Instantaneous voltage and current

You’ve seen “stop motion” if you’ve done much work with a video-cassette recorder (VCR). In fact, you’ve probably seen it if you’ve watched any television sportscasts. Suppose that it were possible for you to stop time in real life, any time you wanted. Then you could examine any instant of time in any amount of detail that would satisfy your imagination.
Recall that an ac sine wave has a unique, characteristic shape, as shown in Fig. 1. This is the way the graph of the function y
sin x appears on the coordinate plane. (The abbreviation sin stands for sine in trigonometry.) Suppose that the peak voltage is plus or minus 1 V, as shown. Further imagine that the period is 1 second, so that the frequency is 1 Hz. Let the wave begin at time t = 0. Then each cycle begins every time the value of t is a whole number; at every such instant, the voltage is zero and positive going.
If you freeze time at t = 446.00 seconds, say, the voltage will be zero. Looking at the diagram, you can see that the voltage will also be zero every so-many-and-a-half seconds; that is, it will be zero at t = 446.5 seconds. But instead of getting more positive at these instants, the voltage will be swinging towards the negative.

Fig. 1: A sine wave with period 1 second and frequency 1 Hz.

If you freeze time at so-many-and-a-quarter seconds, say t = 446.25 seconds, the voltage will be + 1 V. The wave will be exactly at its positive peak. If you stop time at so-many-and-three-quarter seconds, say t = 446.75 seconds, the voltage will be exactly at its negative peak,  1 V.
At intermediate times, say, so-many-and-three-tenths seconds, the voltage will have intermediate values.