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PARAMOUNT SCHOOL SYSTEM

Subject: Physics – I

Unit 8: Magnetism

EXERCISE

MULTIPLE CHOICE QUESTIONS

QI. Choose the best possible option.

If a bar magnet is cut in half it will become ___________.

A. a monopole B. unmagnetized

C. the same magnet D. magnet of less strength

Which one is the quickest method to magnetize a material?

A. strike with hammer B. moving into magnetic field

C. Stroking the opposite pole D. putting inside a current carrying coil

Earth’s magnetic field intensity is___________.

A. constant everywhere B. very high at equator

C. very low at poles D. varies place to place

The cause of the Earth’s magnetic field is___________.

A. rotational motion of Earth B. spinning of Earth

C. Pull of the Sun D. motion of ions in the core

Material which is the best one for making a permanent magnet:

A. Soft iron B. nickel C. cobalt D. steel
Material which is the best one for making an electromagnet:

A. Soft iron B. nickel C. cobalt D. steel
A sensitive magnetic material is to be shielded by the external magnetic field. It should be kept inside a box of ___________.

A. Soft iron B. plastic C. steel D. wood
Magnetic field lines ___________.

A. are farthest at poles B. intersect each other

C. are closed D. do not pass in vacuum

When two current-carrying wires in the same direction are placed parallel near each other, due to the magnetic field produced by each wire they___________.

A. repel each other B. attract each other

C. have no effect on each other D. stop moving the current through them

Which of the following material is ferromagnetic?

A. silver B. copper C. aluminum D. nickel

SHORT RESPONSE QUESTIONS

QII. Give a short response to the following questions

1. Can two magnetic field lines intersect each other? Justify your answer.

No, two magnetic field lines cannot intersect each other. If they did, it would imply two different directions of the magnetic field at a single point, which is impossible. Therefore, magnetic field lines are always smooth and never cross.

2. A freely suspended magnet always points along north-south direction. Why?

A freely suspended magnet always points in the north-south direction because it aligns itself with Earth’s magnetic field. The magnet’s North Pole points towards the Earth’s magnetic South Pole, which is near the geographic North Pole, due to the magnetic forces acting on it.

3. What is the neutral zone or field free region of the magnetic field?

The neutral zone, or field-free region, is an area where the magnetic field strength is zero. This occurs when the magnetic fields from two opposite poles cancel each other out, resulting in no net magnetic force in that region.

4. Is there any material which does not have any magnetic behavior? Justify your answer.

Yes, diamagnetic materials do not exhibit any magnetic behavior because they have zero net magnetic field per atom. When exposed to an external magnetic field, they develop a weak, temporary magnetic moment in the opposite direction, thus showing no permanent magnetism.

5. A proton is also a charged particle and spins like an electron. Why its effect is neglected in study of magnetism?

The effect of a proton is neglected in the study of magnetism because it has a much larger mass than an electron, resulting in a significantly weaker magnetic moment. Therefore, the magnetic influence of electrons, which have a much higher magnetic moment, dominates in magnetic materials.

6. What is the geomagnetic reversal phenomenon? Explain.

The geomagnetic reversal is when Earth’s magnetic field flips, the magnetic north and south poles switch places. This happens because the flow of molten iron in Earth’s core changes, affecting the magnetic field. These reversals don’t happen often and can take thousands to millions of years, leaving patterns in rocks that show the past positions of the poles.

7. Why the Earth spins about its geographical axis instead of its magnetic axis? Explain.

The Earth spins about its geographical axis because this axis is defined by the Earth’s physical shape and rotation, not its magnetic field. The magnetic axis is tilted at an angle of 11.3° from the geographical axis due to the flow of molten iron in the Earth’s core. This difference causes the compass needle to point slightly away from the geographical North Pole, aligning instead with the magnetic poles.

8. Why the Earth’s geographical and magnetic axis are not coincident? Explain.

The Earth’s geographical and magnetic axes are not coincident because they are defined by different factors. The geographical axis is based on the Earth’s physical rotation, while the magnetic axis is created by the flow of molten iron in the Earth’s core, known as the “dynamo effect”. This flow generates a magnetic field that is tilted at an angle of 11.3° from the geographical axis, causing the magnetic poles to be slightly offset from the true North and South Poles.

9. What is the difference between paramagnetic and ferromagnetic materials?

Paramagnetic Materials	Ferromagnetic Materials
Have a small net magnetic field per atom.	Have a large net magnetic field per atom.
Weakly attracted by a magnetic field.	Strongly attracted by a magnetic field.
Do not retain magnetism after removal of the external field.	Can retain magnetism and become permanent magnets.
Examples: Aluminum, platinum.	Examples: Iron, nickel, cobalt.

10. At what factors the strength of the magnetic field of an electromagnet depends?

The strength of the magnetic field of an electromagnet depends on:

Number of Turns of Wire:

More turns in the coil increase the magnetic field strength.

Electric Current:

Higher current through the coil produces a stronger magnetic field.

Core Material:

Using a soft iron core enhances the magnetic field compared to air or other materials.

11. Draw magnetic field lines of two solenoids placed near each other:

i) Facing same poles to each other:

ii) Facing opposite poles to each other

LONG RESPONSE QUESTIONS

QIII. Give a detailed response to the questions given below.

1. Define and explain the magnetism.

Magnetism:

Magnetism is a force of attraction or repulsion between magnetic materials.
It is caused by the motion of electric charges and the alignment of magnetic domains.

Magnetic Materials:

Materials like iron, cobalt, and nickel exhibit magnetism.
They can either become magnets or be attracted by magnets.

Poles of a Magnet:

A magnet has two poles: a north pole and a south pole.
Like poles repel each other, while opposite poles attract.

Magnetic Field:

The region around a magnet where magnetic forces are experienced is called the magnetic field.
Magnetic field lines flow from the North Pole to the South Pole outside the magnet.

Applications of Magnetism:

Magnetism is used in various technologies, including compasses and electronic devices.
Earth’s magnetic field acts like a giant bar magnet, protecting life from cosmic rays and solar radiation.

2. What is the domain theory of magnetism? Explain.

Domain Theory of Magnetism:

Magnetic materials consist of small regions called magnetic domains.
Each domain acts as a tiny magnet with its own north and south poles.

Unmagnetized Material:

In an unmagnetized material, domains are randomly oriented, canceling out their magnetic effects.

Magnetization Process:

When placed in an external magnetic field, the domains align in the same direction.
This alignment causes the material to become magnetized.

Permanent Magnetization:

If domains remain aligned after removing the external magnetic field, the material becomes a permanent magnet.

Temporary Magnetization:

If domains return to random orientations, the material is only temporarily magnetized.

Significance:

Explains how some materials are magnetic and others are not.
Helps in understanding the behavior of magnetic materials in different conditions.

3. Explain magnetic field strength and magnetic shielding.

Magnetic Field Strength:

The magnetic field strength refers to the intensity of the magnetic field produced by a magnet or current.
It indicates how strong the magnetic effect is in a given region of space.
For example, the Earth’s magnetic field is relatively weak, even though it extends millions of kilometers into space. The strength of the Earth’s magnetic field also varies from place to place.

Magnetic Shielding:

Magnetic shielding is the process of blocking or reducing magnetic fields in a specific area.
It involves placing sensitive materials inside a box made of magnetic materials like soft iron, which can absorb magnetic lines of force.
This shielding protects sensitive equipment or areas from the influence of external magnetic fields, ensuring that they are not affected by unwanted magnetic interference.

4. Explain the magnetic field of a bar magnet and that of a solenoid, also compare them.

Magnetic Field of a Bar Magnet:

A bar magnet produces a magnetic field that extends around it.
The field lines emerge from the north pole of the magnet and curve around to enter the South Pole, forming closed loops.
These magnetic field lines are densest near the poles, indicating stronger magnetic forces at the ends of the bar magnet.
The field pattern resembles a butterfly shape, with lines never crossing each other.

Magnetic Field of a Solenoid:

A solenoid is a coil of wire that generates a magnetic field when an electric current passes through it.
The magnetic field inside the solenoid is uniform and strong, with field lines running parallel to each other.
The field lines emerge from one end of the solenoid (acting as a North Pole) and re-enter at the other end (acting as a South Pole), similar to a bar magnet.
Outside the solenoid, the field lines loop back, but they are much weaker compared to the field inside.

Comparison:

Bar Magnet	Solenoid
Naturally magnetized material.	Electric current flowing through the coil.
Similar to a dipole (bar magnet field).	Similar to a bar magnet when current is flowing.
Fixed strength based on material.	Adjustable strength by changing the current.
Non-uniform field, strongest at poles.	Uniform and strong field inside the coil.
North and south poles are permanent.	Poles can switch if the current direction changes.

5. Define induced magnetism. Also explain some of the methods for induced magnetism.

Induced Magnetism:

Induced magnetism refers to the process where a material, which is not naturally a magnet, becomes magnetized when placed in an external magnetic field. The magnetic domains within the material align in the direction of the applied field, causing it to act like a magnet temporarily. This induced magnetism disappears once the external magnetic influence is removed, especially in materials like soft iron.

Methods for Inducing Magnetism:

Stroking Method:

A magnetic material (e.g., steel) is repeatedly stroked in one direction with one pole of a permanent magnet.
This aligns the domains within the material, magnetizing it temporarily.
Hammering Method:
The material (like a steel bar) is placed in a strong magnetic field and gently hammered.
The vibrations help the magnetic domains align with the external magnetic field, magnetizing the bar.
Heating the bar before hammering can increase magnetization.

Solenoid (Using Electric Current):

Wrapping a wire coil (solenoid) around a magnetic material and passing a direct current through it creates a magnetic field.
This field aligns the domains within the material, making it magnetized as long as the current flows.
This method is commonly used to create electromagnets, which lose their magnetism when the current stops.

Heating Method:

Generally used to demagnetize a material.
Heating increases the motion of aligned domains, causing them to misalign and lose their magnetization.
However, recent discoveries like the “magnetic See beck effect” show that heating can sometimes generate a magnetic field.

6. Differentiate between permanent and temporary magnets.

Permanent Magnet	Temporary Magnet
Permanently magnetized	Temporarily magnetized (only when current flows)
Made of hard magnetic materials (e.g., Alnico, steel)	Made of soft magnetic materials (e.g., soft iron)
Magnetic strength does not change	Magnetic strength can be varied by adjusting the current
Poles cannot be altered	Poles can be reversed by changing the current direction
Examples: Iron ore, cobalt, nickel	Examples: Iron nails, screws, solenoids

7. Explain some uses of electromagnets and temporary magnets.

Electromagnets:

Electric Motors: Electromagnets are used in electric motors to convert electrical energy into mechanical energy.
Magnetic Cranes: Used for lifting heavy metallic objects in industries like scrap yards.
Relays and Switches: Electromagnets control the opening and closing of electrical circuits in devices like relays.
Magnetic Lifts: Used in transport systems for lifting and moving objects.
Speakers and Microphones: In audio devices, electromagnets help in producing sound by vibrating diaphragms.

Temporary Magnets:

Magnetic Recording Devices: Used in devices like tape recorders where temporary magnetization is used to record sound.
Magnetic Induction Heating: Temporary magnets are used in induction heating processes in industries.
Electromagnetic Clutches: Used in automotive applications where temporary magnetism helps engage or disengage mechanical components.

8. Explain the three types of magnetic materials.

Diamagnetic Materials:

Magnetic Behavior: These materials are weakly repelled by a magnetic field and do not retain magnetism once the external magnetic field is removed.

Example: Bismuth, copper, and graphite.

Properties: The magnetic susceptibility is negative, meaning they create an opposing magnetic field when exposed to an external one.

Paramagnetic Materials:

Magnetic Behavior: These materials are weakly attracted to a magnetic field and only exhibit magnetism in the presence of an external field.

Example: Aluminum, platinum, and manganese.

Properties: The magnetic susceptibility is positive, but the effect is weak and only temporary; they lose their magnetism when the external field is removed.

Ferromagnetic Materials:

Magnetic Behavior: These materials are strongly attracted to magnetic fields and can become permanent magnets after magnetization.

Example: Iron, cobalt, and nickel.

Properties: The magnetic susceptibility is very high, and they can retain magnetism even after the external field is removed, making them ideal for permanent magnets.

9. Explain the Earth’s magnetic field. Also relate Earth’s geographical and magnetic poles.

The Earth’s magnetic field is a natural magnetic field that surrounds the Earth and extends into space. It is generated by the motion of molten iron and other metals in the Earth’s outer core, which creates electric currents. These currents produce a magnetic field, similar to how a bar magnet works, with a north and South Pole.

Earth’s Magnetic Field:

Source: The Earth’s magnetic field is generated by the movement of molten iron and nickel in the outer core of the Earth, creating a dynamo effect.

Shape: The magnetic field lines emerge from the Earth’s magnetic south pole and curve around to re-enter at the magnetic north pole, forming a protective shield called the magnetosphere.

Importance: The magnetic field protects Earth from harmful solar radiation and cosmic rays, and it also enables navigation using compasses.

Geographical Poles:

These are the fixed points where the Earth’s axis of rotation intersects the surface, i.e., the North Pole and South Pole.

Magnetic Poles:

These are the points on Earth’s surface where the magnetic field lines are vertical. The magnetic poles are not fixed and can shift over time due to changes in the Earth’s core.

Relation:

The Earth’s geographical and magnetic poles do not coincide. The magnetic poles are located some distance away from the geographical poles and move over time due to changes in the Earth’s magnetic field. The magnetic poles are also not aligned perfectly with the rotational axis of the Earth.

10. Explain how birds and other migrating animal use Earth’s magnetic field to navigate.

Birds and other migrating animals use Earth’s magnetic field as a natural navigation system. This phenomenon, called bio-magnetism, helps them travel long distances during migration by detecting the Earth’s magnetic field and using it to guide their routes. Birds use different techniques to navigate.

Iron Crystals in Beaks:

Pigeons and some other birds have iron crystals in their beaks, allowing them to sense Earth’s magnetic field. This gives them a “nose for north” to locate their destinations accurately.

Cryptochrome-4 Molecule in Eyes:

Certain migrating birds have a molecule called cryptochrome-4 in their eyes, which is sensitive to Earth’s magnetism. This helps them visualize magnetic fields and align themselves with the right direction.

Migratory Animals:

Species like sea turtles and whales also use Earth’s magnetic field to navigate during their seasonal migrations. By sensing the polarity of north and south, they determine their paths.