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).