Question Video: Describing Direct Current | Nagwa Question Video: Describing Direct Current | Nagwa

Question Video: Describing Direct Current Science

Which of the following sentences correctly describes direct current? [A] Direct current has a variable magnitude and always has the same direction. [B] Direct current has a variable magnitude and its direction can reverse. [C] Direct current has a constant magnitude and its direction can reverse. [D] Direct current has a constant magnitude and always has the same direction.

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Video Transcript

Which of the following sentences correctly describes direct current? (A) Direct current has a variable magnitude and always has the same direction. (B) Direct current has a variable magnitude and its direction can reverse. (C) Direct current has a constant magnitude and its direction can reverse. (D) Direct current has a constant magnitude and always has the same direction.

Clearing some space at the top of our screen, say that we have a graph of current, that’s 𝐼, plotted versus time. We’ve set up our graph so that current can have positive as well as negative values. This just means that the current can change direction. In other words, charge would flow in different directions in a circuit. If we were to plot a current described by answer option (A) on our graph, that current might look like this. Say it starts out positive and then changes in value, that is, changes in magnitude. But since the current is always the same sign, positive, it always points in the same direction.

Next, let’s plot the current described in option (B). Current with a variable magnitude whose direction can reverse might look like this. Sometimes this current is positive, and sometimes it’s negative. That shows that it changes direction. We also see that the magnitude or size of this current changes over time; it’s variable.

If we were to make a curve described by answer option (C), that could look like this. The magnitude or size of this current is always the same, it’s constant, but its direction does reverse. Finally, let’s make a curve for answer option (D). A current with constant magnitude always pointing in the same direction could look like this. It appears as a horizontal line, and this is a sign that this current is direct current.

The two conditions for a current to be direct current are that its magnitude must be constant and it always has the same direction. We choose answer option (D).

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