Video Transcript
The picture shows the circuit
symbol for a bulb. Complete the following
sentence. If there is an electric potential
difference across the bulb, then blank. (A) The total number of electrons
in the bulb will increase. (B) The total number of electrons
in the bulb will decrease. (C) There will be an electric
current in the bulb.
First, let’s ignore that the bulb
is a part of this problem. Instead, simply consider a
conductive wire with a potential difference between two ends. We know that an electric potential
difference causes electrons in a conductive wire to move from one end to the
other. Suppose that the electric potential
difference is across the length of the wire so that the direction in which electrons
move along the wire is from left to right along the wire. This movement of electrons along
the wire is an electric current.
Now that we know how the electric
potential difference will affect the electrons in the wire, we need to consider how
a bulb might change the situation. A light bulb can contain a
filament, which is a piece of conducting wire made of a different substance to the
connecting wires of a circuit. The connecting wires are usually
made of copper, whereas a filament of a bulb is usually made of tungsten. Now, rather than say that there is
a potential difference across the ends of the wire, let’s now say that there is a
potential difference across the ends of the filament. This way we only need to consider
what happens in the filament.
Let’s recall that an electric
potential difference across the contacts of a filament does work on electrons in the
filament as they travel along the filament. The energy of the electrons is
increased by the work done on them by the electric potential difference. As electrons travel through the
filament, they transfer to the filament the energy that was transferred to them by
the electric potential difference across the filament. The filament then releases this
energy as light. It is important to understand that
the filament releases energy as light because of electrons transferring energy when
they pass through the filament. The filament does not release
energy of light because of increasing or decreasing the number of electrons in the
filament.
In fact, the number of electrons in
a filament does not change when there is a potential difference across its ends. For each electron that moves into
the filament at one end, one other electron moves out of the filament at the
opposite end. So, the number of electrons in the
bulb will neither increase nor decrease. We can therefore eliminate answer
options (A) and (B). We have already seen though that
the movement of the electrons along the filament means that there is an electric
current through it.
We see then that the correct answer
is option (C). If there is an electric potential
difference across the bulb, then there will be an electric current in the bulb.
