Question Video: Reading a Temperature from a Thermometer Physics

The figure shows a thermometer that has been left outside in the shade on a hot day. What temperature reading does the thermometer give to the nearest degree?

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Video Transcript

The figure shows a thermometer that has been left outside in the shade on a hot day. What temperature reading does the thermometer give to the nearest degree?

We can see that we’ve been given this figure that shows a thermometer, which is a device that can be used to measure temperature. We can notice that at the bottom of this thermometer there’s a reservoir or bulb that’s filled with something red. And this red substance then extends some way up the scale of the thermometer. This red substance is mercury, which is a liquid that expands when its temperature increases.

Now mercury expands with temperature in a known and predictable way. So, depending on the temperature of this bulb, the mercury unit will expand a certain amount. And as a result of this expansion, it will rise up the tube in the thermometer. The amount that it expands determines the height that the mercury rises to. And so, the height of the mercury in the thermometer tube indicates the temperature.

We can notice that we’ve got a scale drawn on the side of this thermometer. There are big marks on the scale of 5-degree increments. And these marks are labeled with numbers. Then between each of these big marks, there are four of these smaller marks. Since these four small marks separate this five-degree region into one, two, three, four, five smaller regions, then we know that each one of these small marks corresponds to one degree. We also need to take some care over the particular temperature scale that’s being used here. We can recall that there are several different temperature scales, including the Fahrenheit, the Celsius, and the Kelvin scale.

Looking at the top of the thermometer, we can see that we’ve got this degree C written on it. The C here stands for Celsius, and it tells us that this thermometer measures temperatures on the Celsius scale. That means that a temperature measured with this thermometer must have units of degrees Celsius. And so, each small mark on the thermometer corresponds to one degree Celsius. In order to read off the temperature in units of degrees Celsius that this thermometer is measuring, we just need to see how far up the temperature scale this red line comes. The line comes up to this height here, which is actually partway between two marks on the temperature scale.

Let’s now have a closer look at what’s going on in this region of the thermometer. Looking now at this zoomed-in view, we can see more clearly the height that the mercury rises to. That height is above this small mark, which is two small marks below the 30-degree Celsius mark, and below this small mark, which is one mark below 30 degrees Celsius. Since we know that each small mark corresponds to one degree Celsius, then we know that the upper small mark, which is one mark below 30 degrees Celsius, must be 29 degrees Celsius and the one below this, the lower mark, must be 28 degrees Celsius.

Since the mercury reaches a height in the tube that’s between these two small marks, then we know that this thermometer is measuring a temperature that’s between 28 degrees Celsius and 29 degrees Celsius. Since the thermometer scale only has marks at one-degree increments, then the resolution of this thermometer is not great enough to give an answer to more precision than this. That’s okay though because the question is asking us for the temperature reading given to the nearest degree.

If we look closely at the height of the mercury in this tube, we can notice that it’s ever so slightly closer to the mark above it at 29 degrees Celsius than it is to the mark below it at 28 degrees Celsius. That means that the temperature being measured by this thermometer must be slightly closer to 29 degrees Celsius than it is to 28 degrees Celsius. This means that our answer then is that, to the nearest degree, the temperature reading given by this thermometer is equal to 29 degrees Celsius.

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