Question Video: Describing Electric Using a Current-Time Graph Science

The graph below shows the electric current over time for two different circuits. Which one of the following sentences is correct? [A] Line 1 represents an alternating current and line 2 represents a direct current. [B] Line 1 represents a direct current and line 2 represents an alternating 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? (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.

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