Selina Concise Solutions for ICSE Class 9 Physics Chapter 10 Magnetism

Exercise 10(A)

Question 1S. What is lodestone? State its properties and use.
Answer: Lodestone is an ore of iron oxide (\( \text{Fe}_3\text{O}_4 \)). This ore attracts small pieces of iron and it sets itself along a definite direction when it is suspended freely. It is a natural magnet which was used for the navigation by the mariners.
In simple words: Lodestone is a natural magnetic rock found in the ground. Sailors used it like an early compass because it always points in the same direction when hanging from a string.

📝 Teacher's Note: You can demonstrate the "directive property" by hanging a bar magnet from a silk thread. Show how it always returns to the North-South position after being disturbed.

🎯 Exam Tip: Remember the chemical formula of lodestone is \( \text{Fe}_3\text{O}_4 \). Mentioning "directive" and "attractive" properties is key for full marks.

Question 2S. What are natural magnets? State their limitations.
Answer: The pieces of lodestone found in nature are called the natural magnets. Limitations of a natural magnet are as listed below:
(i) They are irregular and odd shaped.
(ii) They are not magnetically very strong.
In simple words: Natural magnets are just magnetic rocks. Because they come from nature, they have weird shapes and aren't very strong compared to the magnets we make in factories.

📝 Teacher's Note: Use the irregularity of a natural rock to explain why they aren't suitable for high-precision scientific instruments.

🎯 Exam Tip: Always list "weak magnetic field" and "irregular shape" as the primary drawbacks of natural magnets.

Question 3S. What is an artificial magnet? Why are they required?
Answer: An artificial magnet is a magnetized piece of iron (or other magnetic material). Artificial magnets are required because natural magnets have odd and irregular shape and they are not magnetically very strong. Artificial magnets can be given desired shape and made very strong.
In simple words: Artificial magnets are human-made. We create them because we can make them into any shape we want (like bars or horseshoes) and make them much stronger than rocks.

📝 Teacher's Note: Show examples of different shapes: Bar, Horseshoe, and Button magnets. Explain that these are made by stroking iron with a magnet or using electricity.

🎯 Exam Tip: The ability to "customize shape" and "control strength" are the two reasons why artificial magnets are preferred.

Question 4S. What happens when an iron rod and a copper rod are placed near a bar magnet?
Answer: Iron rod is magnetized when placed near a bar magnet by magnetic induction, while copper rod is not magnetized.
In simple words: Iron "catches" magnetism just by being near a magnet. Copper is not a magnetic material, so it doesn't care if a magnet is nearby.

📝 Teacher's Note: This is a great way to introduce magnetic vs. non-magnetic materials. Only ferromagnetic substances like Iron, Cobalt, and Nickel respond to induction.

🎯 Exam Tip: Use the term "magnetic induction" to explain how the iron rod becomes temporary magnetic.

Question 5S. How can you distinguish between a magnet and a soft iron bar of similar size?
Answer: A magnet when suspended freely will rest only in north-south direction, but the soft iron bar will rest in any direction.
In simple words: A magnet is like a compass—it always wants to point North. A plain piece of iron has no preference and will stay pointing wherever you leave it.

📝 Teacher's Note: This is the most reliable "non-destructive" test. Repulsion is another test, but it requires having another known magnet.

🎯 Exam Tip: Mention the "directive property" as the distinguishing factor between a magnet and an unmagnetized iron bar.

Question 6S. Fill in the blanks:
Answer: (a) Poles, (b) Attract, repel, (c) At the middle, (d) North – South
In simple words: Magnets are strongest at their ends (poles), and opposites attract while likes push away.

📝 Teacher's Note: Demonstrate the "middle" property using iron filings; show how almost no filings stick to the exact center of a bar magnet.

🎯 Exam Tip: Remember: Attraction can happen between a magnet and iron, but Repulsion only happens between two magnets.

Question 7S. Describe the behavior of a freely suspended magnet.
Answer: If a small magnet is suspended by a silk thread such that it can swing freely then it rests itself in the geographic north-south direction.
In simple words: A hanging magnet acts like a compass needle. It will swivel until it points toward the North and South poles of the Earth.

📝 Teacher's Note: Why a silk thread? Silk is non-torsional, meaning it doesn't try to untwist itself and influence the magnet's movement.

🎯 Exam Tip: Use the term "Geographic North-South" to be precise in your description.

Question 8S. What is induced magnetism?
Answer: The magnetism acquired by a magnetic material when it is kept near (or in contact with) a magnet, is called induced magnetism.
In simple words: If you leave a nail stuck to a magnet for a while, the nail starts acting like a magnet too. It "borrows" the power from the real magnet.

📝 Teacher's Note: This is a temporary phenomenon. When the inducing magnet is removed, the induced magnetism mostly disappears in soft iron.

🎯 Exam Tip: In diagrams, show that the end of the material near the magnet's pole develops an "opposite" polarity.

Question 9S. Define magnetic induction.
Answer: The process in which a piece of magnetic material acquires the magnetic properties temporarily in presence of another magnet near it is called the magnetic induction. When a piece of iron is placed near or in contact with a magnet, the piece of iron becomes a magnet i.e., it acquires the property of attracting iron filings when they are brought near its ends. Thus, a piece of iron behaves as a magnet as long as it is kept near (or in contact with) a magnet.
In simple words: This is the name for the "invisible magic" where a magnet teaches a piece of iron how to be a magnet just by being nearby.

📝 Teacher's Note: This proves that a magnet can influence materials even without touching them, through its magnetic field.

🎯 Exam Tip: Emphasize the word "temporary" in your definition, as the material loses magnetism once the magnet is taken away.

Question 10S. Why are iron nails attracted to a magnet?
Answer: When iron nails are brought near one end of a magnet, the nearer end of piece acquires an opposite polarity by magnetic induction. Since unlike poles attract each other, therefore, iron nails are attracted towards the end of the magnet. Thus, the iron nail first becomes a magnet by induction and then it is attracted.
In simple words: A magnet first turns a nail into a tiny magnet with the opposite "face" (pole). Since opposites like each other, they pull together and stick.

📝 Teacher's Note: This leads to the famous phrase: "Induction precedes attraction."

🎯 Exam Tip: Use the logic: 1. Induction creates opposite pole \( \implies \) 2. Opposite poles attract each other.

Question 11S. Explain the following observations:
(a) Why do the pointed ends of two pins hung from the same pole move apart?
(b) Why can several soft iron pins form a chain from a magnet pole?
(c) Why does a piece of soft iron attract a magnetic needle?
Answer:
(a) When two pins are hung by their heads from the same pole of a magnet, they acquire same polarity. Because like poles repel each other, their pointed ends move apart.
(b) Several soft iron pins can cling one below the other from the pole of a magnet because the magnet induces magnetism in an iron nail which gets attracted by the magnet and clings to it. This magnetized nail magnetizes the other nail near it by magnetic induction and attracts it. This process continues until force of attraction on first nail is sufficient to balance the total weight of all nails in chain.
(c) When a piece of soft iron is placed a little distance away from the needle, the needle induces magnetism to the piece of soft iron. Thus, soft iron piece starts behaving like a magnet and it attracts the magnetic needle towards it.
In simple words: (a) The pins become identical twins; twins often fight and push away! (b) It's like a relay race where magnetism is passed from one nail to the next. (c) The needle "convinces" the iron to be a magnet so they can pull together.

📝 Teacher's Note: For part (a), draw two vertical pins with 'N' at the top and 'S' at the bottom. The two 'S' poles at the bottom will clearly repel.

🎯 Exam Tip: For the "nail chain" question, remember to mention that gravity eventually wins over the weakening induced magnetism.

Question 12S. Describe what happens to an iron bar during induction.
Answer: The iron bar acquires magnetism due to magnetic induction. If the magnet is removed, the iron bar loses its magnetism.
In simple words: The iron bar is a "temporary helper." It acts like a magnet while the main magnet is there, but goes back to being a normal bar once the magnet leaves.

📝 Teacher's Note: This distinguishes "Soft Magnetic Materials" (like soft iron) from "Hard Magnetic Materials" (like steel), which would stay magnetic.

🎯 Exam Tip: Note that "soft iron" is used for temporary magnets specifically because it loses magnetism easily.

Question 13S. How long does induced magnetism last?
Answer: Induced magnetism is temporary as it lasts as long as the magnet causing induction remains in it vicinity.
In simple words: It's a short-term power. The iron bar only has its magnetic "charge" while the real magnet is close by.

📝 Teacher's Note: Vicinity means the area surrounding the magnet where its force can be felt.

🎯 Exam Tip: Use the word "temporary" in your answer to define induced magnetism.

Question 14S. Why is it said that induction precedes attraction?
Answer: When a piece of magnetic material is brought near a magnet, it first becomes a magnet by induction and then it is attraction. Thus, we say that induction precedes attraction.
In simple words: A magnet doesn't just pull iron; it first turns the iron into a magnet, then the two magnets pull toward each other. The "turning" happens first.

📝 Teacher's Note: This is a very important logical concept. Attraction only occurs between *two* magnets (one can be temporary). This is why repulsion is the only sure test of a permanent magnet.

🎯 Exam Tip: Explain that induction creates an opposite pole in the iron, which is then attracted to the inducing pole.

Question 15S. Define a magnetic field line.
Answer: A magnetic field line is a continuous curve in a magnetic field such that tangent at any point of it gives the direction of the magnetic field at that point.
In simple words: Magnetic field lines are invisible paths that show which way a tiny compass would point if you moved it around a magnet.

📝 Teacher's Note: These lines are imaginary models used to visualize the magnetic force. They were first introduced by Michael Faraday.

🎯 Exam Tip: The keyword is "tangent." The direction of the field is along the tangent to the field line.

Question 16S. State the properties of magnetic field lines.
Answer: Properties of magnetic field lines:
1. They are closed and continuous curves.
2. They are directed from the North Pole towards the South Pole outside the magnet.
3. The tangent at any point on a field line gives the direction of magnetic field at that point.
4. Two magnetic lines never intersect each other.
In simple words: These lines go in loops from North to South, they show the direction of force, and they never cross each other like a tangled mess.

📝 Teacher's Note: Use the analogy of "highway lanes"—the lines stay in their own path and never crash into each other.

🎯 Exam Tip: Memorize these four points. They are very frequent in 3 or 4-mark questions.

Question 17S. Why do iron filings form a specific pattern around a magnet?
Answer: The iron filings take up a definite pattern (curved lines). This happens because each piece of iron filing becomes a magnet to the magnetic induction of the magnet. It thus experiences a force in the direction of magnetic field of the bar magnet at that point and aligns itself along curved lines.
In simple words: Every tiny piece of iron dust becomes a tiny magnet. They all point along the invisible force lines of the big magnet, making the invisible field visible to our eyes.

📝 Teacher's Note: You can easily demonstrate this with a shaker of iron filings and a piece of paper over a bar magnet. It's a classic classroom moment.

🎯 Exam Tip: Mention "magnetic induction" to explain why the filings behave like tiny magnets.

Question 18S. Describe the method of plotting magnetic field lines using a compass needle.
Answer: Method of plotting the magnetic field lines using a compass needle:
Fix a sheet of paper on a drawing board by means of board pins. Place a small compass needle at position 1 as shown in fig (a) and looking from the top of the needle, mark two pencil dots exactly at two ends of the needle. Then move the compass needle to position 2 in such a way that one end of needle coincides with the second pencil dot. Repeat the process of moving the compass needle to positions 3, 4,… to obtain several dots. On joining the different dots, you will get a straight line. Thus one line of magnetic field of earth is traced.
This process is repeated starting from a different point and tracing out another line of magnetic field. In this manner, several lines of magnetic field can be drawn. Each line should be labeled with an arrow from the south pole of the needle towards the north pole to indicate the direction of the magnetic field.
In simple words: You place a compass, mark where it points with dots, move it, and mark again. Connecting these dots draws the map of the Earth's invisible magnetic force.

📝 Teacher's Note: For Earth's magnetic field, the lines are parallel and straight over a small area. This is called a "uniform magnetic field."

🎯 Exam Tip: Remember to mention that the arrow points from the "South Pole of the needle towards the North Pole."

Question 19S. Why do two magnetic field lines never intersect each other?
Answer: No two magnetic field lines can intersect each other. If they do, there would be two directions of the field at that point which is not possible.
In simple words: If lines crossed, a compass needle wouldn't know which way to point at the crossroads! Since a needle can only point in one direction at a time, the lines can never cross.

📝 Teacher's Note: Use the analogy of a person at a fork in the road—you can only walk down one path at a time; you can't go both ways simultaneously.

🎯 Exam Tip: The phrase "two directions of the field at one point is not possible" is the standard required explanation.

Question 20S. Sketch the magnetic field lines when (a) like poles face each other and (b) unlike poles face each other.
Answer:
(a) The North Pole of two magnets is facing each other. So, the field lines will push away from each other.
(b) The North Pole of one magnet is facing the South Pole of the other. So, the field lines will go from the North Pole to the South Pole.
In simple words: When like poles meet, the force lines curve away like they are avoiding each other. When unlike poles meet, the lines jump across and connect the two magnets.

📝 Teacher's Note: Point out the "Neutral Point" in diagram (a) where the lines are furthest apart. This is a zone with no magnetic force.

🎯 Exam Tip: Arrows on field lines MUST always point from North to South.

Question 21S. State two evidences of existence of earth’s magnetic field.
Answer: Two evidences of existence of earth’s magnetic field:
1. A freely suspended magnetic needle always rests in geographic north-south direction.
2. Neutral points are obtained on plotting the field lines of a magnet.
In simple words: We know Earth is a giant magnet because compasses always point North, and we find special "quiet spots" where a regular magnet's power is canceled out by the Earth's magnetism.

📝 Teacher's Note: Mention that birds like pigeons use this field to find their way home over long distances.

🎯 Exam Tip: "Freely suspended needle" and "Neutral points" are the two best evidence points to write in an exam.

Question 24S. What is the magnitude of the magnetic field at a neutral point? Why?
Answer: Magnitude of magnetic field at neutral points is zero. It is so because at these points, the magnetic field of the magnet is equal in magnitude to the earth’s horizontal magnetic field, but it is in opposite direction. Hence, they cancel each other.
In simple words: A neutral point is a "quiet zone." It's like a tug-of-war where both sides pull with exactly the same strength, so nothing moves. The magnetic power at that spot is zero.

📝 Teacher's Note: Use a vector diagram to show how two equal arrows pointing in opposite directions result in zero net force.

🎯 Exam Tip: Specify that it is the "earth's horizontal magnetic field" that is being canceled for full marks.

Question 25S. What can be concluded if the magnetic field at a point is zero?
Answer: It can be concluded that magnetic field at that point is zero. This is because the earth’s magnetic field at that point is neutralized by the magnetic field of some other magnetized material.
In simple words: If you find a spot where a compass doesn't point North, it means another magnet nearby is pushing back just as hard as the Earth.

📝 Teacher's Note: This spot is called a "Null point" or "Neutral point."

🎯 Exam Tip: Use the word "neutralized" to describe how the two fields interact.

Question 26S. Explain why neutral points are important and how they are detected.
Answer: Neutral points are the points where the magnetic field of the magnet is equal in magnitude to the earth’s horizontal magnetic field, but it is in opposite direction. Thus the net magnetic field at the neutral points is zero.
Since the net magnetic field is zero at neutral points, the compass needle remains unaffected (i.e. it comes to rest pointing in any direction) at these points and hence, they can be detected.
In simple words: These "dead zones" are useful because they let us measure the strength of the Earth's own magnetic field. You find them by moving a compass around until the needle just spins or stays anywhere you leave it.

📝 Teacher's Note: Demonstrate this in the lab. When the needle doesn't have a "favorite" direction, you are exactly at the neutral point.

🎯 Exam Tip: Mention that the compass needle is "unaffected" or "in any direction" at a neutral point.

Question 27S. Where will neutral points be found if (i) North pole of magnet faces geographic North, (ii) North pole faces geographic South?
Answer:
(i) Neutral points will be in east-west direction.
(ii) Neutral points will be north-south direction.
In simple words: Depending on how you turn the magnet, the "dead zones" will show up on either the sides of the magnet or at the ends.

📝 Teacher's Note: This is a standard practical observation. Sketch both scenarios on the blackboard to help students visualize the locations.

🎯 Exam Tip: North-North alignment leads to side (equatorial) neutral points. North-South alignment leads to end (axial) neutral points.

Question 28S. Describe the magnetic field of the Earth in terms of uniformity and location of neutral points.
Answer: (a) Uniform, (b) Zero and (c) On either side of the magnet in east and west.
In simple words: Over a small area, Earth's magnetism is even and steady (uniform).

📝 Teacher's Note: This part (a) specifically describes the Earth's field as "Uniform" over small regions.

🎯 Exam Tip: "Uniform field" means the lines are straight, parallel, and equidistant.

Question 1M. Two similar magnetic poles
(a) Attract each other
(b) Repel each other
(c) Neutralize each other
(d) None of the options
Answer: (b) Repel each other
In simple words: Two Norths (or two Souths) will push away from each other.

📝 Teacher's Note: This is the Basic Law of Magnetism.

🎯 Exam Tip: Always look for the word "repel" when poles are "similar" or "like."

Question 2M. Magnetic field lines of earth over a small area are
(a) Parallel equidistant straight lines
(b) Curved lines
(c) Converging lines
(d) Intersecting lines
Answer: (a) Parallel equidistant straight lines
In simple words: Over a tiny patch of ground, the Earth's giant magnetic force looks like perfectly straight and even lanes.

📝 Teacher's Note: This represents a uniform magnetic field.

🎯 Exam Tip: Remember: Uniform field = Parallel and Equidistant.

Exercise 10(B)

Question 1S. What is an electromagnet?
Answer: An electromagnet is a temporary strong magnet made from a piece of soft iron when current flows in the coil wound around it. It is an artificial magnet.
In simple words: It's a magnet you can turn on and off with a switch! It only works when electricity is flowing through the wire.

📝 Teacher's Note: Demonstrate this using a battery, a wire, and an iron nail. It's the most effective way to teach the concept of temporary magnetism.

🎯 Exam Tip: Mention "soft iron" and "temporary" to get full marks for the definition.

Question 2S. Which material is used for preparing an electromagnet?
Answer: The material used for preparing an electromagnet is soft iron.
In simple words: Soft iron is used because it becomes a magnet very quickly and loses its power just as fast when you turn the electricity off.

📝 Teacher's Note: Contrast this with steel, which keeps its magnetism even after the power is off, making it a "permanent" magnet.

🎯 Exam Tip: Soft iron is the standard core material for temporary magnets like those in electric bells.

Question 3S. How is an electromagnet made and what does its strength depend on?
Answer: An electromagnet is made by winding an insulated copper wire around a soft iron core either in the shape of a solenoid or U-shape.
The strength of magnetic field of an electromagnet depends on:
1. The number of turns of wire wound around the coil, and
2. The amount of current flowing through the wire.
In simple words: Wrap wire around an iron bar. To make it stronger, wrap it more times or pump more electricity through it.

📝 Teacher's Note: Use the formula \( B \propto N \cdot I \). This is a directly proportional relationship for both turns and current.

🎯 Exam Tip: These two factors are the most frequently asked "variables" in electromagnet questions.

Question 4S. Draw the circuit diagram for a solenoid electromagnet.
Answer:
In simple words: This shows wire wrapped around a bar. When you close the switch, the iron bar turns into a magnet with North and South poles.

📝 Teacher's Note: The "Right-hand grip rule" can be used to determine the N and S poles of the solenoid based on the current direction.

🎯 Exam Tip: Label the components like "Core", "Coil", and "Battery" for full credit on diagrams.

Question 5S. How can the strength of a solenoid electromagnet be increased?
Answer: By increasing the number of turns of winding in the solenoid, the strength of the electromagnet can be increased.
In simple words: More loops of wire equals a more powerful magnet.

📝 Teacher's Note: Explain that each turn adds its own magnetic field to the total, making the overall field much stronger.

🎯 Exam Tip: Always mention "increasing turns" or "increasing current" as the two main methods.

Question 6S. Identify the device formed and state one use.
Answer: The device formed is an electromagnet. Use: For separating the magnetic substances such as iron from other debris.
In simple words: It's an electromagnet. Scrap yards use them to pick up old car pieces while leaving the plastic and rubber behind.

📝 Teacher's Note: Electromagnets are also used in electric bells, relays, and MRI machines.

🎯 Exam Tip: Separation of magnetic materials is a classic textbook example of industrial use.

Question 7S. Draw a U-shaped electromagnet.
Answer:
In simple words: This is a U-shaped bar with wire coiled around both ends. It puts both magnetic poles close together, making it very strong for lifting.

📝 Teacher's Note: Explain that in a U-shaped electromagnet, the coils must be wound in opposite directions on the two arms to create a North and a South pole.

🎯 Exam Tip: Label the poles (N and S) at the very tips of the U-bar.

Question 8S. List the ways to increase the strength of an electromagnet.
Answer: The strength of an electromagnet can be increased by following ways:
1. Increasing the number of turns of winding in the solenoid.
2. Increasing the current through the solenoid.
In simple words: More wire loops or more electricity will both make the magnet stronger.

📝 Teacher's Note: This repeats a key concept—repetition in textbooks usually means it's a very important exam topic.

🎯 Exam Tip: If the question is worth 2 marks, list both points clearly.

Question 9S. Where is an electromagnet used?
Answer: The electromagnet is used in an electric relay.
In simple words: An electric relay uses a magnet to flip a switch that controls a much larger machine.

📝 Teacher's Note: Relays are common in cars and large industrial circuits to safely switch high-power machines using a low-power switch.

🎯 Exam Tip: "Electric Bell" and "Loudspeaker" are other good examples to remember.

Question 10S. State two advantages of an electromagnet over a permanent magnet.
Answer:
1. An electromagnet can produce a strong magnetic field.
2. The strength of the magnetic field of an electromagnet can easily be changed by changing the current in its solenoid.
In simple words: Electromagnets can be made way stronger than permanent magnets, and you can change their power level whenever you want.

📝 Teacher's Note: Another major advantage is that you can turn an electromagnet OFF, which you cannot do with a permanent magnet.

🎯 Exam Tip: The ability to "control strength" is a key technical advantage.

Question 11S. Differentiate between an electromagnet and a permanent magnet.
Answer:

In simple words: Electromagnets are "adjustable" and temporary. Permanent magnets are "fixed" and stay magnetic forever.

📝 Teacher's Note: Permanent magnets are made of steel because steel keeps its magnetism (has high retentivity), while soft iron loses it easily.

🎯 Exam Tip: Presenting differences in a table like this is the best way to ensure you don't miss any comparison points.

Question 12S. Why is soft iron used as the core of an electric bell?
Answer: The soft iron bar acquires the magnetic properties only when an electric current flows through the solenoid and loses the magnetic properties as the current is switched off. Hence, soft iron is used as the core of the electromagnet in an electric bell.
In simple words: A bell needs to pull and let go of the hammer very quickly. Soft iron allows the magnet to turn ON and OFF instantly so the bell can ring fast.

📝 Teacher's Note: If steel were used, the hammer would stick to the magnet and the bell would only go "ding" once! This illustrates the importance of "temporary" magnetism.

🎯 Exam Tip: The core point is the "instant loss of magnetic properties" when the current is cut.

Question 13S. What happens if an a.c. source is used for an electric bell instead of a battery?
Answer: If an a.c. source is used in place of a battery, the core of the electromagnet will get magnetized, but the polarity at its ends will change. Since attraction of armature does not depend on the polarity of the electromagnet, the bell will still ring on pressing the switch.
In simple words: Even if the electricity keeps swapping directions, the magnet will still pull on the metal hammer, so the bell will still work fine.

📝 Teacher's Note: This is a sophisticated point. Both North and South poles of a magnet attract unmagnetized iron, so the alternating polarity doesn't stop the mechanical ringing.

🎯 Exam Tip: Focus on the fact that "attraction is independent of polarity" for unmagnetized iron pieces.

Question 14S. Why is soft iron used for the armature of an electric bell?
Answer: The material used for making the armature of an electric bell is soft iron which can induce magnetism rapidly.
In simple words: Soft iron responds instantly to magnetic force, which helps the bell hammer move exactly when it's supposed to.

📝 Teacher's Note: Rapid induction and rapid demagnetization are the two reasons why soft iron is the king of electric bell parts.

🎯 Exam Tip: Mention "rapid induction of magnetism" for the armature material choice.

Question 1M. Electromagnets are made of
(a) steel
(b) copper
(c) soft iron
(d) none of the options
Answer: (c) soft iron.
In simple words: Soft iron is the standard material for magnets that need to turn on and off.

📝 Teacher's Note: This is a common basic recall question.

🎯 Exam Tip: Don't confuse the material of the *core* (iron) with the *wire* (copper).

Question 2M. The strength of an electromagnet can be increased by
(a) increasing the number of turns of coil
(b) increasing the current through the coil
(c) both (a) and (b)
(d) none of the options
Answer: (c) both (a) and (b)
In simple words: More wire loops and more electricity are the two main recipes for a stronger magnet.

📝 Teacher's Note: This is a synthesis of the factors mentioned in Solution 8S.

🎯 Exam Tip: Always look for "both" or "all of these" in questions about multiple determining factors.