Magnetism and Electromagnetism MCQ Question and Answer

Answer.3. A line of magnetic flux that does not follow the designed path

Stray magnetic field or magentic leakage is defined as the magnetic flux that does not follow the useful path. It can create an unwanted magnetic field where no magnetic field is required.

Answer.2. High permeability and low hysteresis loss

Permeability:-  It is the property of a material by virtue of which it allows itself to be magnetised. Permeabilty of a magnetic material indicates the ability of that material to allow magnetic flux to exist in it for certain magnetising force. A good magnetic material should have High permeability.

Hysteresis loop measures the energy dissipated due to hysteresis which appears in the form of heat and so raises the temperature of that portion of the magnetic circuit which is subjected to magnetic reversal, The shape of the hysteresis loop depends on the nature of the magnetic material.

Materials subjected to rapid reversal of magnetism should have high permeability and low hysteresis loss.

• Hysteresis loss is associated with the cyclic magnetization and demagnetization of the material as the magnetic field changes with the sinusoidally varying current.
• The hysteresis loss is proportional to the area of the hysteresis loop and therefore in order to reduce the loss the material of the core should have a smaller hysteresis loop area

Answer.4. Through which the magnetic lines of force can pass very easily

Permeability refers to the ease with which a material can pass magnetic lines of force.

Answer.2. Rectangular

Explanation

• If the domain walls are easy to move, the coercive field is low, it is easy to magnetize the material, Such a material is called a soft magnetic material.
• If it is difficult to move the domain walls, the coercive field is large and the material is magnetically hard.

Fig.shows the nature of hysteresis loop of hard magnetic material (steel).The Hard magnetic material has a wider hysteresis loop as shown in the figure below and results in a large amount of energy dissipation and the demagnetization process is more difficult to achieve.

Property of Hard Magnet Material

• Hard magnetic materials have large hysteresis loss due to larger hysteresis loop area.
• in these materials, the domain wall movement is difficult because of the presence of impurities and crystal imperfections and it is irreversible in nature.
• The coercivity and retentivity are large. Hence, these materials cannot be easily magnetized and demagnetizer
• Hard magnetic materials are used to produce permanent magnet.
• Hard Magnetic Material has Low Susceptibility and Permeability.
• The hysteresis loop of hard mateirals is rectangular in shape.
• Magnetic Energy Stored is High.
• The eddy current losses are high.
• They are suitable for making permanent magnet e.g Alnico

Answer.3. Ferromagnetic

Explanation:-

Permeability is a measure of how easy it is to establish the flux in a material. Ferromagnetic materials have high permeability and hence low Reluctance.

Ferromagnetic materials have a relative permeability greater than unity and generally very high. An additional important property of ferromagnetic materials is the dependence of magnetization on the level of the external field. Thus, magnetization in ferromagnetic materials is a nonlinear process.

Answer.4. Resistance of the coil

Explanation:-

Whenever there is relative motion between a magnet and a conducting coil, an emf is induced in the coil,

e= −N(dφ/dt)

where

dφ= the rate of change of the flux.

Hence, emf induced does not depend on resistance of the coil.

Conclusion:  Whenever there is a relative motion between a loop (or coil) and a magnet, an emf is induced in the loop (or coil) which is called INDUCED EMF. This phenomenon is called electromagnetic induction. If the loop or coil is a closed circuit, then an electric current also flows through the coil due to this induced emf, and it is known as induced current. It should be noted here that induced emf does not depend upon the resistance of the circuit but induced current definitely depends on it.

Answer.1. It Will depend on the radius of the circle

The magnetic lines of force are perpendicular to the plane of the circular wire loop. The lines of force are circular near the wire loop but practically straight near the center of the loop. In the middle of the loop, the magnetic field is uniform for a short distance on either side.

The magnetic field at the center of the circular coil

B = μ_onI/2r

B ∝ I/r

Hence When two ends of a circular uniform wire are joined to the terminals of a battery, the field at the center of the circle depends on the radius of the circle.

Answer.2. 0.1 N

Explanation:-

Current of two parallel conductors, I₁ and I₂ = 100 Amps

Distance (d) = 20 mm = 20 × 10^-3 m

The force between two parallel conductor per unit length

$\begin{array}{l} F = \frac{{{\mu _0}{I_1}{I_2}}}{{2\pi d}}\\ \\ = \frac{{4\pi \times {{10}^{ – 7}} \times 100 \times 100}}{{2\pi \times 20 \times {{10}^{ – 3}}}}\\ \\ = 0.1\;N \end{array}$

Answer.4. Current, magnetic field, and direction of the force on a conductor

Fleming’s left-hand rule

Fleming’s left-hand rule tells us the direction of the force on the wire. Hold the thumb and first two fingers of your left hand so that they are at right angles to each other.

If the first finger points in the direction of the magnetic field the second finger points in the direction of the current, then the thumb points in the direction of the thrust (force).

Answer.1. Decreases with an increasing cross-sectional area of material

Explanation

Magnetic Reluctance or “Magnetic Resistance” is analogous to resistance in an electrical circuit (although it does not dissipate magnetic energy). Magnetic Reluctance is the property of the magnetic circuit which offers opposition to the flow of magnetic flux.

In likeness to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance.

The Reluctance (S) of a magnetic circuit is equal to the ratio of the “Magneto Motive Force” (m.m.f) in a passive magnetic circuit and the Magnetic Flux in this circuit.

S = mmf/φ

The reluctance of a uniform magnetic field can be calculated as

S =  (l/µ_oµ_rA) AT/wb

where

l = the length of the circuit in meters.

µ_o = permeability of free space in H/m. ,

µ_r = relative magnetic permeability of the material, (dimensionless).

A is the cross-sectional area of the circuit in square meters.

Since magnetic reluctance is inversely proportional to the area, therefore, reluctance decrease with an increase in the cross-sectional area.