Frank Brothers Solutions for ICSE Class 9 Physics Chapter 8.4 Electricity And Magnetism

Page No: 323

Question 1. Name the two kinds of electric charges.
Answer: Positive and negative are the two kind of charges.
In simple words: Just like magnets have north and south ends, electricity comes in two types called positive (+) and negative (-).

📝 Teacher's Note: Use the analogy of battery terminals to help students visualize positive and negative charges in everyday objects.

🎯 Exam Tip: Always mention both types of charges to get full marks in introductory physics questions.

Question 2. What is the net charge on an atom?
Answer: Net charge on an atom is zero.
In simple words: An atom is like a balanced scale; it has the same amount of positive and negative parts, so they cancel each other out.

📝 Teacher's Note: Explain that the number of protons (positive) equals the number of electrons (negative) in a neutral atom.

🎯 Exam Tip: Use the word "neutral" or "zero" to describe the overall charge of an atom.

Question 3. How many electrons are present in \( \text{Na}^+ \)?
Answer: 10 is the number of electrons present in \( \text{Na}^+ \).
In simple words: A normal Sodium atom has 11 electrons, but when it becomes positive (\( \text{Na}^+ \)), it loses one, leaving only 10.

📝 Teacher's Note: Remind students that a positive charge means an electron was lost, not a proton gained.

🎯 Exam Tip: For any positive ion, subtract the charge value from the atomic number to find the electron count.

Question 4. State the difference between an atom and an ion in terms of charge.
Answer: An atom does not have any net charge while ion is either a positive charge or a negative charged.
In simple words: An atom is perfectly balanced (neutral), but an ion is an atom that has gained or lost electrons, making it "unbalanced" or charged.

📝 Teacher's Note: Show the transformation of an atom to an ion using a simple addition/subtraction diagram of electrons.

🎯 Exam Tip: Clearly distinguish that atoms are neutral while ions are specifically charged particles.

Question 5. What is the reason for electrification when two bodies are rubbed?
Answer: The friction between two bodies when they are rubbed against each other is the reason of electrification.
In simple words: When you rub two things together, friction "knocks" electrons off one surface and onto the other, creating a static charge.

📝 Teacher's Note: Use the balloon-on-hair example to demonstrate this concept practically in class.

🎯 Exam Tip: The keyword here is "friction" or "transfer of electrons."

Question 6. What is a lightning conductor?
Answer: A lightening conductor is a device used to protect the large buildings against lightening during a thunderstorm.
In simple words: It is a metal rod on top of a building that catches lightning and safely carries the electricity into the ground so the building doesn't get damaged.

📝 Teacher's Note: Explain that it provides a low-resistance path to the Earth (earthing).

🎯 Exam Tip: Mention that it protects "large buildings" and works by providing a safe path to the ground.

Question 7. Why is thunder heard after lightning is seen?
Answer: The speed of light is very fast as compared to the speed of sound in air. So, the thunder is seen fast and then we hear the sound of thunderstorm.
In simple words: Light travels much faster than sound. It's like seeing a distant drummer hit a drum before the "boom" reaches your ears.

📝 Teacher's Note: Mention the speeds (light is \( 3 \times 10^8 \) m/s, sound is ~340 m/s) to show the huge difference.

🎯 Exam Tip: Focus your answer on the comparison of the speeds of light and sound.

Question 8. What is the use of an electroscope?
Answer: Electroscope is used to determine the nature of the charge and to detect the presence of charge on the body.
In simple words: It is a scientific tool that tells us if an object has electricity on it and whether that electricity is positive or negative.

📝 Teacher's Note: Demonstrate a gold-leaf electroscope if available; seeing the leaves move is very impactful for students.

🎯 Exam Tip: Remember the two functions: detection of charge and determination of the nature of charge.

Question 9. What happens when a positively charged rod is brought near a positively charged electroscope cap?
Answer: When a positively charged rod is brought near the positively charged cap then, the positive charge on the cap will spread over because of repulsion and some of them will reach to the leaves. Now, the leaves will have the same positive charge, they will repel each other and hence, leaves will diverge.
In simple words: Since both the rod and the cap are positive, they push more positive charge down to the leaves. Because both leaves become even more positive, they push away from each other and open wider.

📝 Teacher's Note: Emphasize the rule "Like charges repel" to explain why the leaves move apart.

🎯 Exam Tip: Use the term "diverge" to describe the movement of the electroscope leaves.

Question 10. Define direct current.
Answer: The flow of electrons in a particular direction is called direct current.
In simple words: Direct current (DC) is electricity that flows like a one-way street, always moving in the same direction.

📝 Teacher's Note: Compare DC (batteries) with AC (wall sockets) to provide real-world context.

🎯 Exam Tip: The key phrase is "particular direction" or "constant direction."

Question 11. State two differences between primary and secondary cells.
Answer: The two differences between primary and secondary cell are:
(i) Primary cells cannot be charged again while secondary cells can be charged again and again.
(ii) In primary cells, chemical reaction is irreversible while in secondary cells, chemical reaction is reversible.
In simple words: Primary cells are use-and-throw (like a remote battery), while secondary cells are rechargeable (like your phone battery).

📝 Teacher's Note: Use the example of a standard AA battery versus a phone battery to make the distinction clear.

🎯 Exam Tip: Focus on "rechargeability" and "reversibility of chemical reactions" as the two main points.

Question 12. What is the S.I. unit of current?
Answer: S.I. unit of current is ampere.
In simple words: Just like we measure weight in kilograms, we measure the "strength" or flow of electricity in Amperes.

📝 Teacher's Note: Briefly mention that the symbol for Ampere is 'A'.

🎯 Exam Tip: Ensure the spelling of "ampere" is correct; it is a very common one-mark question.

Question 13. What is a battery?
Answer: A battery is the made up of multiple cells.
In simple words: A single "battery" you buy at a store is often just one cell. When you connect several of them together, they form a "battery."

📝 Teacher's Note: Draw the circuit symbol for a cell versus a battery to show the difference visually.

🎯 Exam Tip: Define a battery as a "combination" or "series" of cells.

Question 14. Is current a scalar or vector quantity?
Answer: Current is a scalar quantity.
In simple words: Even though electricity flows in a direction, it follows simple math rules (like adding numbers) rather than arrow-math (vectors).

📝 Teacher's Note: This is a tricky concept. Explain that current doesn't follow vector addition laws, which is why it's a scalar.

🎯 Exam Tip: Always state "scalar" for electric current in school exams.

Question 15. Define electric current.
Answer: Current is defined as the rate of flow of charge.
In simple words: Current is a measure of how much electricity passes through a wire every second.

📝 Teacher's Note: Introduce the formula \( I = \frac{Q}{t} \) where Q is charge and t is time.

🎯 Exam Tip: Use the word "rate" in your definition, as it implies the involvement of time.

Question 16. Which is bigger: a milliampere or a microampere?
Answer: A milliampere is bigger than microampere.
In simple words: A milliampere is \( \frac{1}{1,000} \) of an Ampere, while a microampere is much smaller at \( \frac{1}{1,000,000} \).

📝 Teacher's Note: Show the prefixes: Milli (\( 10^{-3} \)) and Micro (\( 10^{-6} \)).

🎯 Exam Tip: Remember that "milli" is 1,000 times larger than "micro."

Question 17. What is another name for a variable resistor?
Answer: Rheostat is other name of variable resistor.
In simple words: A rheostat is a device, like a volume knob, that lets you change how much electricity can flow through a circuit.

📝 Teacher's Note: Show a picture or physical rheostat to explain how sliding the contact changes the resistance.

🎯 Exam Tip: Learn the spelling of "Rheostat" carefully.

Question 18. Name two sources of current.
Answer: A cell and battery are the two sources of current.
In simple words: Things like AA batteries or the battery in a car provide the electricity needed for circuits to work.

📝 Teacher's Note: You can also mention solar cells or generators as alternative sources.

🎯 Exam Tip: Cell and battery are the simplest and most common answers for this question.

Question 19. Is a voltmeter the same as a voltameter?
Answer: No, voltmeter is not the same as voltameter.
In simple words: They sound similar but are different. A voltmeter measures voltage, while a voltameter is used for chemical experiments with electricity.

📝 Teacher's Note: Clarify that a Voltmeter is for circuits, while a Voltameter (or Coulometer) is for electrolysis.

🎯 Exam Tip: Pay attention to the spelling "meter" vs "ameter" to distinguish the two devices.

Question 20. What is the use of an ammeter?
Answer: Ammeter is used to measure the magnitude of current in the circuit.
In simple words: An ammeter acts like a "flow meter" for electricity, showing exactly how many Amperes are flowing through a wire.

📝 Teacher's Note: Mention that an ammeter must always be connected in series.

🎯 Exam Tip: Use the phrase "magnitude of current" to sound more precise.

Question 21. How do electrons move in a conductor with the help of a battery?
Answer: Electrons flow inside the metallic conductor and a battery and cell helps in the movement of electrons in it.
In simple words: The battery acts like a pump that pushes the electrons through the metal wire.

📝 Teacher's Note: Explain that the chemical energy in the battery provides the potential difference needed to push the electrons.

🎯 Exam Tip: State that the battery provides the necessary "push" or potential difference for electron flow.

Question 22. Compare the resistance of a voltmeter and an ammeter.
Answer: Voltmeter has high resistance than the ammeter.
In simple words: A voltmeter has very high resistance so it doesn't "steal" any electricity, while an ammeter has very low resistance so it doesn't "block" the flow.

📝 Teacher's Note: Ideally, an ammeter should have zero resistance and a voltmeter should have infinite resistance.

🎯 Exam Tip: Voltmeter = High Resistance; Ammeter = Low Resistance. This is a very common comparison question.

Question 23. What is an artificial magnet?
Answer: An artificial magnet is the magnetized piece of iron or steel.
In simple words: It's a man-made magnet created by turning ordinary metal into a magnet using electricity or other magnets.

📝 Teacher's Note: Contrast this with natural magnets like lodestone to help students understand the difference.

🎯 Exam Tip: Mention "iron or steel" as the materials commonly used to make artificial magnets.

Question 24. How is a voltmeter connected in a circuit?
Answer: Voltmeter is always connected in parallel to the circuit.
In simple words: You connect it "across" the component you want to measure, like placing a bridge over a river.

📝 Teacher's Note: Explain that parallel connection is necessary because it measures potential difference *between* two points.

🎯 Exam Tip: The keyword is "parallel." Never connect a voltmeter in series.

Question 25. How is an ammeter connected in a circuit?
Answer: Ammeter is always connected in series in the circuit.
In simple words: You connect it directly in the path of the electricity, so all the flow goes right through it.

📝 Teacher's Note: Explain that series connection ensures the device measures the *total* current flowing through that path.

🎯 Exam Tip: The keyword is "series." Connecting an ammeter in parallel can blow a fuse or damage the meter.

Question 26. What is the S.I. unit of resistance?
Answer: S.I unit of resistance is ohm.
In simple words: We measure how much a wire "resists" or blocks electricity in units called Ohms.

📝 Teacher's Note: Introduce the Greek symbol Omega (\( \Omega \)) used to represent ohms.

🎯 Exam Tip: Ohm is the unit. Ensure you know the symbol \( \Omega \) as well.

Question 27. Define 1 ohm resistance.
Answer: The resistance of conductor is 1 ohm when a current of 1 ampere flows through it when the potential difference across it is 1 volt.
In simple words: If 1 Volt of "push" causes 1 Ampere of "flow" through a wire, we say that wire has 1 Ohm of resistance.

📝 Teacher's Note: Use Ohm's Law \( R = \frac{V}{I} \) to derive this definition for the students.

🎯 Exam Tip: Mention all three units: 1 ohm, 1 ampere, and 1 volt to get full marks.

Question 28. Calculate the current if 0.8 C of charge flows in 5 seconds.
Answer: \( I = Q/t \)
\( \implies I = 0.8/5 \)
\( \implies I = 0.16 \text{ Ampere} \).
In simple words: We divide the total charge by the time it took to flow to find the current.

📝 Teacher's Note: Remind students to always check that time is in seconds and charge is in Coulombs before calculating.

🎯 Exam Tip: Always write the formula \( I = \frac{Q}{t} \) and the final unit (Ampere) to ensure maximum points.

Question 29. What is the function of a rheostat?
Answer: Rheostat is an instrument to control the current in the circuit.
In simple words: It acts like a slider that lets you turn the "strength" of the electricity up or down.

📝 Teacher's Note: Explain that it works by changing the length of the wire through which current flows, thus changing resistance.

🎯 Exam Tip: The main function is to "control" or "vary" the current.

Question 30. What is a horse-shoe magnet and why is it useful?
Answer: Horse-shoe magnet is the shape of the magnet. This shape helps in bringing the poles near to each other and make the strong magnet.
In simple words: It's a magnet bent into a 'U' shape. Because the North and South ends are close together, the magnetic pull is much stronger in that gap.

📝 Teacher's Note: Contrast this with a bar magnet where poles are far apart, making the field weaker at any single point.

🎯 Exam Tip: State that bringing poles closer increases the magnetic strength in the gap.

Question 31. Define induced magnetism.
Answer: The magnetism acquired by a magnetic material when it is kept near a magnet is called induced magnetism.
In simple words: When you stick a paperclip to a magnet, that paperclip temporarily becomes a magnet itself—this is called "induction."

📝 Teacher's Note: Use a chain of paperclips to demonstrate how one magnetizes the next through induction.

🎯 Exam Tip: The key part of the definition is that the material is "near" a magnet, not necessarily touching it.

Question 32. What are the poles of a magnet?
Answer: The points of the magnet where attraction appears the maximum are called poles of magnet and they are situated at ends of the magnet.
In simple words: The ends of a magnet are its "strong spots." These are the North and South poles where the magnetic pull is most powerful.

📝 Teacher's Note: Sprinkle iron filings over a magnet to show how they cluster mostly at the ends.

🎯 Exam Tip: Define poles as the regions of "maximum attraction."

Question 33. State the differences between primary and secondary cells.
Answer: The differences between primary and secondary cell are:
(i) Primary cells cannot be charged again while secondary cells can be charged again and again.
(ii) In primary cells, chemical reaction is irreversible while in secondary cells, chemical reaction is reversible.
In simple words: Primary cells are disposable, while secondary cells can be used many times by recharging them.

📝 Teacher's Note: This is a repeat of Question 11, which highlights its importance in the curriculum.

🎯 Exam Tip: Focus on the "irreversible" vs "reversible" nature of the chemical reactions.

Question 34. Define an electric cell.
Answer: A cell is a source of energy which converts the chemical energy into electrical energy. The cell consists of two electrodes in the form of conducting rod immersed in the solution called electrolyte.
In simple words: A cell is a chemical "power pack" that uses a special liquid and two metal rods to produce electricity.

📝 Teacher's Note: Explain the three parts: two electrodes (anode/cathode) and the electrolyte.

🎯 Exam Tip: Mention the energy conversion: Chemical Energy \( \rightarrow \) Electrical Energy.

Question 35. Calculate the number of electrons in 1 Coulomb of charge.
Answer: \( I = Q/t = n e/t \)
So, \( n = I.t / e = 1 \cdot 1 / 1.6 \times 10^{-19} = 6.25 \times 10^{18} \)
So, \( 6.25 \times 10^{18} \) will be the number of electrons.
In simple words: Since each electron carries a tiny charge, it takes a massive amount of them (over 6 quintillion!) to make up just 1 Coulomb.

📝 Teacher's Note: The charge of one electron \( e \) is approximately \( 1.6 \times 10^{-19} \) Coulombs. This is a fundamental constant.

🎯 Exam Tip: Memorize the value \( 6.25 \times 10^{18} \) as it is a standard result for 1C of charge.

Question 36. Calculate the charge \( Q \) if \( I = 4.5 \text{ A} \) and \( t = 2.5 \text{ s} \).
Answer: \( Q = I \cdot t = 4.5 \times 2.5 = 11.25 \text{ C} \).
In simple words: Multiplying the current by the time gives you the total amount of electric charge that passed through.

📝 Teacher's Note: Rearranging \( I = \frac{Q}{t} \) gives \( Q = I \times t \). Ensure units are in Amperes and Seconds.

🎯 Exam Tip: Don't forget the unit 'C' (Coulomb) for the final answer.

Question 37. What is the use of a key in a circuit?
Answer: A key is used to put the current on and off in the circuit.
In simple words: A "key" is just another name for a switch. It opens or closes the path for electricity.

📝 Teacher's Note: Explain that "on" means a closed circuit and "off" means an open circuit.

🎯 Exam Tip: A "key" and a "switch" perform the same function in circuit diagrams.

Question 38. Define electrical resistance. What is its S.I. unit?
Answer: The effect to rate of flow of current by the wire is called its electrical resistance. Its S.I unit is ohm.
In simple words: Resistance is how much a wire tries to stop the flow of electricity. It's like friction for electrons.

📝 Teacher's Note: Use the water-in-a-pipe analogy: a thin pipe has higher resistance than a thick one.

🎯 Exam Tip: The S.I. unit is always "ohm." Make sure to mention it.

Question 39. On what factors does the resistance of a wire depend?
Answer: The resistance of the wire depends on the following factors:

• Length of the wire
• Cross-section of the wire
• Temperature of the wire.
• Material of the wire.

In simple words: Resistance changes based on how long the wire is, how thick it is, how hot it gets, and what it's made of (like copper vs. iron).

📝 Teacher's Note: Discuss how length increases resistance while thickness decreases it.

🎯 Exam Tip: Listing these four factors is a standard 2-3 mark question.

Question 40. How does temperature affect the resistance of a wire?
Answer: With increase in temperature of the wire, the resistance of wire increases.
In simple words: As a wire gets hotter, the atoms inside jiggle more, making it harder for electrons to pass through easily.

📝 Teacher's Note: This applies to most metals; for semiconductors, the behavior is different, but for this level, focus on metals.

🎯 Exam Tip: Remember: Higher Temperature = Higher Resistance for conductors.

Question 41. If a wire's resistance is 2 ohms and its length is doubled, what happens?
Answer: The resistance of wire will become 4ohm.
In simple words: Since resistance is directly linked to length, if you make the wire twice as long, it becomes twice as hard for electricity to get through.

📝 Teacher's Note: Resistance is directly proportional to length (\( R \propto L \)).

🎯 Exam Tip: If length doubles, resistance doubles. Simple math!

Question 42. Explain the construction and use of a variable resistor.
Answer: Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available.
A variable resistance is a device used to change the resistance and hence the current flowing through a circuit; without changing the voltage.
In simple words: It's a device with a sliding part that changes the length of the wire the electricity must travel through. This lets you control the flow of current.

📝 Teacher's Note: Use a dimmer switch as a real-life example of a variable resistor.

🎯 Exam Tip: Mention that it changes current "without changing the voltage" of the source.

Question 43. Distinguish between a galvanometer and an ammeter.
Answer: Galvanometer is used to detect the direction of current as well as to detect very weak current. Ammeter is used to measure the magnitude of an electric current in the circuit.
In simple words: A galvanometer is a super-sensitive "direction finder," while an ammeter is a "measuring tool" for stronger flows.

📝 Teacher's Note: Explain that a galvanometer can be converted into an ammeter by adding a shunt resistance.

🎯 Exam Tip: Keywords: Galvanometer = "detect weak current/direction"; Ammeter = "measure magnitude."

Question 44. Distinguish between a voltmeter and a voltameter.
Answer: Voltmeter is used to measure the potential difference between two points in the circuit. Voltameter is an instrument for measuring the voltaic electricity passing through it, by its effect in decomposing water or some other chemical compound acting as an electrolyte.
In simple words: A voltmeter is a tool for circuit testing, whereas a voltameter is a chemical container used for electrolysis.

📝 Teacher's Note: This distinction is important for preventing confusion in laboratory settings.

🎯 Exam Tip: Voltmeter measures "Volts" (potential difference); Voltameter measures electricity through "chemical decomposition."

Question 45. What is a natural magnet? State its properties.
Answer: The magnets (pieces of lodestone) found in nature is called natural magnet. The properties of magnet are:

• Attractive property
• Directive property
• like poles repel while unlike poles attract.

In simple words: Natural magnets are rocks found in the ground that can pull iron and always point North-South if you hang them.

📝 Teacher's Note: Explain "directive property"—how a freely suspended magnet always aligns with Earth's magnetic field.

🎯 Exam Tip: Always list these three standard properties when asked about magnets.

Question 46. What are the two usual shapes of magnets?
Answer: The two usual shapes of magnet are bar magnet and horse shoe magnet.
In simple words: Magnets usually come as straight bars or 'U' shapes called horseshoe magnets.

📝 Teacher's Note: Mention that the horseshoe shape is more powerful in the gap because the poles are closer.

🎯 Exam Tip: Be prepared to draw both shapes in your answer sheet.

Question 47. Explain magnetic induction. Why does it cause attraction?
Answer: The mechanism in which magnetism acquired by the magnetic material when kept near a magnet is called magnetic induction. When the magnetic material get magnetized, the pole developed on its side will be opposite to the magnet’s pole taken near to it. So, opposite poles attract each other. So, induction causes attraction.
In simple words: When you bring a magnet near a nail, it turns the nail into a magnet with an opposite end. Since opposites attract, the nail is pulled toward the magnet.

📝 Teacher's Note: This is why magnets attract unmagnetized pieces of iron—they magnetize them first!

🎯 Exam Tip: The explanation must mention that "opposite poles" are developed on the near side of the material.

Question 48. How do we draw magnetic field lines?
Answer: We will start drawing the magnetic field of lines from the north pole of magnet. When a small compass is placed at a point near the north pole of the magnet, the needle turns round on its pivot and settle in the fixed direction. At other points away from the pole the needle settles tangent to the drawn field line. By plotting the directions indicated by the needle, we can draw magnetic field of lines.
In simple words: You use a tiny compass and mark dots where the needle points. By connecting these dots, you see the invisible "force paths" around a magnet.

📝 Teacher's Note: Remind students that field lines always travel from North to South outside the magnet.

🎯 Exam Tip: Field lines are always curves and they never cross each other.

Question 49. In which direction does a freely suspended magnet rest?
Answer: It will always rest in north-south direction. The diagram is as follows. The compass is telling that the magnet is pointing in north-south direction.
In simple words: A magnet hanging on a string acts like a compass; it will always swivel until one end points North and the other points South.

📝 Teacher's Note: This is due to Earth's own magnetic field. The magnet aligns with the Earth's magnetic lines.

🎯 Exam Tip: Always specify "North-South" as the direction for a suspended magnet.

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Question 50. Define magnetic field lines.
Answer: The magnetic field of lines are the lines which get formed by joining the poles of the magnet and they are the continuous curves in which tangent drawn to it will give the direction of magnetic field.
In simple words: They are invisible maps of the magnetic force. If you put a compass on a line, it will point exactly along that curve.

📝 Teacher's Note: Emphasize that these lines are "imaginary" but represent a real physical force field.

🎯 Exam Tip: Use the keyword "continuous curves" to describe field lines.

Question 51. What happens to the magnetic strength if a magnet is broken into two halves?
Answer: By breaking the magnet into two halves, magnetic strength will always get half.
In simple words: If you snap a magnet in two, each smaller piece will have half the original pulling power.

📝 Teacher's Note: Also note that breaking a magnet creates two new, smaller magnets, each with its own N and S pole.

🎯 Exam Tip: Remember that breaking a magnet doesn't separate poles; it just creates two smaller, weaker magnets.

Question 52. What is the ratio of magnetic strength if a magnet is broken in half?
Answer: By breaking the magnet into two halves, magnetic strength will always get half. So, the ratio of magnetic strength will be 1:2.
In simple words: The new piece has 1 part strength compared to the original 2 parts of the whole magnet.

📝 Teacher's Note: This assumes an ideal break where mass is divided exactly in two.

🎯 Exam Tip: The ratio is 1:2 when comparing the broken piece to the original whole.

Question 53. If a charged body A touches neutral body B, what happens?
Answer:

• The charge gained by the B will be positive charge
• The B will be charged by the charging by contact. The charge on A will spread on B.

In simple words: When a "hot" object with electricity touches a "cold" one, the electricity spreads across both of them.

📝 Teacher's Note: This is called "conduction." The total charge is conserved but distributed over a larger surface.

🎯 Exam Tip: Charging by "contact" always results in both bodies having the *same* type of charge.

Question 54. List the stages of charging a gold leaf electroscope by induction.
Answer: Electroscope is the device used to detect the presence of charge and the nature of charge on the body. The four stages of the charging a gold leaf electroscope by induction are:
(i) Bring a positively charged rod near the cap of an electroscope. The cap will acquire the negative charge on it by induction. The leaves will diverge
(ii) Earth the electroscope by touching the cap with finger.
(iii) Remove the finger and keep the positively charged rod as such.
(iv) Now, take away the rod. The leaves will diverge because of charges induced by induction.
In simple words: You use a charged rod to push charges around without touching, then use your finger to let some charges escape or enter, leaving the device permanently charged.

📝 Teacher's Note: This method is useful because the resulting charge on the electroscope is the *opposite* of the rod used.

🎯 Exam Tip: Carefully follow the order: Bring rod \( \rightarrow \) Touch (Earth) \( \rightarrow \) Remove finger \( \rightarrow \) Remove rod.

Question 55. What is electrostatic induction?
Answer: Electrostatic induction is the process of charging a body by bringing another charged body near to it. When the X will come near to body B, the opposite charge will be acquired by body B. So, negative charge will be acquired. Similarly, by electrostatic induction, A will acquire the opposite charge of B so A will acquire the positive charge.
In simple words: It is like a "magic trick" where a charged object makes another object charged just by being near it, without ever touching.

📝 Teacher's Note: Explain that induction involves the redistribution of internal charges within the material.

🎯 Exam Tip: The key is that the two bodies do *not* touch during induction.

Question 56. Describe a lightning conductor and define resistance.
Answer:
(i) Lightening conductor is a device used to safeguard the large buildings against lightening of thunderstorm. Normally, they are fitted on the top of tall buildings as a safety device. It consist of few metal spikes fitted to the highest part of building and connected to thick copper strip which is buried inside the earth.
(ii) Resistance of the conductor: It is the property of the conductor by virtue of which it obstructs the flow of current in the circuit.
In simple words: A lightning rod is a safety spike that sends lightning into the ground. Resistance is simply how much a wire "fights" against the electricity trying to go through it.

📝 Teacher's Note: Explain that the "spikes" help dissipate the charge into the air more effectively.

🎯 Exam Tip: For the lightning conductor, mention the "thick copper strip" and "earthing."

Question 57. Compare the brightness of bulbs in series and parallel circuits.
Answer:

• When two bulbs will connect in series then resistance of the circuit will increase so the brightness of the bulbs will decrease.
• When the two bulbs will be connected in parallel then resistance of circuit will decrease so the bulbs brightness will increase.

In simple words: In a series circuit, bulbs have to share the "push," so they are dim. In parallel, each bulb gets the full push, so they are bright.

📝 Teacher's Note: Use the analogy of water pipes: series is like one long pipe with two hurdles; parallel is like two separate pipes side-by-side.

🎯 Exam Tip: Bulbs are always brighter in parallel than in series for a given power source.

Question 58. State the differences between conductors/insulators and primary/secondary cells.
Answer:
(i) Differences between Conductor and insulators:
Conductors allow passing of electric current through it. Insulator does not allow passing of current through it. Silver, Gold are good examples of conductor of electricity. Rubber, Cotton are good examples of insulators.
(ii) Flow of electrons is the flow of negative charges inside the conductor while flow of conventional current is the flow of positive charges inside a conductor.
(iii) Primary cells are the cells in which chemical reaction is irreversible so they can’t be charged again and again. While, secondary cells are the cells in which chemical reaction is reversible and can be charged again and again.
In simple words: Metals conduct; rubber blocks. Real electricity is moving electrons (negative), but old textbooks call it "conventional" (positive). Some batteries are disposable, others are rechargeable.

📝 Teacher's Note: Clarify that conventional current is just a historical convention—the real movement is electrons in the opposite direction.

🎯 Exam Tip: Silver and Gold are the best conductors, but Copper is most common in real life.

Question 59. State the properties and laws of magnetism.
Answer:
(i) Following are the properties of magnet:
Like poles of magnet repel each other while unlike poles attract. Magnet have an attractive property means it attract the magnetic material towards it. Magnet have the directive property means when it is held freely, it always point towards north-south direction.
(ii) The Laws of Magnetism: Like poles repel each other and unlike poles attract each other.
(iii) Magnetic materials get attracted towards the magnet but non-magnetic materials does not get attracted towards the magnet. Natural magnets are the pieces of lodestone which are found in nature while artificial magnet is the magnetized piece of iron or steel. Iron has high susceptibility and low retentivity means it acquires the magnetism faster and removes it soon as well. While, steel has low susceptibility and high retentivity.
In simple words: Magnets pull iron and point North-South. Iron is easy to magnetize but loses it fast; steel is hard to magnetize but keeps it forever.

📝 Teacher's Note: Explain "retentivity" as the ability of a material to stay a magnet after the source is removed.

🎯 Exam Tip: The "Law of Magnetism" is simply that likes repel and unlikes attract.

Question 60. Describe methods of magnetization.
Answer:
(i) Magnetization of Iron Bar by Single Touch Method: Place the iron piece on the table and stroke it with bar magnet from A to B with one end of the magnet. When the magnet reaches B it is lifted and brought back to A to repeat the stroke. After few strokes, we will notice that A has acquired the same polarity as the pole rubbed against it.
(ii) Magnetization of steel needle using electric current: Wind a length of copper wire around a steel needle and connect it to battery and switch. After the current is passed through the copper wire for some time, we will notice that steel needle got magnetized.
In simple words: You can make a magnet by either "rubbing" metal with another magnet in one direction or by "wrapping" it in wire and passing electricity through it.

📝 Teacher's Note: The electrical method produces much stronger magnets (electromagnets).

🎯 Exam Tip: For the single-touch method, emphasize that the magnet must be lifted and returned to the start, not rubbed back and forth.

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Question 61. Explain magnetic induction and how to test for magnetism.
Answer:
(i) Magnetic induction is the process in which magnetism is acquired by the magnetic material when a magnet is brought near it.
(ii) How to test: If we had a magnet then we will bring the magnet near to steel, if there is force of repulsion or attraction is experienced then steel was magnetized otherwise not. We will held the steel freely, if it points to north-south direction only then it is magnetized otherwise not.
(iii) Magnetic Meridian: It is the vertical plane containing the magnetic axis of a freely suspended magnet at rest under the earth’s field. Geographic Meridian: It is the vertical plane containing the geographic north and south poles of the earth and passing through the given place in the surface of earth.
(iv) Angle of declination: The angle between geographic and magnetic meridian is called angle of declination. Angle of Dip: The angle between the horizontal and earth’s magnetic field is known as angle of dip.
In simple words: Induction is turning metal into a magnet by being near one. To test a magnet, see if it repels another magnet or points North-South when hanging.

📝 Teacher's Note: Repulsion is the *only* sure test of magnetism because attraction can happen with unmagnetized iron too.

🎯 Exam Tip: Remember: "Repulsion is the sure test for magnetism."

Question 62. Answer various reasoning questions on static electricity and magnets.
Answer:

• When a nylon shirt, blouse or stockings are removed then they get rubbed with the hairs and due to friction between hairs and nylon, charges get induced on them and attractive force get induced between them.
• The rubbed ebonite rod contains more electrons so it acquires the negative charge so a charged is acquired by it. But, in the rubbed metal rod, electrons are transferred but through hands and body they pass to the earth so no charges will be there while metal rod is held in hand.
• Soft iron is used to make electromagnets because it has high susceptibility and low retentivity, it get magnetized very faster and loses magnetism as soon as the inducing magnet is removed.
• Two steel needles hanging from the lower end of the vertical bar magnet do not hang vertical but will diverge because they got the like poles on it and like poles repel each other so they diverge.
• If magnetic material is brought near to the magnet then it get attracted although its not magnetized but the magnetic material will be repelled only in case it is magnetised. So repulsion is the sure test of magnetism.

In simple words: Clothes crackle because of friction. You can't charge a metal rod by hand because the electricity flows through your body into the ground. Steel needles on a magnet push away from each other because they both get the same magnetic charge.

📝 Teacher's Note: Explain that our bodies are conductors, which is why static charges leak away if we hold a metal object.

🎯 Exam Tip: Use the term "retentivity" when explaining why soft iron is preferred for electromagnets.

Question 63. Define a neutral point and show magnetic fields of like/unlike poles.
Answer:
(i) Neutral point is the point where magnetic field of the magnet is equal to the magnitude of earth’s horizontal magnetic field, but in opposite direction.
(ii) Magnetic interactions:
In simple words: A neutral point is a "dead zone" where the magnet's pull and Earth's pull perfectly cancel each other out.

📝 Teacher's Note: At a neutral point, a compass needle will not point in any specific direction; it will stay wherever you put it.

🎯 Exam Tip: Define neutral point by mentioning that the magnet's field and Earth's field are "equal and opposite."

Question 64. State two disadvantages of setting up large hydroelectric dams.
Answer: Disadvantages of setting up large dams for hydro-electricity power station:

• It leads to large catchment area being submerged
• displacement of people from their places.

In simple words: Building huge dams floods massive areas of land and forces entire villages of people to move away from their homes.

📝 Teacher's Note: Discuss the environmental impact on local ecosystems and fish migration as well.

🎯 Exam Tip: Focus on "environmental submergence" and "human displacement" as the two primary social/environmental costs.

Question 65. Suggest improved practices for the use of energy.
Answer: Improved practices in the use of energy:

• Use the improved cook stove which will save the bio-mass energy
• Use the improved solar panels and solar furnaces.

In simple words: We can save energy by using better stoves that don't waste wood and by switching to solar power whenever possible.

📝 Teacher's Note: Explain that "bio-mass energy" usually refers to using wood, crops, or animal waste as fuel.

🎯 Exam Tip: Mentioning "Solar energy" is always a strong answer for energy-saving questions.

Question 66. What are the functions of the Bureau of Energy Efficiency (BEE)?
Answer: The functions of bureau of energy efficiency setup are:

• control and keep an eye on consumption pattern
• develop appliances suitable for the energy change
• organize posters and slogans for developing awareness in society.

In simple words: The BEE is a group that watches how much power a country uses, invents energy-saving machines, and teaches people how to save electricity.

📝 Teacher's Note: Show the students a "Star Rating" label on a home appliance as a real-world example of BEE's work.

🎯 Exam Tip: Use terms like "consumption pattern" and "awareness" to describe their role.

Question 67. How can energy be saved at school and at home?
Answer:

• At School: Use of solar cells to light up the electricity in schools and do rain water harvesting, keep the electrical appliances off after the needed period of time.
• At Home: Use of bio energy for cooking and lightening, use CFLs instead of filament bulbs, keep the electrical appliances off after the needed period of time.

In simple words: Turn off lights and fans when you leave a room. Use solar power at school and energy-saving bulbs (CFLs) at home.

📝 Teacher's Note: Discuss the difference between CFL/LED bulbs and old filament bulbs (which waste much more heat).

🎯 Exam Tip: "Switching off appliances when not in use" is a universal answer for both categories.

Question 68. Show diagrams of magnetic field lines.
Answer: (i) Bar magnet field lines (ii) Unlike poles field lines.
In simple words: The lines loop out of the North end and go back into the South end, showing the path a small magnet would follow.

📝 Teacher's Note: Remind students that the lines are closest together near the poles where the force is strongest.

🎯 Exam Tip: Ensure your arrows on the field lines always point away from North and toward South.