Junior Engineer (Electrical/Electronics) MCQs – Series 10 (Q111-120)
Q111. According to Faraday’s law of electromagnetic induction, the induced EMF in a coil is proportional to:
- A) The rate of change of magnetic flux linkage
- B) The resistance of the coil
- C) The cross-sectional area of the coil only
- D) The magnitude of the magnetic flux itself
Answer: A
Faraday’s law states e = -N(dΦ/dt); induced EMF depends on how fast the flux linkage changes, not its absolute value.
Q112. Lenz’s law states that the direction of an induced current always opposes:
- A) The change in magnetic flux that produced it
- B) The applied source voltage
- C) The resistance of the circuit
- D) The direction of the existing current
Answer: A
Lenz’s law is a consequence of energy conservation: the induced current creates a flux that opposes the change causing it.
Q113. The SI unit of magnetic flux density is the:
- A) Ampere-turn
- B) Weber
- C) Henry
- D) Tesla
Answer: D
Magnetic flux density (B) is measured in Tesla (T), while magnetic flux (Φ) itself is measured in Weber (Wb).
Q114. According to Ohm’s law, the current through a conductor is:
- A) Directly proportional to resistance
- B) Directly proportional to voltage and inversely proportional to resistance
- C) Independent of the resistance
- D) Inversely proportional to voltage
Answer: B
Ohm’s law: I = V/R, so current increases with voltage and decreases as resistance increases, at constant temperature.
Q115. The purpose of an earthing (grounding) system in an electrical installation is mainly to:
- A) Reduce the supply voltage
- B) Improve the system’s power factor
- C) Provide a safe low-resistance path for fault current and protect against electric shock
- D) Increase the current-carrying capacity of conductors
Answer: C
Earthing ensures fault currents are safely diverted to ground, operating protective devices quickly and minimizing shock hazard to personnel.
Q116. A circuit breaker is selected with a breaking capacity higher than the calculated fault level mainly to ensure:
- A) Lower purchase cost
- B) A higher voltage rating than required
- C) Safe interruption of the maximum possible fault current
- D) Reduced tripping time only
Answer: C
The breaking capacity must exceed the prospective fault current so the breaker can safely interrupt the circuit under worst-case fault conditions.
Q117. The force experienced by a current-carrying conductor placed in a magnetic field is given by:
- A) F = I²R
- B) F = P/V
- C) F = BIL sinθ
- D) F = V/I
Answer: C
The force on a current-carrying conductor in a magnetic field follows F = BIL sinθ, where θ is the angle between the conductor and the field.
Q118. The permeability of free space (μ0) has an approximate value of:
- A) 1.6 × 10⁻¹⁹ C
- B) 4π × 10⁻⁷ H/m
- C) 9 × 10⁹ N·m²/C²
- D) 8.85 × 10⁻¹² F/m
Answer: B
μ0 = 4π × 10⁻⁷ H/m is a fundamental constant used in magnetic field calculations; the other values correspond to permittivity, electron charge, and Coulomb’s constant respectively.
Q119. In standard electrical safety practice, the color code commonly used for the earth (protective) conductor is:
- A) Blue
- B) Red
- C) Green/Yellow
- D) Black
Answer: C
The dual green-and-yellow color coding is the internationally recognized standard for identifying protective earth conductors.
Q120. The standard test used to verify the insulation strength of electrical equipment against high voltage stress is called the:
- A) Polarity test
- B) Load test
- C) High voltage (dielectric) withstand test
- D) Continuity test
Answer: C
The dielectric/high-voltage withstand test applies a specified overvoltage to confirm the insulation can safely withstand stress without breakdown.
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