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Magnetism is the ability of a magnet to attract or repel objects. It is caused due to the movement of electric charge. A magnet, for example, is a material that generates an invisible magnetic field around it, and this field causes a force called magnetism, which pulls materials like iron towards it. When matter is exposed to magnetism, it reacts in a specific way. The presence of a magnetic field triggers the synthesis of a range of magnetic materials.
Very Short Answer Questions [1 Mark Questions]
Ques: A magnetic material's permeability is 0.9983. What kind of magnetic materials does it represent?
Ans- It denotes materials that are diamagnetic.
Ques: Where on the Earth's surface is the angle of dip 90°?
Ans- On Earth's surface, the angle of dip at the magnetic poles is 90°.
Ques: How does the intensity of a paramagnetic material's magnetization change as the applied magnetic field increases?
Ans- The intensity of magnetization rises as the applied magnetic field rises.
Ques: What type of magnetic material is used in the manufacture of permanent magnets?
Ans- Permanent magnets are made from materials with a high coercivity.
Ques: Is it possible to accelerate neutrons in a cyclotron? Why?
Ans- In a cyclotron, neutrons cannot be accelerated. Because a neutron is neutral, and a cyclotron can only accelerate charged particles.
Ques: Write a key property of a paramagnetic substance.
Ans- It progresses from weaker to stronger magnetic field zones.
Ques: Permanent magnets should be made of steel, while electromagnets should be made of soft iron. Give one explanation.
Ans- Steel's retentivity is higher than that of soft iron.
Ques: Define the angle of inclination caused by the earth's magnetic field at a specific location.
Ans- The angle of inclination is the angle formed by the earth's magnetic field at a certain location with the magnetic meridian's horizontal line.
Short Answer Questions [2 Marks Questions]
Ques: What are permanent magnets? Give one example.
Ans- Permanent magnets are materials that retain their ferromagnetic properties for an extended period of time at normal temperature.
Example: Steel
Ques: A charged particle's energy does not change while it moves through a homogeneous magnetic field. Why?
Ans- In a homogeneous magnetic field, the force acting on a charged particle is always perpendicular to the charge's motion. Because the magnetic field does not work on the charge, the energy of the charged particle remains unchanged.
Ques: A magnetic material's susceptibility is –2.6 × 10-5 . Identify the magnetic substance and describe its two qualities.
Ans- Because the susceptibility of a magnetic material is negative, it is diamagnetic.
The properties are:
- It tends to migrate slowly from stronger to weaker portions of a non-uniform magnetic field.
- A diamagnetic rod in free fall aligns itself perpendicular to the field.
Ques: Define a neutral point. Draw lines of force when two identical magnets are placed at a defined distance apart with their N-poles facing each other and locate the neutral points.
Ans- A neutral point is a location near a magnet where the earth's magnetic field is completely balanced by the magnetic field of the magnet.
The figure:
The cross represents the neutral point.
Ques: A vector's specification requires three quantities. Identify the three independent quantities that are traditionally used to describe the earth's magnetic field.
Ans- The three independent quantities commonly used to specify the earth's magnetic field are as follows:
- Magnetic declination
- Dip Angle
- The horizontal component of the Earth's magnetic field.
Ques: At long distances, the Earth's field deviates significantly from its dipole shape (greater than about 30,000 km). What agencies could be to blame for this distortion?
Ans- Because of the presence of the ionosphere, the Earth's field deviates significantly from its dipole shape over long distances (greater than about 30,000 km). The field of single ions modifies the earth's field in this region. These ions generate the magnetic field associated with them while in motion.
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Long Answer Questions [3 Marks Questions]
Ques: When kept in an external magnetic field, how is the behaviour of a diamagnetic substance different from that of a paramagnetic?
Ans-
- A diamagnetic specimen would gravitate toward the weaker part of the field, whereas a paramagnetic specimen would gravitate toward the stronger part.
- A magnet repels a diamagnetic specimen, while a paramagnetic specimen travels towards the magnet.
- The diamagnetic is positioned perpendicular to the field, whereas the paramagnetic is aligned along the field.
Ques: Define a material's magnetic susceptibility. Name two elements, one of which is susceptible to positive stimuli and the other to negative stimuli. What does the term "negative susceptibility" mean?
Ans- Magnetic susceptibility is a property of a substance that determines how quickly it may be magnetised in a magnetic field.
It's also the ratio of the material's intensity of magnetisation (I) to the intensity of the magnetising field (H).
\(\chi_{m}=\frac{\mathrm{I}}{\mathrm{H}}\)
Para-magnetic material has a positive susceptibility.
Two elements are: Al, Ca
Diamagnetic material has a negative susceptibility.
Two elements are: Bi, Cu.
Negative susceptibility indicates diamagnetic character of the substance.
Ques: (i) Make a list of two properties of a material used to make permanent magnets.
(ii) Why are ferromagnetic materials used in the core of an electromagnet?
Ans- (i) The following are two features of a material used to make permanent magnets:
- High retentivity, resulting in a powerful magnetic field.
- High coercivity, so that high magnetic fields, temperature changes, or small mechanical damage do not remove its magnetism.
(ii) Ferromagnetic materials are used in the core of electromagnets because they have a high initial permeability, which allows for substantial magnetisation even with a tiny magnetising field, and a low resistivity, which reduces eddy current losses.
Ques: Write the difference between the magnetic properties of soft iron and steel.
Ans:
| Soft iron | Steel |
|---|---|
| High Magnetic Permeability | Less Magnetic Permeability |
| Small area of hysteresis loop | Large area of hysteresis loop |
| Coercivity is low | Coercivity is high |
Ques: Show how magnetic field lines behave in the presence of paramagnetic and diamagnetic substances using diagrams. What is the best way to describe this unique feature?
Ans- Magnetic field lines in presence of paramagnetic substance:
Magnetic field lines in presence of diamagnetic substance:
The relative magnetic permeability (µr) of a paramagnetic substance is in the range of µr > 1 and for diamagnetic substance it is µr < 1.
Very Long Answer Questions [5 Marks Questions]
Ques: a) Explain Gauss's law of magnetism. Describe its significance.
b) Write down the four most important properties of magnetic field lines caused by a bar magnet.
Ans- a) According to Gauss's Law of Magnetism, the total flux of the magnetic field through any closed surface is always zero.
This law implies the absence of magnetic monopoles or magnetic field lines forms closed loops.
b) The following are the four properties of magnetic field lines:
- Magnetic field lines always form closed loops that are continuous.
- The direction of the net magnetic field at a given position is represented by the tangent to the magnetic field line at that point.
- The magnitude of the magnetic field increases as the number of field lines crossing per unit area increases.
- Magnetic field lines do not cross.
Ques: a) Why does a cooled paramagnetic sample have more magnetization (for the same magnetising field)?
b) Why, on the other hand, is diamagnetism almost temperature independent?
c) Is a ferromagnetic material's permeability independent of the magnetic field? If not, is it more appropriate for lower or higher fields?
d) At every point, magnetic field lines are nearly normal to the surface of a ferromagnet. (This is analogous to static electric field lines being parallel to the surface of a conductor at all times.) Why?
e) Is a paramagnetic sample's maximum possible magnetization of the same order of magnitude as a ferromagnet's magnetization?
Ans- a) The thermal motion of molecules is random, and it becomes more random as temperature rises. Because of this, dipole alignments are disrupted at high temperatures. This disruption is reduced as the temperature drops. As a result, when the temperature of a paramagnetic sample is reduced, or cooled, the magnetization increases.
b) The induced dipole moment in a diamagnetic substance is always opposite to the magnetising field in the presence of a magnetising field. As a result, a change in temperature that causes a change in the internal motion of the atoms has no effect on a material's diamagnetism.
c) The permeability of ferromagnetic materials is affected by the magnetic field applied. The hysteresis curve shows that it is greater for a lower field and vice versa.
d) The permeability of ferromagnetic material is greater than or equal to one. As a result, magnetic field lines are nearly normal to the surface of such materials at all points.
e) A paramagnetic sample's maximum magnetization can be of the same order of magnitude as a ferromagnet's magnetization. For saturation, high magnetising fields are required.
Ques: a) The magnetic field of the Earth varies from place to place in space. Does it change over time? If so, how long does it take for it to change noticeably?
b) Iron is known to exist in the earth's core. However, geologists do not believe that this is the source of the earth's magnetism. Why?
c) The charged currents in the earth's core's outer conducting regions are thought to be responsible for the earth's magnetism. What could be the 'battery' (i.e., the source of energy) to keep these currents going?
d) During its 4 to 5 billion-year history, the earth may have even reversed the direction of its field several times. How can geologists know about the earth's field from such a long time ago?
e) The magnetic field in interstellar space is extremely weak, on the order of 10-12 T. Is it possible for such a weak field to have any significance? Explain
Ans a)- The magnetic field of the Earth changes over time. It takes a few hundred years for a significant amount to change. The change in the earth's magnetic field over time cannot be overlooked.
b)- The Earth's core is made up of molten iron. This type of iron is not magnetic. As a result, this is not regarded as a source of Earth's magnetism.
c)- The energy that keeps the currents running in the earth's core's outer conducting areas comes from radioactivity in the earth's interior. These charged currents are thought to be responsible for the magnetism of the Earth.
d)- Several times in Earth's 4 to 5 billion-year history, the direction of its field has been reversed. During the solidification of rocks, these magnetic fields were weakly recorded. The analysis of this rock magnetism can provide information about the geomagnetic history.
e)- Charged particles moving in a circle can be bent by an extremely weak magnetic field. For a large radius path, this may not be noticeable. The passage of charged particles can be affected by deflection in the vast interstellar space.
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