9th Class Physics Chapter 8 Magnetism Long Questions

Question 1

How can you identify whether an object is a magnet or a magnetic material?

Accepted Answer

Magnet and magnetic material A magnet attracts both another magnet (opposite pole) and plain magnetic material such as iron, so attraction alone cannot tell them apart. Method Suspend a bar magnet freely so that it can turn.; Bring one end of the object near the N-pole of the suspended magnet. Suppose it is attracted: the end is either an S-pole or plain iron.; Now bring the same end near the S-pole of the suspended magnet.; If it is attracted again, the object is only a magnetic material: it is attracted to both poles because magnetism is induced in it.; If it is repelled, the object is a magnet: its end is an S-pole. Repulsion is the real test to identify a magnet.

Question 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.

Accepted Answer

Strength of a magnetic field The strength of a magnetic field is proportional to the number of lines of force passing through unit area held at right angles to the lines. Where the lines are close together the field is strong; where they spread out it is weak. Bar magnet Combinations of magnets (a) Unlike poles facing: the lines run straight across the gap from N to S and are crowded: a strong field between them, and the magnets attract.; (b) Like poles facing: the lines push away from each other; at point X they cancel. X is a neutral point, where the field is zero.; (c) Horseshoe magnet: between its poles the lines are straight, parallel and evenly spaced: a strong, nearly uniform field (except near the edges). Close lines: strong field. Far apart: weak field. No lines (neutral point): zero field.

Question 3

What is a circuit breaker? Describe its working with the help of a diagram.

Accepted Answer

Definition Circuit breaker: A safety switch that passes up to a certain maximum current safely and switches OFF the circuit automatically if the current becomes too large. Diagram Construction A copper strip carries the current to the switch contacts.; An iron armature, pivoted, holds the contacts closed against a spring.; A coil round an iron core (an electromagnet) is in series with the circuit. Working The current flows along the copper strip, through the armature and the coil of the electromagnet.; With a normal current, the electromagnet is too weak to move the armature.; If the current becomes excessive (a short circuit or overload), the electromagnet becomes strong enough to pull the armature.; The armature turns about the pivot, the spring pulls the contacts apart and the circuit is broken.; After the fault is repaired, the breaker is reset by hand. Unlike a fuse, a circuit breaker does not need replacing: it is simply reset.

Question 4

A magnet attracts only a magnet. Explain the statement.

Accepted Answer

The statement is not correct A magnet attracts both other magnets (their opposite poles) and magnetic materials such as iron, nickel and cobalt. Why a magnet attracts iron The field of the magnet passes into the unmagnetised iron and induces magnetism in it.; The near end of the iron becomes the opposite pole: the S-pole of the magnet induces an N-pole at the near end and an S-pole at the far end.; Opposite poles attract, so the iron is pulled towards the magnet.; The induced magnetism of soft iron disappears when the magnet is removed. A magnet attracts magnets and magnetic materials; it repels only magnets.

Question 5

Differentiate between paramagnetic, diamagnetic and ferromagnetic materials with reference to the domain theory.

Accepted Answer

Origin of magnetism Every electron spins and moves in an orbit, and each of these motions produces a tiny magnetic field. The way they combine decides the type of material. Diamagnetic materials The fields of the orbital and spin motions add up to zero, so the atom has no resultant field. Examples: copper, bismuth, water. Paramagnetic materials The fields support each other, so each atom is a tiny magnet (a magnetic dipole). The atoms point at random, so the material is only weakly attracted. Examples: aluminium, lithium. Ferromagnetic materials In iron, steel, nickel and cobalt, the spins of large groups of neighbouring atoms (about 10¹⁶) line up parallel by themselves, forming domains about 0.1 mm across, each a strong small magnet. Unmagnetised: the domains point at random and cancel, so the material shows no magnetism.; In an external field: domains along the field grow at the expense of the others, and the others rotate towards the field.; Result: the material becomes a magnet with N and S poles.; Soft iron's domains return to random when the field is removed (temporary); steel's stay aligned (permanent). Diamagnetic, no resultant field, none, Cu, Bi, water; Paramagnetic, tiny magnet, none, Al, Li; Ferromagnetic, tiny magnet, yes, aligned groups, Fe, Ni, Co, steel

Question 6

Why are ferromagnetic materials suitable for making magnets?

Accepted Answer

Domains In ferromagnetic materials (iron, steel, nickel, cobalt) large groups of neighbouring atoms have their electron spins lined up parallel by nature. These groups form domains, each about 0.1 mm across and each a strong little magnet. Easy to magnetise strongly Because the atoms in each domain are already aligned, an external field only has to turn and grow the domains, not line up each atom one by one. So a moderate field produces a very strong magnet. Para- and diamagnetic materials have no domains and can hardly be magnetised. Temporary or permanent, as needed Soft iron: domains align easily and return to random easily, which is ideal for electromagnets, relays and transformer cores.; Steel and alloys: domains are hard to align but then stay aligned, which is ideal for permanent magnets. Only ferromagnetic materials have domains, so only they can be made into useful magnets.

Question 7

What is an electromagnet? How can its strength be increased? Give its uses.

Accepted Answer

Definition Electromagnet: A soft-iron core that becomes a magnet when a current flows through a coil of wire wound round it. Diagram Working While the current flows, the coil's field magnetises the soft-iron core strongly. When the current stops, the core loses its magnetism, so an electromagnet is a temporary magnet that can be switched on and off. Its poles follow the right-hand grip rule; reversing the current swaps them. Increasing its strength Increase the current (use more cells).; Increase the number of turns of the coil.; Use a soft-iron core. Uses Electric bells and telephone receivers.; Magnetic relays and circuit breakers.; Cranes that lift scrap iron.; Maglev trains.; Recording heads of tapes and hard disks.

Question 8

Describe the methods of making a magnet. State the right-hand grip rule.

Accepted Answer

1. Single-touch method A steel bar is laid on a table and stroked from one end to the other, many times, always in the same direction with the same pole of a permanent magnet, lifting it high above the bar on the way back. 2. Double-touch method Unlike poles of two magnets are placed together at the centre of the bar and stroked outwards towards opposite ends at the same time, many times. It is more effective than single touch. In both, the end where the stroke finishes gets the pole opposite to the stroking pole. 3. Electrical method The steel bar is placed inside a solenoid of several hundred turns and a direct current is passed for a short time. This is the strongest and quickest method. Right-hand grip rule Right-hand grip rule: Grip the solenoid with the right hand so that the fingers curl in the direction of the current; the thumb then points towards the N-pole.

Question 9

What is demagnetisation? Describe three methods of demagnetising a magnet. How are magnets stored?

Accepted Answer

Demagnetisation Destroying the magnetism of a magnet by putting its domains back into random directions. 1. Heating Strong heating makes the atoms vibrate violently, which breaks up the order of the domains; the magnet loses its magnetism quickly. 2. Hammering Beating a magnet with a hammer (or dropping it repeatedly) jolts the domains out of alignment. 3. Alternating current The magnet is placed inside a solenoid carrying A.C. and pulled out slowly. The field keeps reversing and grows weaker as the magnet leaves, so the domains end up at random. This is the best method. Storing magnets Store them in pairs with unlike poles side by side and soft-iron keepers across the ends, so the field forms a closed loop. Do not drop or heat them.

Question 10

Describe the construction and working of a magnetic relay. How does a maglev train use electromagnets?

Accepted Answer

Magnetic relay Relay: A switch worked by an electromagnet, which lets a small current switch on a circuit carrying a large current. Construction An input circuit: a cell, a switch and a coil on a soft-iron core.; A pivoted iron armature near the core, held back by a spring.; Contacts in a second (output) circuit carrying a large current. Working Closing the input switch sends a small current through the coil, which becomes an electromagnet.; It pulls the armature, which closes the contacts of the output circuit.; The high-current device (a motor or a car's starter) starts. Opening the switch lets the spring open the contacts. The person works only the safe low-current circuit, which is the purpose of a relay. Maglev train It has no wheels. Levitation electromagnets lift it slightly above the guideway, and propulsion electromagnets attract and repel it forward. With no friction from the track it moves at about 400 km h⁻¹, smoothly and quietly.

Question 11

Describe the uses of permanent magnets in a moving-coil loudspeaker and an A.C. generator.

Accepted Answer

Moving-coil loudspeaker A voice coil fixed to a paper cone is slipped over one pole of a strong permanent magnet.; The alternating current from the amplifier flows through the coil.; The current in the magnet's field feels a force that keeps reversing, so the coil and the cone vibrate.; The vibrating cone produces sound waves in the air, matching the current. A.C. generator A coil is rotated between the poles of a permanent magnet.; The magnetic field through the coil keeps changing as it turns.; An e.m.f. is induced between the ends of the coil (electromagnetic induction).; Connected to a circuit, it drives an alternating current, which reverses every half turn. Other uses D.C. motors; Door catchers; Compasses; Separating iron from scrap

Question 12

Explain magnetic recording and magnetic shielding.

Accepted Answer

Magnetic recording Sound, video and data are stored by magnetising a material coated with iron oxide, using an electromagnet called a head. Tape: sound is changed into a varying current; in the recording head it magnetises the moving tape in a matching pattern. On playback the pattern produces the current again.; Hard disk: the write head magnetises tiny spots on a spinning disk in one of two directions, storing 1s and 0s; the read head detects the direction. A disk finds any data almost instantly, while a tape may take minutes. Magnetic shielding Permeability: The ability of a material to let lines of force pass through it. Sensitive devices are enclosed in a box of soft iron. Its permeability is very high, so the lines of force crowd into the iron and pass round the inside, which stays almost free of the field.

Type	Atom	Domains	Examples
Diamagnetic	no resultant field	none	Cu, Bi, water
Paramagnetic	tiny magnet	none	Al, Li
Ferromagnetic	tiny magnet	yes, aligned groups	Fe, Ni, Co, steel

Long Questions with Answers

Set A: Textbook comprehensive questions 6 questions

How can you identify whether an object is a magnet or a magnetic material?

Magnet and magnetic material

Method

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.

Strength of a magnetic field

Bar magnet

Combinations of magnets

What is a circuit breaker? Describe its working with the help of a diagram.

Definition

Diagram

Construction

Working

A magnet attracts only a magnet. Explain the statement.

The statement is not correct

Why a magnet attracts iron

Differentiate between paramagnetic, diamagnetic and ferromagnetic materials with reference to the domain theory.

Origin of magnetism

Diamagnetic materials

Paramagnetic materials

Ferromagnetic materials

Why are ferromagnetic materials suitable for making magnets?

Domains

Easy to magnetise strongly

Temporary or permanent, as needed

Set B: More important long questions 6 questions

What is an electromagnet? How can its strength be increased? Give its uses.

Definition

Diagram

Working

Increasing its strength

Uses

Describe the methods of making a magnet. State the right-hand grip rule.

1. Single-touch method

2. Double-touch method

3. Electrical method

Right-hand grip rule

What is demagnetisation? Describe three methods of demagnetising a magnet. How are magnets stored?

Demagnetisation

1. Heating

2. Hammering

3. Alternating current

Storing magnets

Describe the construction and working of a magnetic relay. How does a maglev train use electromagnets?

Magnetic relay

Construction

Working

Maglev train

Describe the uses of permanent magnets in a moving-coil loudspeaker and an A.C. generator.

Moving-coil loudspeaker

A.C. generator

Other uses

Explain magnetic recording and magnetic shielding.

Magnetic recording

Magnetic shielding

The rest of Chapter 8

Long Questions from other Physics chapters