Question Video: Distinguishing between Direct and Alternating Currents on a Graph of Current against Time Science

The graph shows the electric current over time for two different circuits. Which one of the following sentences is correct? [A] Line 1 represents a direct current and line 2 represents an alternating current. [B] Line 1 represents an alternating current and line 2 represents a direct current. [C] There is not enough information to tell which line represents which type of current.

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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.