# Basic Electrical Engineering Questions and Answers – Ohm’s Law for a Magnetic Circuit

This set of Basic Electrical Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Ohm’s Law for a Magnetic Circuit”.

1. Ohm’s law for magnetic circuits is _________
a) F=ϕS
b) F=ϕ/S
c) F=ϕ2S
d) F=ϕ/S2

Explanation: Ohm’s law for magnetic circuits states that the MMF is directly proportional to the magnetic flux where reluctance is the constant of proportionality.

2. What happens to the MMF when the magnetic flux decreases?
a) Increases
b) Decreases
c) Remains constant
d) Becomes zero

Explanation: Ohm’s law for the magnetic circuit’s states that the MMF is directly proportional to the magnetic flux hence as the magnetic flux decreases, the MMF also decreases.

3. Calculate the MMF when the magnetic flux is 5Wb and the reluctance is 3A/Wb.
a) 10At
b) 10N
c) 15N
d) 15At

Explanation: We know that:
F=ϕS
Substituting the given values from the question, we get MMF = 15At.

4. A ring having a cross-sectional area of 500 mm2, a circumference of 400 mm and ϕ=800microWb has a coil of 200 turns wound around it. Calculate the flux density of the ring.
a) 1.6T
b) 2.6T
c) 3.6T
d) 4.6T

Explanation: ϕ=BA => Flux density B = ϕ/A
Substituting the values, we get B=1.6T.

5. A ring having a cross-sectional area of 500 mm2, a circumference of 400 mm and ϕ=800microWb has a coil of 200 turns wound around it. Relative permeability of ring is 380. Calculate the reluctance.
a) 1.68 * 10-4A/Wb
b) 1.68 * 104 A/Wb
c) 1.68 * 106 A/Wb
d) 1.68 * 10-6A/Wb

Explanation: Reluctance=l/(µ*A) = l/(µrµ0*A)
Substituting the values, we get Reluctance=1.68*106 A/Wb.

6. A ring having a cross-sectional area of 500 mm2, a circumference of 400 mm and ϕ=800microWb has a coil of 200 turns wound around it. Relative permeability of ring is 380. Calculate the magnetomotive force.
a) 1442At
b) 1342At
c) 1432At
d) 1344At

Explanation: Reluctance=l/(µ*A) = l/(µrµ0*A)
Substituting the values, we get Reluctance S=1.68*106 A/Wb.
F=ϕS
Substituting the given values, we get F = 1344At.

7. A ring having a cross-sectional area of 500 mm2, a circumference of 400 mm and ϕ=800microWb has a coil of 200 turns wound around it. Relative permeability of ring is 380. Calculate the magnetising current.
a) 6.7A
b) 7.7A
c) 7.6
d) 6.1A

Explanation: Reluctance = l/(µ*A) = l/(µrµ0*A)
Substituting the values, we get Reluctance S=1.68*106 A/Wb.
F=ϕS Substituting the given values, we get F = 1344At.
I=F/N Substituting the values from the question, we get I=6.7A.

8. Can we apply Kirchhoff’s law to magnetic circuits?
a) Yes
b) No
c) Depends on the circuit
d) Insufficient information provided

Explanation: Magnetic circuits have an equivalent to the potential difference of electric circuits. This is the magnetic potential difference which allows us to apply Kirchhoff’s laws to magnetic circuit analysis.

9. What is MMF?
a) Magnetic Machine Force
b) Magnetomotive Force
c) Magnetic Motion Force
d) Magnetomotion Force

Explanation: MMF stands for magnetomotive force. Actually, it is not a force. It is analogous to potential in electric field.

10. The equivalent of the current I in magnetic ohm’s law is?
a) Flux
b) Reluctance
c) MMF
d) Resistance

Explanation: The equivalent of current in magnetic ohm’s law is flux as:
V=IR is equivalent to F=ϕS.

Sanfoundry Global Education & Learning Series – Basic Electrical Engineering.

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