Lesson Explainer: Measuring Temperatures Physics

In this explainer, we will learn how to correctly use thermometers to measure the temperature of liquids and gases.

Temperature is a measure of how hot or cold a substance is. We can measure the temperature of a substance using an instrument called a thermometer. There are two types of thermometer: analog thermometers and digital thermometers.

The diagram below shows an analog thermometer.

Analog thermometers are typically made of glass with a red-colored liquid inside. The liquid rises or falls as the temperature of the environment goes up or down. A temperature scale, normally in degrees Celsius or degrees Fahrenheit or both, is printed on the thermometer.

Looking closely at the thermometer, we see marks corresponding to specific temperature values. The mark closest to the top of the red liquid is the temperature we read.

To measure the temperature of a fluid accurately using an analog thermometer, there are several important things to remember.

  1. Submerge the bottom of the thermometer, called the bulb, in the fluid to be measured. Ensure that the bulb does not touch or get close to any other material such as the sides or bottom of the fluid container; if it does, the thermometer will be measuring the combined temperature of more than one object, rather than the temperature of the substance of interest.
  2. While holding the thermometer steady, give the red liquid time to rise, fall, or stay at the same level. An analog thermometer takes time to adjust to its surroundings. Before we can read out the measured temperature, the height of the red liquid must not be changing at all.
  3. Holding the thermometer upright, we set our vision level with the top of the red liquid. We avoid looking up or down at the top of the liquid, because that can trick our eye into seeing an incorrect level and recording an incorrect temperature.
  4. Looking closely, we may notice that the red liquid has a curved shape to its surface. This is called a meniscus (plural: menisci). Generally, a meniscus can curve upward or downward. Menisci that curve up are called convex, and those curving down are called concave. In either case, when we make a measurement, we look for the middle of the meniscus and use that as the fluid level to compare with the temperature marks on the thermometer.
  5. The temperature mark closest to the level of the middle of the meniscus is the temperature we record as our reading.

A digital thermometer works in much the same way as an analog thermometer. It also has a temperature-sensitive bulb that should be submerged in the liquid or gas being measured. And it too needs some time to reach a stable temperature reading.

Unlike an analog thermometer, a digital thermometer simply reads out the measured temperature. Therefore, errors in interpreting the measured temperature are less likely using a digital thermometer.

Let’s now consider a few examples of taking temperature readings, explaining in some cases why those readings are incorrect.

Example 1: Reading a Thermometer for Temperature in Degrees Celsius

The diagram shows a thermometer being used to measure the temperature of the air outside. What value does the thermometer give for the temperature?

Answer

We want to read the temperature measured by this analog thermometer. That temperature corresponds to the marking that lines up most closely with the top of the red liquid in the thermometer.

Looking closely, we see that the top portion of the red liquid is curved, indicating a meniscus. The middle of the meniscus, where it is flat, lines up with the mark just below 10C. Each mark represents an increment of 1C, so this thermometer is reading a temperature of 9C.

Example 2: Reading Thermometer Temperature to the Nearest Degree

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?

Answer

This thermometer measures temperatures on the Celsius scale. The level of red liquid inside the thermometer indicates which temperature to record. We can see that the level of the liquid lies between two temperature values: 29C and 28C.

The level is closer, though, to the 29C mark. To the nearest degree, then, the thermometer’s temperature reading is 29C.

Example 3: Reading a Temperature Exceeding a Thermometer’s Scale

A thermometer is placed in a pan containing a mixture of fruit juice and sugar that is being heated to be turned into jam. The mixture is heated to over 100C. The thermometer is in the figure shown. What temperature reading does the thermometer give?

  1. 100C
  2. 104C
  3. 105C
  4. It is not possible to get an accurate reading from the thermometer.
  5. 110C

Answer

To read the temperature of a thermometer, we need to find out which mark the red bar lines up with most nearly. That bar indicates the level of the liquid in the thermometer.

A close-up look at this level shows that it is above the highest mark on the thermometer. This nearest mark corresponds to a temperature of 104C, but we do not report this as our answer, because the red bar is outside the range of temperatures for which this thermometer is calibrated. Rather than estimate what the temperature would be if the marks continued up the thermometer or report the temperature of the nearest mark, we need to report that we can not accurately read this temperature on this thermometer.

This corresponds to choice D.

Example 4: Explaining Why a Temperature Reading Is Incorrect

Engy wants to measure the temperature of some water that has been heated to near its boiling point. She uses a thermometer that has been on a table in the room for an hour. She places the bulb of the thermometer in the water and immediately reads the temperature on it, as shown in the diagram. She determines that the temperature of the water is 19C. Which of the following statements explains why this answer is incorrect?

  1. She did not wait for the reading to stabilize. The temperature of the water is actually far higher than 19C.
  2. She should hold the bulb of the thermometer just above the surface of the water. This is where the temperature is highest.
  3. She should submerge the whole thermometer in the water.

Answer

We are told that this water has been heated to near its boiling point, that is, 100C, so we immediately know that a reading of 19C is too low.

19C is approximately room temperature, and we are told that the thermometer has been on a table in the room for one hour. Since Engy reads the measured temperature immediately after putting the bulb in the hot water, we can form an idea of how she obtained her result.

During the time it was on the table, the thermometer stabilized to the ambient air temperature of 19C. When Engy picked up the thermometer and put it in the hot water, she did not allow time for the thermometer to adjust. The reading that she believed corresponded to the temperature of the water indicated instead the temperature of the room.

This line of thinking suggests option A as the best choice.

Option B says Engy should have held the thermometer bulb above the water’s surface. However, she wants to measure the temperature of the water, which requires covering the bulb in the water.

Choice C claims that the proper measurement approach is to submerge the entire thermometer in water, but this is not necessary for a thermometer to measure a temperature accurately. Only the bulb needs to be covered by the material being measured; submerging the entire thermometer would be impractical and would make taking an accurate reading difficult.

Option A best describes the reason for this incorrect measurement.

Example 5: Explaining an Incorrect Temperature Measurement

Amira heats water in a copper container using a Bunsen burner, as shown in the diagram. She wants to measure the temperature of the water. She puts a thermometer in the container so that it is in contact with the side of the container. This leads to a higher temperature reading on the thermometer than the actual temperature of the water. Which of the following statements best explains the reason for this?

  1. The water at the sides of the container is hotter than the water in the middle because it is in direct contact with the hot container. The average temperature of the water is lower.
  2. Copper conducts heat well, and the heat from the Bunsen burner is conducted through the copper of the container. Since the thermometer is in contact with the copper of the container, it is partially measuring the temperature of the container rather than the water.
  3. Convection currents in the water travel up the sides of the container; thus, the water at the sides of the container is hotter than the water in the middle. The average temperature of the water is lower.

Answer

A key fact in our problem statement is that the container holding the water is made of copper. Like all metals, copper is an effective heat conductor. We can expect it to heat up at a faster rate than the water.

At any given moment, then, the temperature of the container is likely higher than the temperature of the water. When Amira puts a thermometer into the water in contact with the container, she exposes the thermometer to this higher temperature.

The thermometer reading will be based on some combination of the water temperature and the container temperature. Since the container temperature is higher, the reading will not accurately reflect the temperature of the water.

The answer best describing this is option B. Note that option A supposes that the water in the container is fairly stagnant, whereas water being heated this way would move and mix throughout the container.

Key Points

  • Analog and digital thermometers are used to measure the temperatures of liquids and gases.
  • To make a correct measurement using an analog thermometer,
    • submerge the thermometer bulb in the fluid being measured;
    • allow the thermometer measurement to stabilize;
    • hold the thermometer upright and position the eye to be level with the top of the red liquid;
    • if a meniscus is present, take the measurement closest to the level of its middle, the flat part of the curve;
    • read the temperature of the mark nearest to this level.
  • For a digital thermometer, follow the first two steps and then read the indicated temperature.

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