Video Transcript
If the emf of a cell does not
change when it is connected to a circuit, which of the following must be true of the
charges on the terminals of the cell? (A) The amount of positive charge
on the positive cell terminal does not change, but the amount of negative charge on
the negative terminal does change. (B) The amount of positive charge
on the positive cell terminal changes, but the amount of negative charge on the
negative terminal does not change. Or (C) the amount of positive
charge on the positive cell terminal does not change, and the amount of negative
charge on the negative terminal does not change.
In circuit diagrams, we usually
represent a cell like this, where each line represents a terminal of the cell. There is one positively charged
terminal and one negatively charged terminal. To answer this question, we’re
going to represent our cell slightly differently, like this. Here, we’ve drawn some of the
charges on each of the terminals. The positive charges are shown in
red, and the negative charges are shown in blue. In this cell, the opposite charges
have been separated. There is an overall positive charge
here, an overall negative charge here, and these regions of charge have a distance
between them.
Recall that opposite charges
attract. There is an attractive force
between positive charges and negative charges that pulls them towards each
other. In order to keep opposite charges
separate and on different terminals of a cell, we need to do work on the
charges. When we do work on the charges, we
can give them enough energy to overcome the attractive force and remain
separate. This creates an electric potential
difference between the two terminals of the cell. This potential difference can also
be called an emf. In short, because there’s a
separation of charge across the terminals of the cell, there is also an emf across
the terminals of the cell. The magnitude of this emf depends
on the amount of charge that has been separated.
If the amount of charge on each
terminal increases, so the positive terminal becomes more positively charged and the
negative terminal becomes more negatively charged, it would take more work to keep
all of the extra positive and negative charges separate. This corresponds to a greater emf
across the terminals of the cell. Similarly, if the amount of charge
on each terminal decreased, less work would need to be done to separate the
charges. This corresponds to a smaller emf
across the terminals.
In this question, we’re told that
the emf of the cell does not change when it is connected to the circuit. This means that the amount of
charge on each terminal does not change. If we look through the options
we’ve been given, we see that this corresponds to option (C). The amount of positive charge on
the positive cell terminal does not change, and the amount of negative charge on the
negative terminal does not change. So, the correct answer to this
question must be option (C).
