The graph below shows the electric
current over time for two different circuits. Which one of the following
sentences is correct? (A) Line one represents an
alternating current and line two represents a direct current. (B) Line one represents a direct
current and line two represents an alternating current. (C) There is not enough information
to tell which line represents which type of current.
Looking at the graph given, we see
line one is the line in red and line two is that in blue. Focusing first on line one, we see
that this line has both positive values of current as well as negative values. This tells us that the current
represented by line one changes direction. Along with this, if we start from a
time of zero seconds, we can see that the amount of time for which the current is in
the positive direction — that’s this time interval here — is equal to the amount of
time the current then switches to point in the negative direction. This pattern then repeats for the
rest of the changes in direction of the current represented by line one. We can say then that this current
reverses direction periodically. That tells us that line one
represents alternating current.
Next, let’s consider line two, the
blue line on our graph. If we were to trace over this line,
we would see that it always has the same positive value of current. This means the current always
points in the same direction, and it always has the same magnitude. That tells us the current
represented by line two is direct current. Answer option (A) then is the one
we’ll choose. Line one on this graph represents
an alternating current and line two represents a direct current.