Video Transcript
The picture shows a positive charge
and a negative charge that are near each other. The charges are fixed in place and
cannot move. Complete the following
sentence. The separation of the positive and
negative charges creates an (A) electric current or (B) electric potential
difference between them.
In this question, we are asked to
decide when there are two charges, one positive and the other negative, fixed at a
small distance apart, whether there is an electric current or an electric potential
difference between them. We know that two opposite charges
produce forces that attract each other. The force that acts on either
charge is towards the other charge. Recall that an electric current is
defined as the motion of electric charge. Usually, for example, in a wire,
all of the electric charges have the same sign and move in the same direction.
The two charges we are considering
have opposite signs. One charge is positive, and one
charge is negative. So, can these two opposite-sign
charges move to produce an electric current? Actually, yes. If opposite-sign charges move in
opposite directions to each other, this produces an electric current. Electric current is positive in the
direction of motion of positive charge. Negative charge moving in the
opposite direction to a positive charge produces an electric current in the same
direction as the current produced by the motion of the positive charge.
It is very important to remember
however that the question states that the charges are fixed in place. They cannot move. If the charges cannot move, then
there can be no motion of charge. And so there cannot be an electric
current. Option (A) is therefore
incorrect.
But what about option (B)? Can there be an electric potential
difference between the charges? Electric potential difference is
defined as the work needed to move a unit charge between two points. And work is done by a force to move
an object in a particular direction. Let’s think about how these charges
became separated. Initially, with no forces holding
them in place, they must have been next to each other. If we want to separate these
charges, we’ll have to apply forces to overcome the attractive forces pulling them
together. When we apply these forces, we’re
doing work on the charges to separate them. And so, by separating these
charges, we have created an electric potential difference between them.
Therefore, answer option (B) is the
correct answer. The separation of positive and
negative charges creates an electric potential difference between them.
