Video Transcript
The diagram below shows an electric
circuit consisting of a cell, a resistor, a voltmeter, and an ammeter. The reading of the voltmeter and
the value of the resistor are both shown on the diagram. What current will the ammeter
measure?
In this question, we are given a
diagram of an electric circuit with various components. And we are asked to find how much
current in the circuit would be measured by the ammeter. In the circuit, we have a cell with
an unlabeled potential difference, a resistor with one ohm of resistance, a
voltmeter attached in parallel on either end of the resistor, and an ammeter with an
unknown reading attached in series, such that it can measure the current through the
wires of the circuit.
In order to find this value of
current, we can calculate it using the values of the other components we’re given in
the circuit. We do so by using a specific
expression of Ohm’s law, which puts its variables in terms of potential
difference. The current 𝐼 is equal to the
potential difference 𝑉 divided by the resistance 𝑅. So, in order to find the current in
this ammeter, which measures the current of the circuit, we just have to find the
potential difference and resistance of the circuit.
At first, this may seem difficult
since we are not given a value for the potential difference supplied by the power
cell. However, we are given a value for
the potential difference across two points on either side of the load resistor in
the circuit. Because there is only this one
resistor in the circuit, it means the entire potential difference on one end and the
other must represent the entire potential difference for the circuit, since there
are no other components that could cause a further difference in the potential. While there is another component in
the circuit, the ammeter, we assume that ideal ammeters have zero resistance, so
they will not cause a change in the potential difference for the circuit.
All of this means that we can now
use this value given by the voltmeter here as the value of potential difference
needed for the Ohm’s law expression, which represents the entire circuit. If we combine this voltmeter value
with the resistance of the resistor, which represents the entire resistance of the
circuit, then we can find the current of the circuit which is what will be measured
by the ammeter.
To do so, let’s set up our
equation. The value of potential difference
𝑉 is one volt, given to us by the voltmeter. The value of resistance 𝑅 is one
ohm, given to us by the resistor. For the units in this equation, we
should note that when we divide volts by ohms, we will get units of amperes. So, one volt divided by one ohm is
equal to one ampere. Therefore, the current that the
ammeter in this circuit will measure will be one ampere.
