The graph below shows the output potential difference of a generator over time. What is the period of the output potential difference?
The graph mentioned is this one here, with time measured in milliseconds on the horizontal axis and output potential difference measured in volts on the vertical axis. What we want to determine from this graph is the period of the output potential difference.
We can recall that the period is the time that it takes the value of a quantity to complete one cycle. Looking at the graph, if we say that the cycle starts at time zero, then one complete cycle will be after the output potential difference has reached its maximum value, its minimum value, and then returned to zero. Since we began our cycle at time zero, the period is just whatever time is represented by this point on the horizontal axis. But to know what that point is, we need to know the horizontal axis scale.
What we do know is that the vertical line all the way to the left represents zero milliseconds and the vertical line in the middle represents 100 milliseconds. In between these two lines, there are an additional nine lines that are evenly spaced, marking off values between zero and 100. So there are 10 evenly spaced steps to get to 100 milliseconds, and each of those steps is then one-tenth of 100 milliseconds, which is 10 milliseconds. So the horizontal axis scale is 10 milliseconds per line.
Going back to our marked cycle, we see that the cycle starts at zero and ends at the second line to the right. Recalling our horizontal axis scale, two spaces is an interval of 20 milliseconds. Since the time it takes for the output potential difference to complete one cycle is 20 milliseconds, the period of the output potential difference is 20 milliseconds.