Lesson Explainer: Measuring Temperatures Physics - Nagwa

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 19∘C. 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 19∘C.
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, 100∘C, so we immediately know that a reading of 19∘C is too low.

19∘C 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 19∘C. 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.