Video Transcript
The diagram shows an electric
circuit consisting of a cell and a bulb. Which of the following statements
is correct? (A) The cell contains the same
number of positive and negative charges, but the material of the cell creates a
buildup of negative charges on the negative terminal. This creates a potential difference
across the circuit, which creates a current in it. (B) The cell contains more negative
charges than positive charges. The negative charges flow from the
negative terminal around the circuit. Once the number of negative charges
in the cell is equal to the number of positive charges, the cell can no longer
produce a current.
To decide which statement is
correct, (A) or (B), let’s analyze the two answers. In option (A), we are told that the
number of negative and positive charges in the cell is the same, while in option
(B), we are told that there are more negative than positive charges. Let’s think about this. We know that a cell has two
terminals; one terminal is called a positive terminal. In the symbol for a cell, this is
represented by the longer, thinner line. The other terminal, the negative
terminal, is represented by the shorter and thicker line.
Negative charges can move freely
through an electric circuit because these charges are electrons that can move
between atoms in the conducting material that forms the circuit. Positive charges do not move in a
conductor. If we connect conductive material,
such as a copper wire, from the negative terminal of a cell to the positive terminal
of a cell, this forms a circuit. Electrons will flow through the
wire, away from the negative terminal toward the positive terminal. The motion of electrons is in the
same direction as an electric current.
Let us now see whether option (A)
or option (B) better describes what actually happens when a cell is connected in a
circuit. Let us suppose that there were more
negative charges than positive charges in the cell, where the negative charges are
electrons. If this was the case, then
connecting the cell to an uncharged circuit would indeed transfer electrons and
hence negative charge to the circuit. Option (B) states that the negative
charges flow from the negative terminal around the circuit. However, if a cell was simply an
object with more negative charges than positive charges, there would be no reason
why the negative charges would accumulate at one end of the cell.
Negative charges would spread out
equally across the whole cell. Negative charges transferred to a
circuit by a cell that was just an object with surplus negative charge would then
transfer negative charges from both terminals of the cell equally. This means that electrons would
move from both the positive and negative terminals of the cell into the circuit. Electrons from the positive
terminal would move toward the negative terminal. Electrons from the negative
terminal would move toward the positive terminal. The motion of electrons away from
the negative terminal would then equal the motion of electrons toward the
terminal. The overall motion of electrons
away from the negative terminal would then be zero. We see then that a cell cannot
produce a current in a circuit just by having more negative charges than positive
charges.
Option (A) states that the cell
contains the same number of positive and negative charges, but the material of the
cell creates a buildup of negative charges on the negative terminal. If option (A) is correct, then the
negative terminal of the cell accumulates more negative charge than positive
charge. This would mean that the cell must
have separated some of the negative charges in the cell from some of the positive
charges in the cell. Option (A) also states that this
creates a potential difference across the circuit, which creates a current in
it.
Recall that when opposite charges
are separated, this produces a potential difference between the separated
charges. As the cell has separated opposite
sign charges, there must be a potential difference across the terminals of the
cell. If opposite ends of a wire are
connected to the opposite terminals of a cell, there must then also be a potential
difference across the wire. A wire contains electrons that can
move through it. Producing a potential difference
across the wire makes electrons in the wire move through the wire in the same
direction. This is an electric current. We see then that option (A)
correctly describes the effect of connecting a cell to a circuit.
