Video Transcript
The graph below shows the electric
current over time for two different circuits. Which one of the following
sentences is correct? Is it (A) line one represents a
direct current and line two represents an alternating current? (B) Line one represents an
alternating current and line two represents a direct current. Or (C) there is not enough
information to tell which line represents which type of current.
The question asks us to look at two
lines on a current–time graph. Each line represents a current in a
time interval for a different circuit, circuit one and circuit two. We must identify whether each line
represents a direct current or an alternating current. We recall that a direct current has
a constant value and direction, whereas an alternating current has a periodically
changing direction. The horizontal axis of the
current–time graph shows the values of time, while the vertical axis shows values of
current.
It is important to notice that the
vertical axis of the graph extends above and below the horizontal axis. Values of current above the
horizontal axis are positive, and values below the horizontal axis are negative
values. The sign of a value of current
corresponds to the direction of the current. A positive-valued current and a
negative-valued current are in opposite directions. We can see that line one is
curved. More specifically, the slope of
line one is positive in some time intervals and negative in other time
intervals. The positive slope and negative
slope time intervals alternate.
We can also see that in some time
intervals the values of current for line one are positive, and in other time
intervals the values of current for line one are negative. The positive current value and
negative current value time intervals alternate. Unsurprisingly then, line one
represents an alternating current. We can see that line two is
horizontal. The values of the current is
constant and positive. This represents a direct
current.
The correct answer is option
(B). Line one represents an alternating
current and line two represents a direct current.
