Question

A resistance of 40 Ω is connected to a source of alternating current rated 220 V, 50 Hz. Find the time taken by the current to change from its maximum value to the rms value :

• A. 2.5 ms
• B. 1.25 ms
• C. 2.5 s
• D. 0.25 s

Answer 1

Answer: (A)

2.5 ms

Answer 2

I = I 0cos(ω t) say
⇒ At maximum ω t 1 = 0 or t 1 = 0
Then at rms value
l=l0l√2
⇒ ω t 2 = $\frac{\pi}{4}$
⇒ ω(t 2 – _ t_1) = $\frac{\pi}{4}$
Δt=$\frac{\pi}{4}$ω=$\frac{\pi T}{4}$×2π
=$\frac{1}{400}$ s or 2.5 ms
The correct option is (A) : 2.5 ms