# Question Video: Measuring the Potential Difference across a Bulb Science

Each of the 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? [A] Diagram A [B] Diagram B [C] Diagram C [D] Diagram D [E] Diagram E

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### 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.