Video Transcript
The picture shows the electrons and
atomic nuclei in a section of copper wire. The blue circles represent
electrons, and the red circles represent atomic nuclei. An electric potential difference is
set up between the left-hand end and the right-hand end of the wire. This creates an electric current in
it. Which of the arrows best shows the
direction of the current in the wire? (A) i, (B) ii, (C) iii, (D) iv.
First, let’s recall that an
electric potential difference can exist between two points. Next, let’s recall that an electric
potential difference between two points causes electric charges to flow from one of
the points to the other. When electric charges flow together
in the same direction, this is called an electric current. Electrons are electrically charged,
and electrons can flow freely between atoms in an electrically conductive substance
such as the copper that the wire in this question is made from.
We can say then that an electric
potential difference across the length of a copper wire results in an electric
current in the wire. The electrons that make up the
current move between the sides of the wire which there is an electric potential
difference across. The electrons all flow in the same
direction. They do not change direction unless
the direction of the electric potential difference across the wire is changed. Notice by the way that we’ve drawn
the electrons moving in the opposite direction to the direction of the current, as
conventional current is in the direction of positive charge flow, while electrons
are negatively charged.
The question states that there is
an electric potential difference between the left-hand side and the right-hand side
of the wire. This means that the direction of
the current must either be from left to right or from right to left. There is no way to tell from the
question whether the direction is from left to right or from right to left. Whether the direction is from left
to right or right to left, we can call the direction a horizontal direction, along
this left-right axis. If we look at the options shown, we
see that option (C), direction iii, shows the direction from left to right, which is
a horizontal direction. The other options show directions
that are either completely or partially vertical.
The correct answer is therefore
option (C) iii.
