Question Video: Measuring the Potential Difference across a Circuit Component Science

Video Transcript

Each of the following diagrams shows a circuit containing a cell, a bulb, a buzzer, and a voltmeter. Which one shows how the voltmeter must be connected to the circuit to measure the potential difference across the bulb only? Is it option (A), option (B), option (C), option (D), or option (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, a light bulb, and a buzzer. We must choose in which diagram the voltmeter will correctly measure the potential difference across the light bulb only.

The word “only” is emphasized to make it clear that we must not choose a circuit which measures the potential difference across the light bulb and the buzzer. 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. And 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 options (A), (B), (C), and (D), we 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 or the buzzer. Condition two has not been met. So these connections cannot be used to measure the potential difference across the bulb.

The circuit in option (A) will in fact correctly measure the potential difference across the buzzer. The circuits in option (B) and option (D) will in fact correctly measure the combined potential difference across the bulb and buzzer. The circuit in option (C) will correctly measure the potential difference across the cell.

For option (E), 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 the bulb and nothing else. Option (E) shows the voltmeter connected correctly.