UPSC Prelims 2004 Q72: Science & Technology – Physics
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer: (d) A is false but R is true
Explanation
> Note for students: As printed, both A and R are false for India (supply is 50 Hz and the direction reverses twice per cycle), so no option is exactly right. (d) is the only code in which A is false, so it is the nearest choice; you may prefer to skip it to avoid a negative mark.
Alternating current in houses
| Part | Verdict |
|---|---|
| A. AC line changes direction 60 times per second | False: current changes direction twice per cycle, so 100 times per second at 50 Hz (120 at 60 Hz) |
| R. Frequency of supplied voltage is 60 hertz | False for India: the supply frequency is 50 Hz (60 Hz is used in the USA) |
A is false, since a 50 Hz supply changes direction 100 times per second, never 60. R is false as well for Indian houses, which get 50 Hz, so no option is perfectly right. Among the given codes, A is false in every case, so code (d) is the only one consistent with A being false.
Answer: (d) A is false but R is true.
Why each option is right or wrong
- (a) Incorrect. A is false, because AC reverses twice every cycle.
- (b) Incorrect. A is false, so both cannot be true.
- (c) Incorrect. A is false, so this code is wrong.
- (d) CORRECT. A is false as the current reverses twice per cycle; R is also not true for India's 50 Hz supply, so this is the closest code.
Key point
India: 50 Hz, 100 reversals per second; USA: 60 Hz, 120 per second.
Remember
Indian AC supply is 50 Hz and reverses 100 times per second.
Common mistakes
- Equating the reversals per second with the frequency.
- Using the 60 Hz US figure for Indian houses.
- Forgetting that each cycle has two reversals.
Exam tip
Direction changes per second = 2 x frequency.
Sources
- NCERT Class XII Physics — Alternating Current
- Indian Electricity Rules, 1956 (frequency of supply: 50 Hz)