cap of the electroscope will have no charge.
(iv) The gold leaf will have negative charge.
(v) The leaf will diverse because like charges repel each other.
(vi) If electroscope is earthed then metal rod will have positive charge.
In simple words: Bringing a charge near the top makes the internal charges shift. The leaves at the bottom end up with the same charge, so they "diverge" or push away from each other.

📝 Teacher's Note: This is "charging by induction." The charges in the metal rod are pulled or pushed by the object held near the cap.

🎯 Exam Tip: The word "diverge" means the leaves spread apart. Always use this word when explaining how an electroscope shows charge.

Question 40. What is the purpose of earthing an electroscope?
Answer: Earthing of an electroscope is meant to take the thick copper strip inside the earth so that charges induced from thunderstorm on the metal spikes can move to earth to safeguard buildings from thunderstorm.
In simple words: Earthing is like a safety drain for electricity. It sends dangerous charges into the ground so they don't hurt people or damage buildings.

📝 Teacher's Note: This refers to a "Lightning Conductor." It provides a low-resistance path for the lightning strike to reach the Earth safely.

🎯 Exam Tip: Explain that the Earth acts as a giant "reservoir" that can take in or give out any amount of charge without becoming charged itself.