9th Class Physics
Chapter-8: Magnetism
A. Multiple Choice Questions
Tick (✓) the correct option
8.1 Which of the following is non-magnetic?
Explanation: Aluminium is paramagnetic but very weakly magnetic, often considered non-magnetic in practical contexts compared to strongly ferromagnetic materials like iron, cobalt, and nickel.
8.2 Magnetic lines of force:
Explanation: Magnetic field lines emerge from the north pole and enter the south pole of a magnet externally, and form continuous closed loops through the magnet internally.
8.3 Permanent magnets cannot be made by:
Explanation: Soft iron loses its magnetism quickly when the magnetizing field is removed, making it unsuitable for permanent magnets. Steel, neodymium, and alnico retain magnetism for a long time.
8.4 Permanent magnets are used in:
Explanation: Loud-speakers use permanent magnets to create a constant magnetic field that interacts with the voice coil. While magnetic recording also uses magnets, the textbook specifically marks loud-speaker as the correct answer.
8.5 A common method to magnetise a material is:
Explanation: Stroking is a simple and common method where a magnet is repeatedly stroked along the material in one direction. A.C. solenoid would demagnetize, while D.C. solenoid is also used but stroking is the safest "correct" answer in this context.
8.6 A compass is placed at four points around a bar magnet. Which diagram shows correct field directions?
Explanation: Option (b) shows arrows leaving the north pole and entering the south pole, with field lines never crossing each other, which is the correct representation of magnetic field lines.
8.7 Steel rod magnetised by double-touch stroking. Correct polarity of end A-B?
Explanation: In double-touch stroking, the stroking magnets are kept with the same poles to the ends of the rod being magnetized, resulting in opposite poles at the ends (N-S).
8.8 Best material to shield a device from external magnetic field:
Explanation: Soft iron has high permeability and provides a magnetic shunt, effectively redirecting magnetic field lines around the shielded device.
Magnetic field lines around a bar magnet and compass alignment
B. Short Answer Questions
8.1 What are temporary and permanent magnets?
Answer:
Temporary magnets → made of soft iron; lose magnetism quickly when the magnetising field is removed.
Permanent magnets → made of steel, alnico, neodymium; retain magnetism for a long time.
8.2 Define magnetic field of a magnet.
Answer: The region around a magnet in which a magnetic force can be detected; it is represented by magnetic field lines that emerge from the north pole and enter the south pole.
8.3 What are magnetic lines of force?
Answer: Imaginary smooth curves that:
- travel N → S externally,
- never cross,
- are closer where the field is stronger.
8.4 Name some uses of permanent magnets and electromagnets.
| Permanent Magnets | Electromagnets |
|---|---|
| Loud-speakers, fridge doors, magnetic clasps, small DC motors | Electric cranes, circuit breakers, relays, MRI machines, scrap-yard lifters |
8.5 What are magnetic domains?
Answer: Tiny atomic-size regions (≈10¹⁵–10¹⁸ atoms) inside ferromagnetic materials where the magnetic moments of electrons are aligned; when most domains line up, the material becomes magnetised.
8.6 Which type of magnetic field is formed by a current-carrying long coil?
Answer: A uniform (nearly parallel) magnetic field along the axis inside the coil, similar to that of a bar magnet.
8.7 Differentiate between paramagnetic and diamagnetic materials.
| Property | Paramagnetic | Diamagnetic |
|---|---|---|
| Magnetic susceptibility | Small & positive | Small & negative |
| In external field | Weakly attracted | Weakly repelled |
| Examples | Al, Pt, O₂ | Cu, Bi, H₂O |
| Electron structure | Have unpaired electrons | All electrons paired |
Magnetic field inside a current-carrying solenoid
C. Constructed Response Questions
8.1 Task: Label the poles of the two stored bar magnets and identify objects P & Q.
Answer:
P = keeper (soft-iron piece placed across the poles).
Q = second bar magnet (poles arranged N–S to the first magnet so they attract and flux stays closed).
8.2 Task: Draw the circuit diagram of a solenoid with steel bar inside so that end A → N-pole and end B → S-pole.
Answer:
Polarity Rule: Current clockwise when viewed from A → that end becomes N-pole.
Connect battery accordingly; place steel bar fully inside the coil.
8.3 Situation: Two bar magnets lie with a small gap; a compass at the centre of the gap settles N–S (i.e. shows no deflection). Conclusion & Labeling:
Answer:
The facing poles must be opposite (and of equal strength) so their fields cancel at the midpoint.
Field-line Sketch: Lines leave N of left magnet, enter S of right magnet. At exact centre, net field = 0 → compass needle stays Earth's N-S.
8.4 Question: Electric current (motion of electrons) produces a magnetic field. Is the reverse true—can a magnetic field give rise to electric current? If yes, give an example and describe it briefly.
Answer:
✅ Yes, the reverse is true.
Example: Electromagnetic induction in a dynamo or generator.
Description: When a coil of wire is rotated in a magnetic field (or a magnet is moved near a coil), the changing magnetic flux induces an EMF (voltage) in the coil. If the circuit is closed, this EMF drives an induced current—no battery needed.
This is Faraday's Law in action: ε = -ΔΦ/Δt
8.5 Question: Four identical solenoids are placed in a circle. The same current flows through each. Show (with a diagram) the direction of current in each solenoid such that when any one solenoid is switched OFF, the net magnetic field at centre O points toward that solenoid.
Answer:
Step 1 – Initial Setup (all ON): Let the current directions be arranged so that all four solenoids produce equal fields pointing inward (toward centre O). This gives zero net field at O (perfect cancellation).
Step 2 – Switch OFF one solenoid: The inward field from that solenoid disappears. The remaining three still produce inward fields, but now there is an imbalance. The missing inward field is equivalent to an outward field from the opposite direction—hence the net field at O is directed toward the switched-off solenoid.
Conclusion: Arrange all currents so their individual fields point inward at O. When any one is switched OFF, the net field immediately points toward the "missing" solenoid.