Get the complete solved exercise of Chapter 8 Magnetism from 9th Class Physics for Punjab Board students. Detailed answers, explanations, and tips to help you understand magnetism concepts
8.1 Which one of the following is not a magnetic material?
Options:
(a) Cobalt
(b) Iron
(c) Aluminium
(d) Nickel
Answer: (c) Aluminium
Explanation:
Cobalt, iron, and nickel are ferromagnetic materials, meaning they exhibit strong magnetic properties. Aluminium, on the other hand, is paramagnetic, meaning it is weakly attracted to a magnetic field but does not retain magnetism.
Tip: Remember the three main ferromagnetic elements: Iron (Fe), Cobalt (Co), and Nickel (Ni). If a metal is not one of these, it is likely non-magnetic or weakly magnetic.
8.2 Magnetic lines of force:
Options:
(a) Are always directed in a straight line
(b) Cross one another
(c) Enter into the north pole
(d) Enter into the south pole
Answer: (d) Enter into the south pole
Explanation:
Magnetic field lines emerge from the north pole and enter the south pole of a magnet. They never cross each other and follow a curved path.
Tip: Remember the rule:
- Magnetic field lines always travel from north to south outside the magnet and from south to north inside the magnet.
8.3 Permanent magnets cannot be made by:
Options:
(a) Soft iron
(b) Steel
(c) Neodymium
(d) Alnico
Answer: (a) Soft iron
Explanation:
Soft iron is highly magnetically permeable but loses its magnetism quickly. Permanent magnets require materials like steel, neodymium, and alnico, which retain magnetism for a long time.
Tip: Soft iron is used in temporary magnets (e.g., electromagnets), whereas materials like steel, neodymium, and alnico are used in permanent magnets.
8.4 Permanent magnets are used in:
Options:
(a) Circuit breaker
(b) Loudspeaker
(c) Electric crane
(d) Magnetic recording
Answer: (b) Loudspeaker
Explanation:
Permanent magnets are essential in loudspeakers because they interact with an electric current to create vibrations and produce sound. Circuit breakers and electric cranes usually use electromagnets, which can be turned on or off as needed.
Tip:
- Loudspeakers, microphones, and some types of electric motors use permanent magnets.
- Electromagnets are used in devices where control over magnetism is needed (e.g., cranes, circuit breakers).
8.5 A common method used to magnetise a material is:
Options:
(a) Stroking
(b) Hitting
(c) Heating
(d) Placing inside a solenoid having A.C current**
Answer: (a) Stroking
Explanation:
A material can be magnetized by stroking it with a permanent magnet in one direction. Hitting or heating disrupts the alignment of magnetic domains, causing demagnetization. An A.C. current in a solenoid does not magnetize a material effectively because the alternating current reverses direction constantly.
Tip: Stroking is an easy method to remember. Another effective method is placing the material inside a solenoid carrying D.C. current.
8.6 Magnetic field direction around a bar magnet:
Answer: (d)
Explanation:
The correct diagram should show magnetic field lines exiting the north pole and entering the south pole of the bar magnet. In the given options, option (d) correctly represents this field direction.
Tip:
- Field lines always go from North to South outside the magnet.
- Inside the magnet, they travel from South to North.
Solutions to MCQs, Short Answer Questions, and Constructed Response Questions
Multiple-Choice Questions (MCQs)
8.7 A steel rod is magnetized by the double touch stroking method. Which one would be the correct polarity of the AB magnet?
Options:
(a) 🔴🔵🔴🔵
(b) 🔵🔴🔵🔴
(c) 🔴🔵🔵🔴
(d) 🔵🔴🔴🔵
Answer: (c) 🔴🔵🔵🔴
Explanation:
In the double-stroke method, two permanent magnets are used to stroke a steel rod from the center outward. The end where the north pole moves becomes the south pole, and the end where the south pole moves becomes the north pole. Based on this principle, option (c) is correct.
Tip:
- Double-stroke method: Stroke from the center to the ends with two magnets in opposite directions.
- Single-stroke method: Use one magnet to stroke in one direction.
8.8 The best material to protect a device from an external magnetic field is:
Options:
(a) Wood
(b) Plastic
(c) Steel
(d) Soft iron
Answer: (d) Soft iron
Explanation:
Soft iron has high magnetic permeability, meaning it can redirect magnetic field lines around sensitive devices, providing effective shielding.
Tip:
- Soft iron is used in electromagnetic shielding to prevent interference.
- Plastic and wood do not block magnetic fields effectively.
**Short Answer
Short Answer Questions
8.1 What are temporary and permanent magnets?
Answer:
- Temporary Magnets: These magnets exhibit magnetism only when influenced by an external magnetic field. Example: Electromagnets.
- Permanent Magnets: These retain their magnetism even after the external magnetic field is removed. Example: Neodymium magnets.
Tip:
- Temporary magnets lose their magnetism easily, while permanent magnets keep it for a long time.
Keywords: Electromagnets, neodymium, retain, lose magnetism
8.2 Define the magnetic field of a magnet.
Answer:
The magnetic field is the region around a magnet where its magnetic force can be detected. It is represented by magnetic field lines that originate from the north pole and end at the south pole.
Tip:
- Stronger near poles, weaker away from the magnet.
Keywords: Region, force, field lines, north to south
8.3 What are magnetic lines of force?
Answer:
Magnetic lines of force are imaginary lines that represent the direction and strength of a magnetic field. They always travel from north to south outside the magnet and south to north inside the magnet.
Tip:
- Field lines never cross each other.
Keywords: Imaginary, direction, never cross, north to south
8.4 Name some uses of permanent magnets and electromagnets.
Answer:
- Permanent Magnets: Used in loudspeakers, electric motors, and refrigerator doors.
- Electromagnets: Used in cranes, electric bells, and MRI machines.
Tip:
- Electromagnets can be turned on and off, permanent magnets cannot.
Keywords: Loudspeaker, electric motor, crane, MRI
8.5 What are magnetic domains?
Answer:
Magnetic domains are small regions inside a material where atomic magnetic moments are aligned in the same direction. When all domains align, the material becomes magnetized.
Tip:
- Magnetism depends on domain alignment.
Keywords: Regions, alignment, magnetized, atomic moments
8.6 Which type of magnetic field is formed by a current-carrying long coil?
Answer:
A solenoid produces a magnetic field similar to a bar magnet, with a north and south pole.
Tip:
- Right-hand rule: Curl fingers in the direction of current, thumb points to the north pole.
Keywords: Solenoid, bar magnet, right-hand rule
8.7 Differentiate between paramagnetic and diamagnetic materials.
Answer:
- Paramagnetic materials: Weakly attracted to a magnetic field (e.g., aluminum, platinum).
- Diamagnetic materials: Weakly repelled by a magnetic field (e.g., copper, bismuth).
Tip:
- Ferromagnetic materials (like iron) are strongly attracted.
Keywords: Weakly attracted, repelled, aluminum, copper
Constructed Response Questions
8.1 Two bar magnets are stored in a wooden box. Label the poles of the magnets and identify P and Q objects.
Answer:
The poles of the bar magnets should be labeled north and south such that opposite poles face each other. The objects P and Q could be soft iron keepers used to prevent demagnetization.
Tip:
- Opposite poles attract, like poles repel.
- Soft iron keepers help retain magnetism.
Keywords: North, south, soft iron, demagnetization
8.2 A steel bar has to be magnetized by placing it inside a solenoid such that end A of the bar becomes N-pole and end B becomes S-pole. Draw a circuit diagram of the solenoid showing the steel bar inside it.
Answer:
To magnetize the steel bar:
- Use a solenoid with a direct current (D.C.) source.
- Apply the right-hand rule (curl fingers in the direction of current, thumb points to the north pole).
Tip:
- A.C. current will not magnetize permanently.
Keywords: Solenoid, D.C. current, right-hand rule, magnetization
8.3 Two bar magnets are lying as shown in the figure. A compass is placed in the middle of the gap. Its needle settles in the north-south direction. Label N and S poles of the magnets. Justify your answer by drawing field lines.
Answer:
The compass aligns with the external magnetic field and points from the north pole of one magnet to the south pole of the other magnet.
Tip:
- A compass always points in the direction of the magnetic field.
Keywords: Compass, north-south, field lines, alignment
Solutions to Questions
Short Answer Questions
8.4 Electric current or motion of electrons produce a magnetic field. Is the reverse process true, that is, does the magnetic field give rise to electric current? If yes, give an example and describe it briefly.
Answer:
Yes, a changing magnetic field can induce an electric current. This is explained by Faraday’s Law of Electromagnetic Induction, which states that a varying magnetic field through a coil generates an electromotive force (EMF), producing current.
Example:
- Electric generators: Rotating a coil inside a magnetic field induces a current.
- Transformers: A changing current in one coil induces a voltage in another coil through a magnetic field.
Tip:
- Current produces a magnetic field (Oersted’s Law).
- Changing magnetic fields induce current (Faraday’s Law).
Keywords: Faraday’s Law, EMF, generators, induction, transformers
8.5 Four similar solenoids are placed in a circle as shown in the figure. The magnitude of current in all of them should be the same. Show by diagram, the direction of current in each solenoid such that when current in any one solenoid is switched OFF, the net magnetic field at the center O is directed towards that solenoid. Explain your answer.
Answer:
To ensure the net magnetic field at the center (O) is directed towards the solenoid that is switched OFF:
- The current directions in solenoids must be arranged symmetrically to produce equal magnetic field contributions at O.
- When one solenoid is turned OFF, the balance is disturbed, making the field at O point towards the inactive solenoid.
Tip:
- Use the right-hand rule: Curl fingers in the direction of current, and the thumb shows the field direction.
Keywords: Solenoid, symmetry, current, right-hand rule, field direction
Comprehensive Questions
8.1 How can you identify whether an object is a magnet or a magnetic material?
Answer:
An object is a magnet if it:
- Attracts and repels another magnet (showing both attraction and repulsion).
An object is a magnetic material if it: - Only attracts a magnet but does not repel it.
Tip:
- A magnet shows repulsion, magnetic materials do not.
Keywords: Attract, repel, magnet, magnetic material, test
8.2 Describe the strength of a magnetic field in terms of magnetic lines of force. Explain it by drawing a few diagrams for the fields as examples.
Answer:
- The strength of a magnetic field is directly proportional to the density of magnetic field lines.
- Stronger field: Closely packed lines (e.g., near poles of a magnet).
- Weaker field: Widely spaced lines (e.g., far from the magnet).
Tip:
- Dense lines = strong field, sparse lines = weak field.
Keywords: Field strength, density, magnetic lines, poles
8.3 What is a circuit breaker? Describe its working with the help of a diagram.
Answer:
A circuit breaker is a safety device that automatically stops current flow when there is an overload or short circuit.
- It uses an electromagnet to detect excessive current.
- When current exceeds a safe limit, the electromagnet pulls the switch, breaking the circuit.
Tip:
- Used in homes, industries, and power plants for safety.
Keywords: Circuit breaker, safety, electromagnet, overload, short circuit
8.4 A magnet attracts only a magnet. Explain the statement.
Answer:
This statement is incorrect because:
- A magnet attracts both magnetic materials (e.g., iron) and other magnets.
- However, only another magnet can repel it, which confirms that an object is truly a magnet.
Tip:
- Attraction does not confirm magnetism; repulsion does.
Keywords: Attraction, repulsion, test, magnetic material
8.5 Differentiate between paramagnetic, diamagnetic, and ferromagnetic materials with reference to the domain theory.
Answer:
| Property | Paramagnetic | Diamagnetic | Ferromagnetic |
|---|---|---|---|
| Behavior in field | Weakly attracted | Weakly repelled | Strongly attracted |
| Magnetic domains | Random, slightly align | Oppose the field | Strongly aligned |
| Examples | Aluminum, platinum | Copper, gold | Iron, cobalt, nickel |
Tip:
- Ferromagnetic materials have strong, aligned domains.
Keywords: Domains, alignment, attraction, repulsion, iron, copper
8.6 Why are ferromagnetic materials suitable for making magnets?
Answer:
Ferromagnetic materials (e.g., iron, cobalt, nickel) are suitable because:
- Their magnetic domains remain aligned after magnetization.
- They have high permeability (easily magnetized).
- They retain magnetism for a long time.
Tip:
- Strong, aligned domains = strong permanent magnet.
