Physics · Alternating Current · NEET
Yes. For AC, the peak value i0 (or V0) is exactly the amplitude of the sine wave. In i = i0 sin(wt), the number i0 sitting in front of the sine is the amplitude, which equals the maximum value the current reaches. NEET may print any of these three words peak value, amplitude, maximum value and they all mean the same i0.
wt is the phase, an angle in radians that grows with time. Here w = 2(pi)f is the angular frequency and t is the time. As t increases, the angle wt sweeps from 0 to 2(pi) over one full cycle, so sin(wt) smoothly rises and falls between +1 and -1. That is why the current keeps changing every instant.
The instantaneous value is the value at a single chosen moment, i = i0 sin(wt), so it keeps changing and can be anything from -i0 to +i0, including zero. The peak value i0 is one fixed number, the highest instantaneous value in the whole cycle. In short: instantaneous = a moving snapshot, peak = the fixed maximum of all those snapshots.
No. The peak value i0 is the top of the wave. The rms value is a kind of effective average and is smaller: i_rms = i0 / sqrt(2) = 0.707 x i0. So a device rated 220 V (rms) has a peak voltage of V0 = 220 x sqrt(2) = about 311 V. Never mix i0 and i_rms in formulas.
Because sin(wt) itself swings between 0 and 1 (and down to -1). When wt = 0, sin = 0, so i = 0. When wt = 90 degrees (pi/2), sin = 1, so i = i0, the peak. This repeating rise and fall is the whole nature of alternating current, unlike steady DC which stays fixed.
The peak value of an alternating current is 5 A and frequency is 60 Hz. How long will the current, starting from zero, take to reach the peak value?
The peak voltage of the ac source is equal to:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
For alternating current, i = i0 sin(wt), and for voltage, v = v0 sin(wt). Here i0 and v0 are the peak values and w = 2(pi)f is the angular frequency. This gives the value at any instant t.
When sin(wt) = 1, that is when wt = 90 degrees or pi/2 radians. At that instant the current equals i0, its peak. Starting from zero, this happens after a time equal to one quarter of the period, T/4.
No. The peak value i0 is fixed by the source; changing the frequency f only changes how fast the wave oscillates (through w = 2(pi)f), not how tall it is. Frequency changes the time to reach the peak (T/4), not the peak value itself.
It is the starting point of the whole AC chapter. Every result about phase, reactance, and power comes from writing v and i as instantaneous sine functions. NEET questions often give v = V0 sin(wt) and expect you to read off V0 (peak) and w (from which you get f and time to peak).
