Power in AC circuits and basic transformers - One Line Questions

1. Consider an AC circuit with voltage V(t) = Vm sin(ωt) and current I(t) = Im sin(ωt - φ). The instantaneous power is P(t) = V(t)I(t). The average power over a cycle is: — (Vm * Im / 2) * cos(φ)
2. In a purely resistive AC circuit, the voltage and current are in phase. In this case, what is the power factor? — 1
3. If the secondary current in an ideal transformer is 2 A and the primary current is 0.5 A, what is the ratio of turns Ns/Np? — 4
4. An AC voltage source of 200 V is connected to the primary coil of a transformer. The secondary coil has 100 turns, and the primary coil has 500 turns. What is the secondary voltage? — 40 V
5. An AC circuit has a voltage of 230 V and a current of 5 A. If the power factor is 0.8, what is the true power consumed? — 920 W
6. If a transformer has Np = 200 turns and Ns = 1000 turns, and the primary voltage is 12 V, what is the secondary voltage? — 60 V
7. If the apparent power in an AC circuit is 1000 VA and the true power is 800 W, what is the reactive power? — 600 VAR
8. A transformer is used to step down the voltage from 220 V to 110 V. If the primary coil has 1000 turns, how many turns does the secondary coil have? — 500 turns
9. If the input power to a transformer is 1000 W and the output power is 950 W, what is the power loss? — 50 W
10. In an AC circuit, the instantaneous power is given by the product of instantaneous voltage and instantaneous current. What is the term for the average value of this instantaneous power over a complete cycle? — True Power (or Real Power)
11. In an AC circuit, the sum of true power and reactive power is equal to: — Apparent power
12. The rating of a transformer is typically given in kVA. Why is it not given in kW? — Because the power factor of the load is not known at the time of manufacturing
13. Which of the following losses in a transformer is independent of the load? — Iron loss
14. Which type of loss in a transformer is primarily responsible for the heat generated in the windings due to the resistance of the wire? — Copper loss (I^2R loss)
15. Reactive power (Q) in an AC circuit is associated with: — Energy stored and returned by reactive components (inductors and capacitors)
16. An autotransformer differs from a conventional transformer in that it: — Has a single winding common to both primary and secondary circuits
17. What is the main advantage of an autotransformer over a two-winding transformer for certain applications? — Reduced size and cost for small voltage changes
18. For maximum efficiency in a transformer, the copper losses should be equal to: — Iron losses (or core losses)
19. In a real transformer, the ratio of secondary voltage to primary voltage (Vs/Vp) is slightly different from the turns ratio (Ns/Np) due to: — Leakage flux and winding resistance
20. A transformer is used to convert high voltage AC to low voltage AC. This is essential for: — Making electricity safe for domestic use
21. If a transformer is overloaded, which type of loss is most likely to increase significantly and cause overheating? — Copper loss
22. In an ideal step-down transformer, the voltage is decreased. What happens to the current? — It increases proportionally
23. An inductive load in an AC circuit causes the current to: — Lag the voltage
24. In an AC circuit, if the voltage leads the current, the power factor is: — Leading
25. In a purely capacitive AC circuit, the current: — Leads the voltage by 90 degrees
26. The primary winding of a transformer is connected to an AC source. If the secondary is open-circuited, the current flowing in the primary is called: — Magnetizing current
27. The power dissipated in a purely inductive or purely capacitive AC circuit is: — Zero
28. A transformer is a device that transfers electrical energy from one AC circuit to another through electromagnetic induction. What is the primary principle behind its operation? — Faraday's Law of Electromagnetic Induction
29. The efficiency (η) of a transformer is defined as: — Output power / (Output power + Losses)
30. What is the formula for true power (P) in an AC circuit, where Vrms is the RMS voltage, Irms is the RMS current, and cos(phi) is the power factor? — P = Vrms * Irms * cos(phi)
31. In a real transformer, the efficiency is always less than 100%. This is due to: — Various types of energy losses (copper, iron, etc.)
32. In an ideal transformer, the ratio of the number of turns in the secondary coil (Ns) to the number of turns in the primary coil (Np) is equal to the ratio of: — Secondary voltage to primary voltage
33. Hysteresis loss in a transformer core occurs due to: — The repeated magnetization and demagnetization of the core material
34. The term 'power factor' represents the ratio of: — True power to apparent power
35. Which component in an AC circuit is responsible for storing energy in an electric field? — Capacitor
36. Which component in an AC circuit is responsible for storing energy in a magnetic field? — Inductor
37. The relationship between apparent power (S), true power (P), and reactive power (Q) in an AC circuit is given by: — S^2 = P^2 + Q^2
38. For an ideal transformer, if the number of turns in the secondary coil is greater than the number of turns in the primary coil (Ns > Np), it is a(n): — Step-up transformer
39. In a city's power distribution system, voltage is stepped up to very high values for transmission over long distances. What type of transformer is used for this purpose? — Step-up transformer
40. The voltage transformation ratio (or turns ratio) of a transformer is defined as k = Ns / Np. If k = 0.5, the transformer is a: — Step-down transformer
41. At the receiving end of the power grid, the high transmission voltage is reduced to a safer level for household use. What type of transformer is used here? — Step-down transformer
42. A transformer with Ns = Np is an: — Isolation transformer
43. A transformer's ability to transfer power from primary to secondary is dependent on: — All of the above
44. What is the purpose of an isolation transformer? — To provide electrical isolation between the primary and secondary circuits without changing the voltage
45. The core of a transformer is usually made of laminated soft iron. Why is it laminated? — To reduce eddy currents
46. What is the main purpose of using a soft iron core in a transformer? — To provide a path of low reluctance for magnetic flux
47. A transformer is rated at 10 kVA. This rating refers to the: — Apparent power output
48. In an ideal transformer, the power in the primary coil is equal to the power in the secondary coil. This implies that: — Vp * Ip = Vs * Is
49. The unit of apparent power in an AC circuit is: — Volt-Ampere (VA)
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