Video Transcript
Each of the following diagrams
shows a circuit containing a cell, a bulb, and a voltmeter. Which one shows how the voltmeter
must be connected to the circuit in order to measure the potential difference across
the bulb? Is it option (A), (B), (C), (D), or
(E)?
The first thing we must understand
is what is asked in this question. We see there are five circuit
diagrams consisting of a cell and a light bulb. And we must choose in which diagram
the voltmeter will correctly measure the potential difference across the light
bulb.
We know that a voltmeter measures
the potential difference between two points in a circuit. Remember that a potential
difference must be between two points. So, in order to measure the
potential difference across the bulb, we are going to need to choose two points at
which to connect the voltmeter that meet the following conditions. One, the points must be on opposite
sides of the bulb. Two, the points must have a path
that connects them, on which there is a voltmeter and nothing else. Three, the points must also have a
second path that connects them, on which there is the bulb and nothing else. Let us look at the circuits in the
options.
For option (A), we see that there
is only a point on one side of the bulb that connects to the voltmeter. Condition one has not been met. So this connection cannot be used
to measure the potential difference across the bulb.
For option (B) and option (C), we
can see that there are points on both sides of the bulb that connect to the
voltmeter. But the path between these points
passes through the cell. Condition two has not been met. So these connections cannot be used
to measure the potential difference across the bulb.
For option (E), we can see that
there are points on both sides of the bulb that connect to the voltmeter. The path between these points
passes through the cell in the circuit so does not meet condition two. There is also a path from the sides
of the voltmeter to the bulb that does not pass through the cell. But this connects to only one side
of the bulb, not to both sides of the bulb. This connection cannot be used to
measure the potential difference across the bulb.
For option (D), we can see that
there are points on both sides of the bulb that connect to a path on which there is
a voltmeter and nothing else. We can also see that these points
are connected by a second path, on which there is a bulb and nothing else. Option (D) shows the voltmeter
connected correctly.
