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CNMS ICSE - 7 Std
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Answer key

Prepared by: learnloophq@gmail.com

Chapter: 08. Magnetism

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SECTION A: MULTIPLE CHOICE QUESTIONS (MCQs)

b [Magnetite (Fe₃O₄)]
c [Each broken part turns into a complete magnet with its own north and south poles.]
c [Brass]
a [Zero]
c [The amount of electric current flowing through the coil]
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SECTION B: FILL IN THE BLANKS

poles
Repulsion
core
induced current
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SECTION C: TRUE OR FALSE QUESTIONS

False (It becomes magnetized when the switch is pressed and current flows.)
False (Permanent magnets are usually made of hard materials and retain their magnetic properties without an external magnetic field.)
True (The end where current flows clockwise acts as the south pole, and the anticlockwise end acts as the north pole.)
True (Current is induced due to the relative motion which changes the magnetic lines of force associated with the coil.)
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SECTION D: EXPLANATION OF TERMS

Electromagnet: A temporary magnet in which a magnetic field is produced through an electric current passing through a coil wound around a soft iron core.
Electromagnetism: The branch of physics which deals with the magnetic effect of electric current.
Electromagnetic induction: The phenomenon of producing an electric current in a conductor due to a change in the magnetic lines of force associated with the conductor.
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SECTION E: DIFFERENTIATE BETWEEN CONCEPTS

1. Differences between Permanent Magnets and Temporary Magnets:
Feature
Permanent Magnets
Temporary Magnets
Magnetic Retention
Retain their magnetic properties even without the influence of an external magnetic field.
Behave as magnets only in the presence of a strong magnetic field (or when electric current is passing).
Type of Material Used
Usually made of hard materials.
Usually made of soft materials.
Examples
Bar magnets, horseshoe magnets.
Electromagnets.
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SECTION F: DIAGRAM-BASED QUESTIONS

When the switch is turned on, the electric current flows through the coil, causing the iron nail to behave like a magnet (an electromagnet) and attract the paper clips.
When the switch is turned off, the paper clips will fall off the nail. This is because the electromagnet is a temporary magnet that loses its magnetic properties as soon as the flow of electric current is stopped.
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SECTION G: SHORT ANSWER QUESTIONS

Repulsion is the surest test because non-magnetic materials made of iron, nickel, or cobalt can still be attracted to a magnet, but they will never be repelled. Only a magnet can repel another magnet.
The strength of an electromagnet depends on:
The amount of current flowing through the coil (directly proportional).
The number of turns per unit length of the coil (directly proportional).
When the magnet is moved away from the coil, the galvanometer needle shows a deflection in the opposite direction, indicating the presence of an induced current flowing in the opposite direction.
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SECTION H: ENVIRONMENTAL AWARENESS & EVERYDAY USE

Electromagnets are used in:
Electrical appliances (such as telephones, electric motors, and electric bells).
Cranes to lift heavy loads of scrap iron and steel.
Bullet trains.
Separation of iron ore and its impurities. ​(Any four of the above or other listed uses like removing steel splinters from wounds are acceptable).
Preference: MRI scans are preferred because they produce clearer, more detailed images of organs and soft tissues, and they do not expose the body to harmful radiation, unlike CT scans. ​Safety Concern: MRI scans generate strong magnetic fields, which can cause problems for people who have metallic implants in their bodies.
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SECTION I: LONG ANSWER QUESTIONS

Construction of an Electric Bell: An electric bell consists of a U-shaped electromagnet, a contact spring, a contact screw, an iron armature, a hammer, and a metal gong, all fitted on a flat wooden or plastic board.
Working of an Electric Bell:
When the push-button switch is pressed, the electric circuit is completed, and current flows from the battery through the electromagnet to the contact screw and back.
As current flows through the U-shaped electromagnet, it becomes magnetized and attracts the iron armature towards itself.
This movement causes the hammer attached to the armature to hit the metal gong, producing a ringing sound.
At the same time, the displacement of the armature pulls it away from the contact screw, breaking the electric circuit.
Since the circuit is broken, current stops flowing, and the electromagnet immediately loses its magnetism.
The contact spring then pulls the armature back to its original position, restoring contact with the contact screw. This completes the circuit again, and the process repeats continuously as long as the switch is kept pressed.
Faraday’s Laws of Electromagnetic Induction:
First Law: A current is induced in a coil when the number of magnetic lines of force associated with the coil changes.
Second Law: The magnitude of the induced current is directly proportional to the rate at which the magnetic lines of force associated with the coil changes.
Ways to increase the magnitude of the induced current:
By increasing the number of turns of the wire in the coil.
By increasing the strength of the magnet used.
By increasing the relative speed of motion between the magnet and the coil.
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