# Is the amplitude of a sine wave increasing or decreasing in this phase?

I'm new to electrical signal, sorry if my question sounds dumb.

Below is a picture from my text book: It says a sine wave with a phase of 0° starts at time 0 with a zero amplitude. The amplitude is increasing.

I am little bit confused here, shouldn't that be decreasing? If you watch this simulation (choose oscillate) below: The single point at time 0 is decreasing.

Is my understanding of how waves propagate incorrect or is the textbook wrong?

• use a pencil and paper to draw the wave, from left to right ... ask yourself the same question as you are drawing ... the drawing action is a simulation of the electrical signal May 8, 2020 at 15:47
• The wording in the book is not great, it can confuse. First it says all three waveforms have the same amplitude, which is right, i.e. A*sin(wt). Then below the picture it says that the amplitude is rising, which obviously means only the instantaneous amplitude of the waveform. Anyway, if it is going to more positive direction when time is increasing, it is rising. May 8, 2020 at 16:21

I think your issue is that you can't correlate the wave animation with the textbook chart one-to-one. The textbook graph shows the amplitude vs. time, so as you go along the x-axis towards the right from origin, the time increases, you are traveling "into the future". The animation is the opposite way of looking at the wave - the point of where the wave is generated (attached to the rod) is "the present", and the wave propagating away is in "the past". When the animation generates a sine wave (stating at zero), the amplitude will be increasing. • Thanks for your answer. So when you mention "the textbook graph shows the amplitude vs. time", do you mean the it is the amplitude of the single point(the green ball)? May 9, 2020 at 1:11
• That's exactly right.
– DSI
May 9, 2020 at 2:13

Your first set of graphs do not represent propagation of a wave. They just show how the amplitude of a signal varies with time.

If you plot the amplitude versus time of the leftmost green dot on your wave-on-a-string you will get the same sinusoidal graph as the first of your sines. That animation is showing what happens to other points as a result of what happens to the first.

Looking at the topmost sine function vs time, if you start at zero the function goes up vs time until it reaches a maximum. That means it is increasing initially.