This set of Electromagnetic Theory Multiple Choice Questions & Answers (MCQs) focuses on “Magnetic Energy and Circuits”.
1. The magnetic energy of a magnetic material is given by
Explanation: The magnetic energy of a material is given by half of the product of the magnetic flux density and the magnetic field intensity. It is represented as BH/2. Since B = μH, we can also write as μH2 or B2/2μ.
2. The induced emf in a material opposes the flux producing it. This is
a) Faraday law
b) Ampere law
c) Lenz law
d) Curie law
Explanation: The induced emf in a material under the influence of a magnetic field will oppose the flux that produces it. This is indicated by a negative sign in the emf equation. This phenomenon is called Lenz law.
3. The energy in a magnetic material is due to which process?
Explanation: The energy in a magnetic material is due to the formation of magnetic dipoles which are held together due to magnetic force. This gives energy to the material. Hence it is due to magnetization process.
4. The flux lines of two energised coils overlapping on each other will give
a) Series aiding
b) Shunt aiding
c) Series opposing
d) Shunt opposing
Explanation: Flux lines are the magnetic lines of force of a magnetic material. Since the flux is overlapping, the total flux of the two coils together will be high. Thus it is an aiding flux. Also this type of overlapping is possible only when the two coils are back to back or in series connection.
5. The resistance in a magnetic material is called as
d) Magnetic resistance
Explanation: The reluctance of a magnetic material is the ability of the material to oppose the magnetic flux. It is the ratio of the magnetic motive force mmf to the flux.
6. Calculate the reluctance of the material with a mmf of 3.5 units and flux of 7units.
Explanation: The reluctance is defined as the ratio of the mmf and the flux. It is given by S = mmf/φ. On substituting mmf = 3.5 and φ = 7, we get S = 3.5/7 = 0.5 units.
7. Which of the following relations is correct?
a) NI = Sφ
b) NS = Iφ
c) Nφ = SI
d) NI = S/φ
Explanation: The reluctance is also defined by the ratio of the current element to the flux. In other words, mmf = NI. Thus S = NI/φ. We get the relation NI = Sφ.
8. Calculate the reluctance of a material with length 2π x 10-4 in air with area 0.5.
Explanation: The reluctance is given by S = L/μ A, where L is the length, A is the area and μ is the permeability. On substituting L = 2π x 10-4, A = 0.5 and μ = 4π x 10-7, we get S = 103/(2×0.5) = 1000 units.
9. Ampere turn is equivalent to which element?
Explanation: Ampere turn refers to the current element, which is the product of the turns and the current. It is given by NI. From the definition of reluctance, S = NI/φ. Thus NI = Sφ is the best equivalent.
10. The line integral of the magnetic field intensity is given by
b) Flux density
d) Current element
Explanation: The line integral of H is given by ∫H. dl. From Ampere law it can be related to the current density and hence the current element NI for a coil of N turns. Thus, ∫H. dl = NI.
Sanfoundry Global Education & Learning Series – Electromagnetic Theory.
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