Video Transcript
Complete the following
sentence. In an electric circuit,
conventional current goes from the blank terminal of a cell to the blank
terminal. (A) Negative, positive; (B)
positive, negative.
This question is asking us in which
direction is the current in a circuit, the current from the negative terminal of the
cell to the positive or vice versa? Why does it matter? Recall that a current is the flow
of charged particles. We know that in a circuit electrons
are the charged particles that move. You might also remember that when a
cell is connected to a circuit, the electron current goes from the negative terminal
of the cell to the positive terminal of the cell.
However, we have to be very careful
here. The question isn’t asking us about
electron current. The question is asking about
conventional current. These are not the same thing. When scientists first discovered
electric current, they did not know that current consisted of moving electrons. They knew that some kind of charged
particle, called a charge carrier, was moving around the circuit, but they weren’t
sure exactly what kind of particle it was or whether it was positively or negatively
charged. In fact, they actually thought that
the positive charges were the ones moving. This is how the term current was
originally defined: the flow of positive charges around a circuit.
Today, we know that this is not
true, but this convention is still often used. So, we call this hypothetical
positive-charge-carrying current a conventional current. To figure out the direction of
conventional current, we must remember what we know about charges attracting and
repelling. Recall that unlike charges
attract. This means a positive charge and a
negative charge will be pulled towards each other. And like charges repel. If you had two positive charges
next to each other, they would push away from each other. Therefore, positive charges would
be attracted to the negative terminal of a cell and repelled by the positive
terminal of the cell.
So, the conventional current must
move away from the positive terminal of the cell and towards the negative
terminal. The correct answer is therefore
(B).