Level 1
Electricity MCQs set 2 for Join Pak Navy: Initial Academic Test Everyday Science — 20 solved questions.
Q1. When resistors are connected in series, which quantity remains the same through all resistors?
Answer: Current
Explanation: In a series circuit, all components share the same single current path, so the same current flows through every resistor in the circuit. Voltage, by contrast, is divided among the resistors in proportion to their individual resistances.
Q2. The heating effect of electric current is used in which of the following devices?
Answer: Electric heater
Explanation: Electric heaters exploit the heating effect of current (Joule heating), where electrical energy is converted to thermal energy as current flows through a high-resistance element, described by P = I²R. Motors and generators convert electrical energy to mechanical energy and vice versa, while transformers transfer electrical energy between circuits without intentional heat generation.
Q3. According to Ohm's Law, the relationship between voltage (V), current (I), and resistance (R) is:
Answer: V = IR
Explanation: Ohm's Law states that the potential difference (voltage) across a conductor equals the product of the current and the resistance: V = IR. This relationship defines resistance as the constant of proportionality between voltage and current for ohmic materials at constant temperature.
Q4. In a series circuit, the total resistance is equal to the:
Answer: Sum of all individual resistances
Explanation: In a series circuit, current flows through each resistor in turn with no branching paths, so the total resistance is simply the arithmetic sum of all individual resistances: R_total = R1 + R2 + R3 + ….
Q5. In a parallel circuit, the voltage across each component is:
Answer: The same for all components
Explanation: In a parallel circuit, each component is connected directly across the same two nodes, meaning all components share the same potential difference (voltage) regardless of their individual resistances. This is in contrast to a series circuit, where the total voltage is divided among components proportional to their resistances.
Q6. The formula for electric power in terms of voltage (V) and current (I) is:
Answer: P = VI
Explanation: Electric power is defined as the rate at which electrical energy is transferred, given by P = VI, where V is the voltage (potential difference) and I is the current. This formula can be combined with Ohm's Law to give the equivalent forms P = I²R and P = V²/R.
Q7. For a parallel circuit with resistors R1 and R2, the total resistance is calculated as:
Answer: R = (R1 × R2) / (R1 + R2)
Explanation: For two resistors in parallel, the reciprocal of the total resistance equals the sum of reciprocals: 1/R = 1/R₁ + 1/R₂; rearranging gives R = (R₁ × R₂)/(R₁ + R₂), always less than the smaller of the two resistances.
Q8. In a series circuit, if one component fails or is removed, the effect on the rest of the circuit is:
Answer: The entire circuit stops working
Explanation: In a series circuit, all components share a single continuous path for current; if any component fails or is removed, the circuit is broken and current cannot flow through any part of it. This is a key disadvantage of series circuits compared to parallel circuits.
Q9. Two resistors of 6 ohms and 3 ohms are connected in parallel. Their combined resistance is:
Answer: 2 ohms
Explanation: Using the parallel formula: R = (6 × 3)/(6 + 3) = 18/9 = 2 Ω; resistors in parallel always give a combined resistance lower than the smallest individual resistor.
Q10. The unit of electrical power is:
Answer: Watt
Explanation: The SI unit of electrical power is the watt (W), defined as one joule of energy transferred per second (1 W = 1 J/s). The joule is the unit of energy, the ampere is the unit of current, and the volt is the unit of potential difference.
Q11. The unit of electrical energy in practical applications is:
Answer: Kilowatt-hour
Explanation: In practical electricity billing and engineering, the kilowatt-hour (kWh) is used as the unit of electrical energy, equal to the energy consumed by a 1 kW device operating for one hour (3.6 MJ).
Q12. Ohm's Law states that the current through a conductor is equal to voltage divided by:
Answer: Resistance
Explanation: Ohm's Law states I = V/R: the current (in amperes) equals the voltage (in volts) divided by the resistance (in ohms); this linear relationship holds for ohmic conductors at constant temperature.
Q13. The unit of electric resistance is:
Answer: Ohm
Explanation: The ohm (Ω) is the SI unit of electrical resistance, defined as the resistance that allows one ampere of current to flow when one volt of potential difference is applied (R = V/I). It is named after German physicist Georg Simon Ohm, who formulated Ohm's Law.
Q14. In a series circuit, the total resistance is equal to:
Answer: The sum of individual resistances
Explanation: In a series circuit, resistors are connected end-to-end along a single path, so the total resistance is simply the arithmetic sum of all individual resistances (R_total = R₁ + R₂ + R₃ + …). This is in contrast to a parallel circuit, where the reciprocal of the total resistance equals the sum of the reciprocals.
Q15. A step-up transformer increases:
Answer: Voltage while decreasing current
Explanation: A step-up transformer increases the output voltage above the input voltage by having more turns on the secondary coil than the primary coil. By the principle of conservation of energy, this voltage increase is accompanied by a proportional decrease in current, keeping power approximately constant.
Q16. An electric generator converts:
Answer: Mechanical energy into electrical energy
Explanation: An electric generator uses electromagnetic induction (Faraday's law) to convert mechanical energy - typically provided by a rotating turbine - into electrical energy. This is the reverse process of an electric motor, which converts electrical energy into mechanical energy.
Q17. Which of the following is a semiconductor material?
Answer: Silicon
Explanation: Silicon has four valence electrons and a band gap of about 1.1 eV, giving it conductivity between metals and insulators; doping with phosphorus (n-type) or boron (p-type) makes it the foundation of transistors, solar cells, and integrated circuits.
Q18. Two resistors of 6 Ω and 3 Ω are connected in parallel. The equivalent resistance is:
Answer: 2 Ω
Explanation: For resistors in parallel, the reciprocal of the equivalent resistance equals the sum of the reciprocals: 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2, so R = 2 Ω. In a parallel circuit, the equivalent resistance is always less than the smallest individual resistance.
Q19. An electric motor converts:
Answer: Electrical energy into mechanical energy
Explanation: An electric motor uses the interaction between a magnetic field and current-carrying conductors to convert electrical energy into mechanical (rotational) energy. This is the reverse of a generator, which converts mechanical energy into electrical energy via electromagnetic induction.
Q20. A fuse in an electric circuit is designed to:
Answer: Melt and break the circuit when current exceeds safe limits
Explanation: A fuse is a safety device containing a thin wire that melts and breaks the circuit when the current exceeds a safe threshold, preventing damage to appliances and wiring from overloads or short circuits. Once blown, a fuse must be replaced, unlike a circuit breaker which can be reset.