# Question Video: Calculating Temperature Difference Using Thermometers Physics

The following figure shows two thermometers that measure different temperatures. What is the difference in the readings on the thermometers?

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### Video Transcript

The following figure shows two thermometers that measure different temperatures. What is the difference in the readings on the thermometers? Is it (A) 17 degrees Celsius, (B) 22 degrees Celsius, (C) 27 degrees Celsius, or (D) 31 degrees Celsius?

In this question, we’re asked to determine the difference in the readings of two thermometers. Let’s notice first of all that the thermometers have zero points in different positions. But they’re both reading the same scale, that is, degrees Celsius. Therefore, the thermometer on the left is only capable of measuring temperatures above the freezing point of water, zero degrees C, while the thermometer on the right can measure temperatures below this.

Now, if we read the temperature of the thermometer on the left, we can see that it indicates a number that is one mark lower than 20 degrees Celsius, which must mean it is measuring 19 degrees Celsius. When we measure the temperature of the thermometer on the right, we have to be a bit more careful, as the fluid in the thermometer is indicating a negative temperature. This means that starting from the negative five degrees Celsius mark, moving down one mark will indicate negative six degrees Celsius, the number going up as the temperature goes down. Counting carefully, we see that the thermometer on the right measures three marks down from negative five degrees Celsius, indicating a temperature of negative eight degrees Celsius.

To find the temperature difference now, we need to subtract the smaller measurement from the larger measurement. This means our difference will look like 19 degrees C minus negative eight degrees C. Remember that when we subtract a negative number, it’s the same as adding that positive number, 19 degrees C plus eight degrees C. We can see then that this result is equal to 27 degrees Celsius.

If we look at the readings and double-check the difference between them, by counting from each of the two zero points, we can see that the difference is indeed 27 degrees Celsius. So the correct answer to what is the difference between these two thermometers is option (C), 27 degrees Celsius.