Magnetostatics
The induced voltage will oppose the flux producing it. State True/False. a) True b) False
Answer: a Explanation: According to Lenz law, the induced voltage acts in such a way that it opposes the flux producing it. This is indicated by a negative sign.
Electric field will be maximum outside the conductor and magnetic field will be maximum inside the conductor. State True/False. a) True b) False
Answer: a Explanation: At the conductor-free space boundary, electric field will be maximum and magnetic field will be minimum. This implies electric field is zero inside the conductor and increases as the radius increases and the magnetic field is zero outside the conductor and decreases as it approaches the conductor.
8. In a static magnetic field only magnetic dipoles exist. State True/False. a) True b) False
Answer: a Explanation: From Gauss law for magnetic field, we get divergence of the magnetic flux density is always zero (ie, Div(B) = 0). This implies the non-existence of magnetic monopole.
In metals which of the following equation will hold good? a) Curl(H) = J b) Curl(J) = dD/dt c) Curl(H) = D d) Curl(J) = dB/dt
Answer: a Explanation: Generally, the Curl(H) is the sum of two currents- conduction and displacement. In case of metals, it constitutes conduction J and in case of dielectrics, it constitutes the displacement current dD/dt.
Find the electric field when the magnetic field is given by 2sin t in air. a) 8π x 10-7 cos t b) 4π x 10-7 sin t c) -8π x 10-7 cos t d) -4π x 10-7 sin t
Answer: a Explanation: Given H = 2sin t. We get B = μH = 4π x 10-7 x 2sin t = 8πx10-7sin t. To get E, integrate B with respect to time, we get 8πx10-7cos t.
In which of the following forms can Maxwell's equation not be represented? a) Static b) Differential c) Integral d) Harmonic
Answer: a Explanation: Maxwell equations can be represented in differential/point form and integral form alternatively. Sometimes, it can be represented by time varying fields called harmonic form.
The Laplacian of the magnetic vector potential will be a) -μ J b) - μ I c) -μ B d) -μ H
Answer: a Explanation: The Laplacian of the magnetic vector potential is given by Del2(A) = -μ J, where μ is the permeability and J is the current density.
Which of the following statements is true? a) E is the cross product of v and B b) B is the cross product of v and E c) E is the dot product of v and B d) B is the dot product of v and E
Answer: a Explanation: The electric field is the cross product of the velocity and the magnetic field intensity. This is given by Lorentz equation.
Find the power of an inductor of 5H and current 4.5A after 2 seconds. a) 25.31 b) 50.62 c) 102.4 d) 204.8
Answer: a Explanation: The energy stored in an inductor is given by E = 0.5 LI^2. Thus, put L = 5 and I = 4.5 and we get E = 0.5 x 5 x 4.5^2 = 50.625 units To get power P = E/t = 50.625/2 = 25.31 units.
Find the inductance when the energy is given by 2 units with a current of 16A. a) 15.6mH b) 16.5mH c) 16.8mH d) 15.8mH
Answer: a Explanation: The energy stored in an inductor is given by E = 0.5 LI^2. To get L, put E = 2 and I = 16 and thus L = 2E/I2 = 2 x 2/16^2 = 15.6mH.
The energy of a coil depends on the turns. State True/False. a) True b) False
Answer: a Explanation: The inductance is directly proportional to square of the turns. Since the energy is directly proportional to the inductance, we can say both are dependent on each other.
Find the inductance of a material with 100 turns, area 12 units and current of 2A in air. a) 0.75mH b) 7.5mH c) 75mH d) 753mH
Answer: a Explanation: The inductance of any material(coil) is given by L = μ N^2A/I. On substituting N = 100, A = 0.12 and I = 2, we get L = 4π x 10^-7 x 100^2 x 0.12/2 = 0.75 units.
Calculate the magnetic energy when the magnetic intensity in air is given as 14.2 units(in 10^-4 order) a) 1.26 b) 2.61 c) 6.12 d) 1.62
Answer: a Explanation: The magnetic energy is given by E = 0.5 μ H2. Put H = 14.2 and in air μ = 4π x 10^-7, we get E = 0.5 x 4π x 10^-7 x 14.2^2 = 1.26 x 10^-4 units.
Calculate the magnetic energy when the magnetic flux density is given by 32 units(in 108 order) a) 4.07 b) 7.4 c) 0.47 d) 7.04
Answer: a Explanation: The magnetic energy is given by E = 0.5 μ H^2 and we know that μH = B. On substituting we get a formula E = 0.5 B^2/μ. Put B = 32 and in air μ = 4π x 10^-7, we get E = 0.5 x 32^2/4π x 10^-7 = 4.07 x 10^8 units.
Find the energy of a coil of inductance 18mH and current passing through it 1.25A.(in 10^-3 order) a) 14.06 b) 61 c) 46.1 d) 28.12
Answer: a Explanation: The magnetic energy possessed by a coil is given by E = 0.5 x LI^2. Put L = 18 x 10^-3 and I = 1.25, thus we get E = 0.5 x 18 x 10^-3 x 1.25^2 = 14.06 x 10^-3 units.
3. Find the magnetic field of a finite current element with 2A current and height 1/2π is a) 1 b) 2 c) 1/2 d) 1/4
Answer: a Explanation: The magnetic field due to a finite current element is given by H = I/2πh. Put I = 2 and h = 1/2π, we get H = 1 unit.
The magnetic flux density is directly proportional to the magnetic field intensity. State True/False. a) True b) False
Answer: a Explanation: The magnetic field intensity is directly proportional to the magnetic field intensity for a particular material (Permeability). It is given by B = μH.
Find the magnetic field intensity of a toroid of turns 40 and radius 20cm. The current carried by the toroid be 3.25A. a) 103.45 b) 102 c) 105.7 d) 171
Answer: a Explanation: The magnetic field intensity of a toroid is given by H = NI/2πrm. Put N = 40, I = 3.25 and rm = 0.2, we get H = 40 x 3.25/2π x 0.2 = 103.45 units.
The magnetic field intensity of an infinite sheet of charge with charge density 36.5 units in air will be a) 18.25 b) 11.25 c) 73 d) 1/36.5
Answer: a Explanation: The magnetic field intensity of an infinite sheet of charge is given by H = 0.5 K, for the point above the sheet and -0.5 K, for the point below the sheet. Here k is the charge density. Thus H = 0.5 x 36.5 = 18.25 units.
The relation between flux density and vector potential is a) B = Curl(A) b) A = Curl(B) c) B = Div(A) d) A = Div(B)
Answer: a Explanation: The magnetic flux density B can be expressed as the space derivative of the magnetic vector potential A. Thus B = Curl(A).
Find the magnetic field intensity of a material with flux density of 24 units in air(in 106 order) a) 19.09 b) 21 c) 25 d) 26.78
Answer: a Explanation: The magnetic flux density is given by B = μ H. To get H, put B = 24 and μ = 4∏ x 10^-7. Thus H = 24/4∏ x 10^-7 = 19.09 x 10^6 units.
Find the flux density of a conductor in the square of the centre of the loop having current 3.14A and radius is 1.414m in air. a) 8π x 10^-7 b) 4π x 10^-7 c) 6π x 10^-7 d) 2π x 10^-7
Answer: a Explanation: The magnetic flux density of a conductor in the square of centre of the loop is given by B = 2√2 μo I/π R. Put I = 3.14 and R = 1.414, we get B = 2 x 1.414 x 4π x 10^-7 x 3.14/π x 1.414 = 8π x 10^-7 units.
Which type of flux will increase the inductance? a) Series aiding b) Series opposing c) Shunt aiding d) Shunt opposing
Answer: a Explanation: The series aiding flux will give maximum inductance to a circuit compared to any other fluxing techniques. This is because all the individual and mutual inductances will get added.
The Faraday's law states about which type of EMF? a) Transformer EMF b) Back EMF c) Generator EMF d) Secondary EMF
Answer: a Explanation: The stationary loop in a varying magnetic field results in an induced emf due to the change in the flux linkage of the loop. This emf is called as induced or transformer EMF.
Find the total flux in a coil of magnetic flux density 12 units and area 7 units. a) 0.84 b) 0.96 c) 8.4 d) 9.6
Answer: a Explanation: The total flux in a coil is defined by φ = BA, where B = 12 and A = 0.07. On substituting these values, we get φ = 12 x 0.07 = 0.84 units.
Find the total flux in a material of flux density 15 units in an area of 24 units. a) 3.6 b) 7.2 c) 9.6 d) 5.4
Answer: a Explanation: The total flux in a material is given by φ = ∫ B.dS. Put B= 15 and ∫dS = 0.24. On substituting, we get φ = 15 x 0.24 = 3.6 units.
Find the flux enclosed by a material of flux density 12 units in an area of 80cm. a) 9.6 b) 12/80 c) 80/12 d) 12/0.8
Answer: a Explanation: The total flux in a material is the product of the flux density and the area. It is given by flux = 12 x 0.8= 9.6 units.
Find the flux contained by the material when the flux density is 11.7 Tesla and the area is 2 units. a) 23.4 b) 12.3 c) 32.4 d) 21.3
Answer: a Explanation: The total flux is given by φ = ∫ B.ds, where ∫ds is the area. Thus φ = BA. We get φ = 11.7 x 2 = 23.4 units.
Find current density J when B = 50 x 10-6 units and area dS is 4 units. a) 9.94 b) 8.97 c) 7.92 d) 10.21
Answer: a Explanation: To get H, H = B/μ = 50 x 10-6/ 4π x 10-7 = 39.78 units. Also H = ∫ J.dS, where H = 39.78 and ∫ dS = 4. Thus J = 39.78/4 = 9.94 units.
Find the magnetic field intensity when the flux density is 8 x 10-6 Tesla in the medium of air. a) 6.36 b) 3.66 c) 6.63 d) 3.36
Answer: a Explanation: We how that, B = μH. To get H = B/μ, put B = 8 x 10-6 and μ = 4π x 10-7. Thus H = 8 x 10-6/ 4π x 10-7 = 6.36 units.
Find the magnetic field intensity when the current density is 0.5 units for an area up to 20 units. a) 10 b) 5 c) 20 d) 40
Answer: a Explanation: We know that ∫ H.dl = I. By Stoke's law, we can write Curl(H) = J. In integral form, H = ∫ J.ds, where J = 0.5 and ds is defined by 20 units. Thus H = 0.5 x 20 = 10 units.
In a magnetic material, always there exist magnetic dipoles as well as monopoles. State True/False. a) True b) False
Answer: b Explanation: A magnetic material possesses only magnetic dipoles. The absence of magnetic monopoles is indicated by the equation Div(B) = 0.
Find the current density on the conductor surface when a magnetic field H = 3cos x i + zcos x j A/m, for z>0 and zero, otherwise is applied to a perfectly conducting surface in xy plane. a) cos x i b) -cos x i c) cos x j d) -cos x j
Answer: b Explanation: By Ampere law, Curl (H) = J. The curl of H will be i(-cos x) - j(0) + k(-z sin x) = -cos x i - zsin x k. In the xy plane, z = 0. Thus Curl(H) = J = -cos x i.
When the rotational path of the magnetic field intensity is zero, then the current in the path will be a) 1 b) 0 c) ∞ d) 0.5
Answer: b Explanation: By Ampere law, Curl(H) = J. The rotational path of H is zero, implies the curl of H is zero. This shows the current density J is also zero. The current is the product of the current density and area, which is also zero.
The value of ∫ H.dL will be a) J b) I c) B d) H
Answer: b Explanation: By Stoke's theorem, ∫ H.dL = ∫ Curl(H).dS and from Ampere's law, Curl(H) = J. Thus ∫ H.dL = ∫ J.dS which is nothing but current I.
9. The magnetic field intensity will be zero inside a conductor. State true/false. a) True b) False
Answer: b Explanation: Electric field will be zero inside a conductor and magnetic field will be zero outside the conductor. In other words, the conductor boundary, E will be maximum and H will be minimum.
Find the current when the magnetic field intensity is given by 2L and L varies as 0->1. a) 2 b) 1 c) 0.5 d) 0
Answer: b Explanation: From Ampere law, we get ∫ H.dL = I. Put H = 2L and L = 0->1. On integrating H with respect to L, the current will be 1A.
Inductance is present in semiconductor. State True/False. a) True b) False
Answer: b Explanation: Inductance property exists only for pure conductors like coil, solenoid, toroid etc. It is not present in semiconductors.
Electromagnetic waves are longitudinal in nature. State True/False. a) True b) False
Answer: b Explanation: Light and other electromagnetic radiations are transverse in nature as they travel at the same speed through a vacuum, such as through space. Such waves vibrate at right angles to the direction of propagation.
Identify the devices that do not use electromagnetic energy. a) Television b) Washing machine c) Microwave oven d) Mobile phones
Answer: b Explanation: Television and mobile phones use the electromagnetic waves as signals. Microwave ovens generate electromagnetic waves (microwaves) for heating the food. Washing machine does not use any EM wave for its operation.
Find the charge density when the electric flux density is given by 2x i + 3y j + 4z k. a) 10 b) 9 c) 24 d) 0
Answer: b Explanation: The charge density is the divergence of the electric flux density by Maxwell's equation. Thus ρ = Div (D) and Div (D) = 2 + 3 + 4 = 9. We get ρ = 9 units.
The charge build up in the capacitor is due to which quantity? a) Conduction current b) Displacement current c) Convection current d) Direct current
Answer: b Explanation: The charge in the capacitor is due to displacement current. It is the current in the presence of the dielectric placed between two parallel metal plates.
3. Calculate the emf when the flux is given by 3sin t + 5cos t a) 3cos t - 5sin t b) -3cos t + 5sin t c) -3sin t - 5cos t d) 3cos t + 5sin t
Answer: b Explanation: The electromotive force is given by Vemf = -dλ/dt. Thus Vemf = -dλ/dt = -(3cos t - 5sin t) = -3cos t + 5sin t.
Find the flux density B when the potential is given by x i + y j + z k in air. a) 12π x 10-7 b) -12π x 10-7 c) 6π x 10-7 d) -6π x 10-7
Answer: b Explanation: The field intensity H = -Grad(V). Since the given potential is a position vector, the gradient will be 3 and H = -3. Thus the flux density B = μH = 4π x 10-7 x (-3) = -12π x 10-7 units.
Find the vector potential when the field intensity 60x2 varies from (0,0,0) to (1,0,0). a) 120 b) -20 c) -180 d) 60
Answer: b Explanation: The field intensity H = -Grad(V). To get V, integrate H with respect to the variable. Thus V = -∫H.dl = -∫60x2 dx = -20x3 as x = 0->1 to get -20.
Find the force due to a current element of length 2cm and flux density of 12 tesla. The current through the element will be 5A. a) 1 N b) 1.2 N c) 1.4 N d) 1.6 N
Answer: b Explanation: The force due to a current element is given by F = BI x L. Thus F = 12 x 5 x 0.02 = 1.2 units.
Find the induced EMF in an inductor of 2mH and the current rate is 2000 units. a) 4 b) -4 c) 1 d) -1
Answer: b Explanation: The induced emf is given by e = -Ldi/dt. Put L = 2 x 10-3 and di/dt = 2000 in the equation. We get e = -2 x 10-3 x 2000 = -4 units.
The inductance is the measure of a) Electric charges stored by the material b) Emf generated by energising the coil c) Magnetic field stored by the material d) Magnetization of dipoles
Answer: b Explanation: The inductance is a property of an electric conductor/coil which measures the amount of emf generated by passing current through the coil.
Calculate the energy when the magnetic intensity and magnetic flux density are 15 and 65 respectively. a) 755 b) 487.5 c) 922 d) 645
Answer: b Explanation: The magnetic energy can also be written as E = 0.5 μH2 = 0.5 BH, since B = μH. On substituting B = 65 and H = 15 we get E = 0.5 x 65 x 15 = 487.5 units.
4. Calculate the magnetic field at a point on the centre of the circular conductor of radius 2m with current 8A. a) 1 b) 2 c) 3 d) 4
Answer: b Explanation: The magnetic field due to a point in the centre of the circular conductor is given by H = I/2a. Put I = 8A and a = 2m, we get H = 8/4 = 2 units.
Find the magnetic field intensity when the magnetic vector potential x i + 2y j + 3z k. a) 6 b) -6 c) 0 d) 1
Answer: b Explanation: The magnetic field intensity is given by H = -Grad(Vm). The gradient of Vm is 1 + 2 + 3 = 6. Thus H = -6 units.
Find the magnetic flux density of a finite length conductor of radius 12cm and current 3A in air( in 10-6 order) a) 4 b) 5 c) 6 d) 7
Answer: b Explanation: The magnetic field intensity is given by H = I/2πr, where I = 3A and r = 0.12. The magnetic flux density in air B = μ H, where μ = 4π x 10-7.Thus B = 4π x 10-7 x 3/2π x 0.12 = 5x 10-6 units.
Find the height of an infinitely long conductor from point P which is carrying current of 6.28A and field intensity is 0.5 units. a) 0.5 b) 2 c) 6.28 d) 1
Answer: b Explanation: The magnetic field intensity of an infinitely long conductor is given by H = I/2πh. Put I = 6.28 and H = 0.5, we get h = 1/0.5 = 2 units.
Find the magnetic flux density when the vector potential is a position vector. a) 1 b) 0 c) -1 d) ∞
Answer: b Explanation: The magnetic flux density is given by B = Curl(A) and A = x i + y j + z k. The curl of the position vector A is i(0) - j(0) + k(0) = 0. Thus the flux density is also zero.
Find the magnetic flux density of the material with magnetic vector potential A = y i + z j + x k. a) i + j + k b) -i - j - k c) -i-j d) -i-k
Answer: b Explanation: The magnetic flux density is the curl of the magnetic vector potential. B = Curl(A). Thus Curl(A) = i(-1) - j(1) + k(-1) = -i - j - k. We get B = -i - j - k.
The magnetic vector potential is a scalar quantity. a) True b) False
Answer: b Explanation: The magnetic vector potential could be learnt as a scalar. But it is actually a vector quantity, which means it has both magnitude and direction.
The permeability and permittivity of air or free space is unity. State true/false. a) True b) False
Answer: b Explanation: The permeability and permittivity of free space or air is always unity. This implies that the air is always ready to store electric or magnetic charges subjected to it.
The time varying electric field E is conservative. State True/False. a) True b) False
Answer: b Explanation: The time varying electric field E(t) is not a closed path. Thus the curl will be non-zero. This implies E(t) is not conservative and the statement is false.
Find the magnetic flux density when a flux of 28 units is enclosed in an area of 15cm. a) 178.33 b) 186.67 c) 192.67 d) 124.33
Answer: b Explanation: The total flux is the product of the magnetic flux density and the area. Total flux = B x A. To get B, put flux/area. B = 28/0.15 = 186.67 units.
Find the work done in an inductor of 4H when a current 8A is passed through it? a) 256 b) 128 c) 64 d) 512
Answer: b Explanation: The work done in the inductor will be W = 0.5 x LI2. On substituting L = 4 and I = 8, we get, W = 0.5 x 4 x 82 = 128 units.
Given the magnetic field is 2.4 units. Find the flux density in air(in 10-6 order). a) 2 b) 3 c) 4 d) 5
Answer: b Explanation: We know that B = μH. On substituting μ = 4π x 10-7 and H = 2.4, we get B = 4π x 10-7 x 2.4 = 3 x 10-6 units.
Find the magnetic flux density B when E is given by 3sin y i + 4cos z j + ex k. a) ∫(4sin z i - ex j - 3cos y k)dt b) -∫(4sin z i - ex j - 3cos y k)dt c) ∫(4sin y i - ex j + 3cos y k)dt d) -∫(4sin y i + ex j + 3cos y k)dt
Answer: b Explanation: We know that Curl (E) = -dB/dt. The curl of E is (4sin z i - ex j - 3cos y k). To get B, integrate the -curl(E) with respect to time to get B = -∫(4sin z i - ex j - 3cos y k)dt.
1. Biot Savart law in magnetic field is analogous to which law in electric field? a) Gauss law b) Faraday law c) Coulomb's law d) Ampere law
Answer: c Explanation: Biot Savart law states that the magnetic flux density H = I.dl sinθ/4πr2, which is analogous to the electric field F = q1q2/4πεr2, which is the Coulomb's law.
Choose the best relation. a) A = -Div(V) b) V = Curl(A) c) H = -Grad(V) d) V = Div(E)
Answer: c Explanation: For any magnetic field, the magnetic field intensity will be the negative gradient of the potential of the field. This is given by H = -Grad(V).
1. For time varying currents, the field or waves will be a) Electrostatic b) Magneto static c) Electromagnetic d) Electrical
Answer: c Explanation: For stationary charges, the field is electrostatic. For steady currents, the field is magneto static. But for time varying currents, the field or waves will be electromagnetic.
Find the Maxwell equation derived from Faraday's law. a) Div(H) = J b) Div(D) = I c) Curl(E) = -dB/dt d) Curl(B) = -dH/dt
Answer: c Explanation: From the Faraday's law and Lenz law, using Stoke's theorem, we get Curl(E) = -dB/dt. This is the Maxwell's first law of electromagnetics.
Find the Maxwell law derived from Ampere law. a) Div(I) = H b) Div(H) = J c) Curl(H) = J d) Curl(B) = D
Answer: c Explanation: From the current density definition and Ohm's law, the Ampere circuital law Curl(H) = J can be derived. This is Maxwell's second law of electromagnetics.
2. Which of the following cannot be computed using the Biot Savart law? a) Magnetic field intensity b) Magnetic flux density c) Electric field intensity d) Permeability
Answer: c Explanation: The Biot Savart law is used to calculate magnetic field intensity. Using which we can calculate flux density and permeability by the formula B = μH.
The H quantity is analogous to which component in the following? a) B b) D c) E d) V
Answer: c Explanation: The H quantity refers to magnetic field intensity in the magnetic field. This is analogous to the electric field intensity E in the electric field.
Using Maxwell equation which of the following cannot be calculated directly? a) B b) D c) A d) H
Answer: c Explanation: The Maxwell equations can be used to compute E,H,D,B and J directly. It is not possible to find the magnetic vector potential A directly.
5. The current element of the solenoid of turns 100, length 2m and current 0.5A is given by, a) 100 dx b) 200 dx c) 25 dx d) 50 dx
Answer: c Explanation: The current element of the solenoid is given by NI dx/L. Put N = 100, I = 0.5 and L = 2 to get, I dx = 100 x 0.5 x dx/2 = 25 dx.
Find the displacement current when the flux density is given by t3 at 2 seconds. a) 3 b) 6 c) 12 d) 27
Answer: c Explanation: The displacement current is given by Jd = dD/dt. Thus Jd = 3t2. At time t = 2, we get Jd = 3(2)2= 12A.
When the electric field travels in +x direction and the EM wave is travelling the -y direction, then the magnetic field will be travelling in which direction? a) +z direction b) -z direction c) Either +z or -z direction d) Does not travel
Answer: c Explanation: The electric field and magnetic field will always travel perpendicular to each other and the EM wave will travel perpendicular to both these fields. In the given condition when E travels in +x direction and wave in -y direction, then the H field that is perpendicular to both components will be travelling in either +z or -z direction.
Find the electric flux density of a material with charge density 16 units in unit volume. a) 1/16 b) 16t c) 16 d) 162
Answer: c Explanation: The electric flux density from Maxwell's equation is given by D = ∫ ρ dv. On substituting ρ = 16 and ∫dv = 1, we get D = 16 units.
Given the vector potential is 16 - 12sin y j. Find the field intensity at the origin. a) 28 b) 16 c) 12 d) 4
Answer: c Explanation: The field intensity is given by H = - Grad(V). The gradient is given by 0 - 12cos y. At the origin, the gradient will be -12 cos 0 = -12. Thus the field intensity will be 12 units.
If ∫ H.dL = 0, then which statement will be true? a) E = -Grad(V) b) B = -Grad(D) c) H = -Grad(Vm) d) D = -Grad(A)
Answer: c Explanation: The given condition shows that the magnetic field intensity will be the negative gradient of the magnetic vector potential.
Calculate the emf when a coil of 100 turns is subjected to a flux rate of 0.3 tesla/sec. a) 3 b) 30 c) -30 d) -300
Answer: c Explanation: The induced emf is given by Vemf = -dλ/dt = -Ndψ/dt. Thus emf will be -100 x 0.3 = -30 units.
Find the turns in an solenoid of inductance 23.4mH , current 2A and area 15cm. a) 900 b) 400 c) 498 d) 658
Answer: c Explanation: The inductance of any material(coil) is given by L = μ N^2A/I. Put L = 23.4 x 10^-3, I = 2 and A = 0.15, we get N as 498 turns.
Find the magnetic field intensity at the radius of 6cm of a coaxial cable with inner and outer radii are 1.5cm and 4cm respectively. The current flowing is 2A. a) 2.73 b) 3.5 c) 0 d) 1.25
Answer: c Explanation: The inner radius is 1.5cm and the outer radius is 4cm. It is clear that the magnetic field intensity needs to be calculated outside of the conductor ie, r>4cm. This will lead to zero, since H outside the conductor will be zero.
Calculate the magnetic field intensity due to a toroid of turns 50, current 2A and radius 159mm. a) 50 b) 75 c) 100 d) 200
Answer: c Explanation: The magnetic field intensity is given by H = NI/2πrm, where N = 50, I = 2A and rm = 1/2π. Thus H = 50 x 2/2π x 0.159 = 100 units.
10. Find the magnetic field when a circular conductor of very high radius is subjected to a current of 12A and the point P is at the centre of the conductor. a) 1 b) ∞ c) 0 d) -∞
Answer: c Explanation: The magnetic field of a circular conductor with point on the centre is given by I/2a. If the radius is assumed to be infinite, then H = 12/2(∞) = 0.
7. Find the magnetic flux density when a point from a finite current length element of current 0.5A and radius 100nm. a) 0 b) 0.5 c) 1 d) 2
Answer: c Explanation: The magnetic flux density is B = μH, where H is given by I/2πr. Put μ = 4π x 10-7, I = 0.5 and r = 10-7, we get B = 4π x 10-7 x 0.5/2π x 10-7 = 1 unit.
Find the magnetic field intensity due to an infinite sheet of current 5A and charge density of 12j units in the positive y direction and the z component is below the sheet. a) 6 b) 0 c) -6 d) 60k
Answer: c Explanation: The magnetic intensity when the normal component is below the sheet is Hy = -0.5 K, where K = 12.Thus we get H = -0.5 x 12 = -6 units.
The current element of the magnetic vector potential for a surface current will be a) J dS b) I dL c) K dS d) J dV
Answer: c Explanation: The magnetic vector potential for the surface integral is given by A = ∫ μKdS/4πR. It is clear that the current element is K dS.
The magnetostatics highly relies on which property? a) Resistance b) Capacitance c) Inductance d) Moment
Answer: c Explanation: The magnetostatics highly relies on the inductance of the magnetic materials, which decides its behavior in the influence of magnetic field.
The Ampere law is based on which theorem? a) Green's theorem b) Gauss divergence theorem c) Stoke's theorem d) Maxwell theorem
Answer: c Explanation: The proof of the Ampere's circuital law is obtained from Stoke's theorem for H and J only.
Identify which of the following is the unit of magnetic flux density? a) Weber b) Weber/m c) Tesla d) Weber^-1
Answer: c Explanation: The unit of magnetic flux density is weber/m2. It is also called as tesla.
Find the magnetic field when the magnetic vector potential is a unit vector. a) 1 b) -1 c) 0 d) 2
Answer: c Explanation: We know that H = -Grad(V), where is a unit vector. The gradient of a constant/unit vector will be zero. Thus the magnetic field intensity will be zero.
Ampere law states that, a) Divergence of H is same as the flux b) Curl of D is same as the current c) Divergence of E is zero d) Curl of H is same as the current density
Answer: d Explanation: Ampere circuital law or Ampere law states that the closed integral of the magnetic field intensity is same as the current enclosed by it. It is given by Curl(H) = J.
The point form of Ampere law is given by a) Curl(B) = I b) Curl(D) = J c) Curl(V) = I d) Curl(H) = J
Answer: d Explanation: Ampere law states that the line integral of H about any closed path is exactly equal to the direct current enclosed by that path. ∫ H.dl = I The point form will be Curl (H) = J.
When the conduction current density and displacement current density are same, the dissipation factor will be a) Zero b) Minimum c) Maximum d) Unity
Answer: d Explanation: Dissipation factor refers to the tangent of loss angle. It is the ratio of conduction current density to displacement current density. When both are same, the loss tangent or the dissipation factor will be unity.
The divergence of which quantity will be zero? a) E b) D c) H d) B
Answer: d Explanation: The divergence of the magnetic flux density is always zero. This is because of the non existence of magnetic monopoles in a magnetic field.
Find the force experienced by an electromagnetic wave in a conductor? a) Electrostatic force b) Magneto static force c) Electro motive force d) Lorentz force
Answer: d Explanation: The electromagnetic wave experiences Lorentz force which is the combination of the electrostatic force and magneto static force. It is given by F = QE + Q(V X B).
2. According to Faraday's law, EMF stands for a) Electromagnetic field b) Electromagnetic force c) Electromagnetic friction d) Electromotive force
Answer: d Explanation: The force in any closed circuit due to the change in the flux linkage of the circuit is called as electromotive force EMF. This phenomenon is called as Faraday's law.
Find the magnetic field intensity due to a solenoid of length 12cm having 30 turns and current of 1.5A. a) 250 b) 325 c) 175 d) 375
Answer: d Explanation: The magnetic field intensity of a solenoid is given by H = NI/L = 30 X 1.5/0.12 = 375 units.
Find the magnetic field intensity due to an infinite sheet of current 5A and charge density of 12j units in the positive y direction and the z component is above the sheet. a) -6 b) 12k c) 60 d) 6
Answer: d Explanation: The magnetic field intensity when the normal component is above the sheet is Hx = 0.5 K, where K = 12. Thus we get H = 0.5 x 12 = 6 units.
6. Find the magnetic field intensity at the centre O of a square of the sides equal to 5m and carrying 10A of current. a) 1.2 b) 1 c) 1.6 d) 1.8
Answer: d Explanation: The magnetic field is given by H = 4I/√2πω. Put I = 10 and ω = 5m. Thus H = 4 x 10/√2π(5) = 1.8 unit.
Which of the following relation will hold good? a) D = μ H b) B = ε E c) E = ε D d) B = μ H
Answer: d Explanation: The magnetic flux density is the product the permeability and the magnetic field intensity. This statement is always true for any material (permeability).
The magnetic vector potential for a line current will be inversely proportional to a) dL b) I c) J d) R
Answer: d Explanation: The magnetic vector potential for the line integral will be A = ∫ μIdL/4πR. It is clear that the potential is inversely proportional to the distance or radius R.
The divergence of H will be a) 1 b) -1 c) ∞ d) 0
Answer: d Explanation: We know that the divergence of B is zero. Also B = μH. Thus divergence of H is also zero.
