Two Marks Questions with Answers

Two Marks Questions with Answers

 

Q.1 What is meant by electric force ? Write expression of it.

Ans. : A force exerted on a static or moving charge Q by a static electric field is known as an electric force. It is denoted by . The expression of an electric force is given by,

The force  is in the direction of  for a positive charge.

 

Q.2 Define magnetic force. Write its expression and significance.

Ans. : The force exerted on a charge Q moving with velocity  in a steady magnetic field   is called magnetic force. It is denoted by . It is given by,

From above expression, it is clear that, the magnitude of the magnetic force depends on magnitudes of Q,  and the sine of angle between . But the direction of  is perpendicular to the plane containing .

 

Q.3 What is Lorentz force equation ? Write its applications.

OR

What is the total force acting on a moving charge, Q in the presence of both electric and magnetic fields ?

OR

Write Lorentz force equation for a moving charge.

AU : May-09

Ans. : If a particle carrying charge + Q and moving with velocity  is present in a region where both electric and magnetic fields are present, then the particle in a motion experiences forces  due to the electric and magnetic fields respectively and the resultant force experienced by a charged particle in motion is given by,

Above equation is called Lorentz Force Equation which relates mechanical force to the electrical force.

The solution of this equation is useful in the determination of electron orbits in magnetron, proton paths in cyclotron and plasma characteristics in Magneto Hydro Dynamic generator (MHD generator).

 

Q.4 What are the points of differences between electric force and magnetic force ?

Ans. :

 

Q.5 Write an expression for force on a differential current element. What will be expression for force on straight conductor in uniform magnetic field ?

AU : Dec.18

Ans. : The force exerted on a differential current element is given by,

If a conductor is straight and placed in the uniform magnetic field, the force exerted is given by,

 

Q.6 A conductor 4 m long lies along the y-axis with the current of 10 A in  direction, if the field is . Tesla calculate the force on the conductor.

Ans. :

 

Q.7 Give an integral expression for the force on a closed circuit a current I in the magnetic field H.

AU : Dec.-04

Ans. : Refer answer of Q.5.

 

Q.8 Give an expression for the force between two current element.

Ans. : The force exerted on current element 1 due to current element 2 is given by,

Similarly the force exerted on current element 2 due to current element 1 is given by,

Q.9 Write the expression for force between two parallel current carrying  conductors. What will be the nature of force depending upon direction of currents through the conductors ?

Ans. : In general the expression for force between two parallel current carrying conductors is given by,

F = (µ₀ µ_r ) I₁ I₂ l / 2πd

where          l = Length of conductor,

I₁ I₂  = Current through conductors

d = Distance of separation between conductors 

Observations : When currents through conductors are in same direction, then conductors experience force of attraction. When the currents through conductors are opposite in direction, the conductors experience force of repulsion.

 

Q.10 Two wires carrying currents of 2 A and 3 A in opposite direction with 5 cm apart from each other in free space. Calculate force between then state nature of force.

Ans . : F = µ I₁ I₂ l / 2πd

As length is not mentioned, the force should be calculated per length as follows.

As the currents are in same direction, the force experienced will be force of attraction.

Q.11 What is meant by magnetic moment?

OR

Define magnetic moment. AU: May-04

OR

Define torque.

Ans. : The moment of force or torque about a specified point is defined as the vector product of the moment arm  and the force . The torque is measured in N.m in Nm (i.e. newton meter). Th mathematical expression is given by,

 

Q.12 Define magnetic dipole moment. Sketch the field due to magnetic dipole.

OR

What is magnetic dipole moment?

AU: Dec.-03

Ans. : The magnetic dipole moment of a current loop is defined as the product of current through the loop and the area of the loop, directed normal to the current loop. From the definition it is clear that, the magnetic dipole moment is a vector quantity. It is denoted by  . The direction of the magnetic dipole moment   is given by the right hand thumb rule. The right hand thumb indicates the direction of the unit vector in which   is directed and the figures represents the current direction. The magnetic dipole moment is given by,

 

Q.13 For 500 turn rectangular coil, 15 cm by 7.5 cm in dimension, current of 5 A flows through it. It is placed in uniform field of B = 0.1 T. Determine magnetic dipole moment and maximum torque.

Ans. : For a single turn of a rectangular coil, area is given by,

S' (15 × 10^-2) (7.5 × 10^-2) = 0.01125 m²

For 500 turn rectangular coil, total area is given by,

S = NS' (500) (0.01125) = 5.625m²

The magnitude of the magnetic moment is given by,

m = (I) (5) = (5) (5.625) = 28.125 Am²

The magnitude of maximum torque is given by,

T_max = mB = (28.125) (0.1) = 2.8125 Nm

 

Q.14 Classify the magnetic materials.

OR

Name magnetic materials.

Ans. :

The material with relative permeability µ_r ≤ 1, it is called diamagnetic. 

The material with relative permeability µ_r  ≥ 1, is called paramagnetic.

The material with relative permeability µ_r >> 1 is called ferromagnetic.

 

Q.15 Define diamagnetic, paramagnetic and ferromagnetic material.

Ans. : The magnetic materials in which the orbital magnetic moment and electron spin magnetic moment cancel each other making net permanent magnetic moment of each atom zero are called diamagnetic materials.

The magnetic materials in which the orbital and spin magnetic moments do not cancel each other resulting in a net magnetic moment of an atom are called paramagnetic materials.

The materials in which the atoms have large dipole moment due to electron spin magnetic moments are called ferromagnetic materials.

 

Q.16 Define hysteresis.

OR

What is meant by hysteresis ? Draw hysteresis

OR

Explain the phenomenon of hysteresis with reference to ferromagnetic materials.

Draw B-H curve classifying magnetic materials.

Ans. : In the ferromagnetic materials, the adjacent atoms line up their magnetic dipole moments in parallel fashion in the lattice. The regions in which large number of magnetic moments lined in parallel are called domains. When an external field is applied, the domains increase their size increasing internal field to a high value. When the external field is removed, the original random alignment of dipole moments is not achieved. Some of the moments remain in a small region which results in residual field or remanant field. This effect is called hysteresis. Iron, nickel and cobalt are the examples of ferromagnetic materials.

 

Q.17 What is meant by soft and hard magnetic materials ?

Ans. : The magnetic material which retains high residual flux density is called hard magnetic material. The material with small values of the residual flux density is called soft material.

 

Q.18 Explain antiferromagnetic material.

Ans. : The materials in which the dipole moments of adjacent atoms line up in antiparallel fashion are called antiferromagnetic materials. The net magnetic moment in such materials is zero. Thus when a specimen of antiferromagnetic material is kept near a strong magnet gets neither attracted nor repelled. This property is observed in materials like many of the oxides, chlorides and sulphides at low temperatures.

 

Q.19 What is meant by ferrimagnetic materials and ferrites ?

Ans. : The materials in which the magnetic dipole moments are lined up in antiparallel fashion, but the net magnetic moment is non-zero are called ferrimagnetic materials. Ferrites are special ferrimagnetic materials having very low electrical conductivity.

 

Q.20 Explain supermagnetic materials.

Ans. : In supermagnetic materials, the ferromagnetic materials are suspended in the dielectric matrix. The important property of the supermagnetic material is that eventhough each particle of it contain large magnetic domains but can not penetrate adjacent particles. The common  examples of supermagnetic materials are magnetic tapes used for audio, video and data recordings.

 

Q.21 Define magnetization.

Ans. : The field produced due to movement of bound charges is called magnetization represented by  . The magnetization is defined as the magnetic dipole moment per unit volume. Its unit is A/m.

 

 

Q.22 What is magnetic susceptibility ?

Ans. : The ratio of magnetization M to the magnetic field intensity H is called magnetic susceptibility. It is denoted by X_m. It is the measure of how susceptible the material is to the magnetic field. By definition  where X_m is dimensionless.

 

Q.23 What is relative permeability of material ?

AU : Dec.-04

Ans. : The ratio of permeability (µ) of a material to the permeability (µ₀) of free space is called relative permeability of material. It is denoted by µ_r. It is a dimensionless quantity. Mathematically it can be expressed as,

µ_r  = µ  / µ₀ = (1 + X_m)

 

Q.24 What is the relationship between magnetic flux density, magnetic field intensity and magnetization ?

Ans. : The relationship between magnetic flux density  , magnetic field intensity  and magnetization   is given by,

 

Q.25 State the boundary conditions of magnetic media.

AU : Dec.-09

OR

Write magnetic boundary conditions.

AU : Dec.-13, May-16

Ans. :          1) The normal component of magnetic flux density  is continuous at boundary between two magnetic media, i.e. Bt_{an 1} / B_{tan 2} = µ₁ / µ₂

2) The tangential component of magnetic field intensity  is continuous at the boundary between two magnetic media, i.e.

H_N1 = H_N2 = µ₂ / µ₁

 

Q.26 Define self inductance.

AU : May-03, 05,15, Dec.-04, 17

Ans. : The ratio of total flux linkage to the current producing that flux is called inductance denoted by L. It is measured in henry (H).

L = (N ϕ / I) H

 

Q.27 Define mutual inductance.

AU ; May-03, 05, 09, 15,Dec.-04

Ans. : With respect to the flux linkage, the mutual inductance between the two coils is defined as the ratio of flux linkage of one coil to the current in other coil. Hence we can write,

 

Q.28 Determine the force per meter length between two long parallel wires A and B separated bp 5 mm in air and each carrying current of 40 A in the same direction.

Ans. : Force between two parallel conductors is given by,

Hence force per meter length is given by,

Both conductors carry equal current so I₁ = I₂ =40 A. The distance between two conductors is d = 5 mm = 5 × l0^-3m. Substituting values, we get,

Q.29 A loop with magnetic dipole moment  lies in a uniform magnetic field of  Calculate torque.

AU : May-11

Ans. : Torque is given by,

 

Q.30 What is the force and its direction on a conductor in + z-axis carrying current I and placed in a magnetic filed  along X-axis ?

Ans. : In general, force on a conductor is given by,

Thus the force will be directed in + y direction.

 

Q.31 What is the expression for the torque experienced by a current carrying loop, placed in magnetic field ?

AU : May-03, 06

Ans.

The torque experienced by a current carrying loop placed in a magnetic filed is given by,

 

Q.32 Find the maximum torque on an 100 turns rectangular coil of 0.2 m by 0.3 m, carrying current of 2 A in the field of flux density 5 Web/m².

AU : May-15

Ans.

Area of single turn rectangular coil is given by,

S₁ = (0.2)(0.3) = 0.06 m²

For 100 turn rectangular coil, total area will be,

S = NS₁ = (100) (0.06) = 6m²

Hence magnitude of maximum torque on rectangular coil is given by,

T = mB = ISB = (2)(6)(5) = 60 Nm

 

Q. 33 What is the expression for inductance of a toroid ?

Ans.

The inductance of a toroid is given by,

L = µN² A / 2 π R   where A = Area of corss-section of a troidal ring

R = Radius of a toroid.

N = Number of turns.

 

Q. 34 A solenoid 3 cm in length carries a current of 200 mA. If solenoid is to produce a magnetic flux density of 4.6 mWb/m² , how many turns of wire are needed ?

AU : May-11

Ans. : The field intensity inside solenoid is given by,

 

Q.35 State Ohm’s law for magnetic circuits.

Ans. : For magnetic circuits, Ohm's law is given by,

m.m.f. = Magnetomotive force

= ϕ R where ϕ is total flux and R is reluctance of circuit.

 

Q.36 Find inductance per unit length of long solenoid of N turns and having length I meters. Assume that it carries current of I amperes.

Ans. :

 Let I be the length of solenoid. Then inductance of a solenoid of N terms carrying current I is given by,

L = ( µN² A / l ) H

Hence the inductance per unit length is given by,

L = L / l ( µN² A / l ²) H/m

 

Q.37 What is the mutual inductance of the two inductively coupled coils with self inductance of 25 mH and 100 mH ?

AU : Dec.-15

Ans. : The mutual inductance is given by,

M = k √L₁L₂

where k is coefficient of coupling

M = k√ (25 × 10^-3)(100 × 10^-3) = (50 × 10^-3)k H

... answer interms of k.

Assuming two coils tightly coupled i.e. k = 1, then M = 50 mH.

 

Q.38 Define reluctance and permeability.

Ans. : The resistance offered by the material to the flow of magnetic flux through it, is called reluctance.

The ability or case with which the magnetic material forces the magnetic flux through a given medium is called the permeability of that medium.

 

Q.39 Write down the steps to calculate inductance of various shapes.

Ans. : Assume current I flowing. From this, calculate the magnetic field strength H. For the given medium, find B from the relation B = µH Find the flux from cross-sectional area of the shape A as ϕ = BA. Then find the flux linkages Nϕ Then dividing flux linkages by the current, the inductance can be obtained.

 

Q.40 What is main effect of eddy current ?

AU : May-19

Ans. : The eddy currents are induced currents in a conductor due to changing magnetic fields. These currents flow in closed loops, in planes perpendicular to the magnetic field. The main effect of these currents is to produce losses causing heat as these currents oppose the change in the magnetic field which created it. These losses reduce efficiency of machines while heat produced can be used for induction heating.

 

Q. 41 What is energy density in magnetic field ?

AU : Dec.-19

Ans . : Energy density in magnetic field =  1 / 2 B² / µ

= 1/2 µH² J/m³