Level 1
Electricity MCQs set 3 for Pakistan Coast Guard / Maritime Security Everyday Science — 20 solved questions.
Q1. Which type of current flows in one direction only?
Answer: DC
Explanation: Direct current (DC) flows in one direction only, from the negative terminal to the positive terminal, maintaining a constant polarity. Alternating current (AC) periodically reverses direction at a fixed frequency (typically 50 or 60 Hz), making it suitable for efficient long-distance transmission.
Q2. Power dissipated in a resistor can be expressed as P = I²R. If the current doubles while resistance stays constant, the power:
Answer: Quadruples
Explanation: Since P = I²R, power is proportional to the square of the current; if the current doubles (2I), the new power is (2I)²R = 4I²R, which is four times the original power. This quadratic relationship means small increases in current produce large increases in heat dissipation.
Q3. Nine volt lantern draws zero point five amp steady approximate resistance?
Answer: Eighteen ohms about
Explanation: Ohm's law states that resistance equals voltage divided by current (R = V/I); for a 9 V source drawing 0.5 A, R = 9 ÷ 0.5 = 18 ohms. The other values result from arithmetic errors in applying the formula.
Q4. Two alike resistors in series across an ideal fixed supply divide voltage equally implying each midpoint node compared with rail reference behaves around?
Answer: Half supply magnitude each if identical halves share evenly simplistic midpoint story
Explanation: When two identical resistors are connected in series across a fixed voltage supply, the voltage divides equally across each resistor, so the midpoint node sits at exactly half the supply voltage. This equal voltage division occurs because both resistors carry the same current and have the same resistance, producing equal voltage drops.
Q5. Same supply voltage unchanged yet resistance doubles yielding steady current becomes?
Answer: Roughly halves following simple linear Ohm proportional story
Explanation: By Ohm's law (I = V/R), if voltage is constant and resistance doubles, the current is halved. The current and resistance are inversely proportional at constant voltage.
Q6. A 12 V battery drives 2 A through a resistor. What power is dissipated?
Answer: 24 W
Explanation: Electrical power is calculated as P = V × I, where V is voltage and I is current. With a 12 V battery driving 2 A, the power dissipated is 12 × 2 = 24 W.
Q7. Two 6 Ω resistors are connected in series. What is the total resistance?
Answer: 12 Ω
Explanation: For resistors in series the total resistance is the sum of individual resistances: R_total = R₁ + R₂ = 6 + 6 = 12 Ω. In a series circuit the same current flows through all components.
Q8. If voltage across a resistor is fixed and resistance is tripled, what happens to current?
Answer: It becomes one third as large
Explanation: Ohm's Law states I = V/R; if voltage is fixed and resistance is tripled (3R), the current becomes I' = V/(3R) = one third of the original current. Resistance and current are inversely proportional when voltage is held constant.
Q9. Household wiring in Pakistan typically uses alternating current near which standard frequency?
Answer: 50 Hz
Explanation: Pakistan, like most of South Asia, Europe, and Africa, uses a 50 Hz alternating current standard for its national grid. The United States and some other countries use 60 Hz, but 50 Hz is the internationally predominant standard.
Q10. A fuse in a circuit mainly protects wiring by doing what during a large overcurrent?
Answer: Melting or opening to stop the current path
Explanation: A fuse contains a thin wire that melts when the current exceeds a safe threshold, opening the circuit and stopping current flow before the wiring can overheat and cause a fire. Once blown, a fuse must be replaced; circuit breakers serve the same protective function but can be reset.
Q11. Two 8 Ω resistors are connected in parallel. What is the equivalent resistance?
Answer: 4 Ω
Explanation: For two identical resistors in parallel, the equivalent resistance is R/2 = 8/2 = 4 Ω. The general formula for parallel resistance is 1/R_eq = 1/R₁ + 1/R₂, which for equal resistors simplifies to R_eq = R/n where n is the number of identical resistors.
Q12. A lamp uses 60 W at 120 V steady. What current does it draw?
Answer: 0.50 A
Explanation: Current is calculated using I = P/V, where P is power in watts and V is voltage. For a 60 W lamp at 120 V, the current is 60 ÷ 120 = 0.50 A.
Q13. You connect a heater for 2 h at 1.5 kW. How much energy did it use?
Answer: 3 kWh
Explanation: Energy in kilowatt-hours equals power in kilowatts multiplied by time in hours: E = P × t = 1.5 kW × 2 h = 3 kWh. Kilowatt-hours are the standard unit used by electricity suppliers for billing purposes.
Q14. Grounding household appliances mainly reduces which risk when insulation fails?
Answer: Risk of dangerous shock if live parts touch the case
Explanation: Grounding (earthing) provides a low-resistance path to the earth so that if a live wire contacts the metal casing of an appliance, fault current flows to earth rather than through a person, triggering the fuse or circuit breaker. Without grounding, touching the casing could deliver a fatal shock.
Q15. In a simple battery-bulb circuit one wire breaks. Why does the bulb go out?
Answer: Current path is interrupted so charge flow stops
Explanation: In a simple series circuit, all components share a single unbroken conducting path; if that path is broken at any point - such as by a broken wire - the circuit is open and current ceases to flow everywhere, extinguishing the bulb. This is the fundamental limitation of series wiring.
Q16. Copper is a good electrical conductor mainly because it has what feature?
Answer: Many mobile charge carriers that can move under a field
Explanation: Copper has a single loosely bound valence electron per atom that is free to move through the metal lattice as a conduction electron when an electric field is applied, giving copper its high electrical conductivity.
Q17. Rubber gloves help protect a worker from shock mainly because rubber is typically a good what?
Answer: Electrical insulator
Explanation: Rubber is an excellent electrical insulator because its electrons are tightly bound and not free to move, preventing the flow of electric current through the material. Workers handling live electrical equipment wear rubber gloves to prevent current from passing through their bodies to earth.
Q18. A transformer needs a changing magnetic flux in its core. Why is AC suited for stepping voltage up or down?
Answer: Because AC voltage changes with time producing changing flux
Explanation: A transformer operates on the principle of electromagnetic induction, which requires a changing magnetic flux through the core to induce a voltage in the secondary coil. Alternating current continuously reverses direction, creating the continuously changing flux needed, whereas direct current produces a static field that cannot induce a sustained secondary voltage.
Q19. A 5 Ω and a 10 Ω resistor are in series across an ideal 30 V battery. What is the current?
Answer: 2 A
Explanation: In a series circuit, total resistance = 5 + 10 = 15 Ω, and by Ohm's law I = V/R = 30/15 = 2 A. The same 2 A flows through both resistors since they are in series.
Q20. Why is it bad practice to connect many high-power appliances to one thin extension cord?
Answer: Total current may exceed safe capacity and overheat the cord
Explanation: Each high-power appliance draws significant current, and connecting many in parallel means the extension cord carries the sum of all their currents; if this total exceeds the cord's rated capacity, resistive heating in the thin wires can cause insulation to melt or ignite. This is the leading cause of electrical fires from overloaded extension cords.