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Explores how thermal energy in a substance is measured.

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This girl has a fever, and it makes her feel miserable. She feels achy and really tired. She also feels hot because her temperature is higher than normal. She has a thermometer in her mouth to measure her temperature.

What Is Temperature?

No doubt you already have a good idea of what temperature is. You might say that it’s how warm or cool something feels. In physics, temperature is defined as the average kinetic energy of the particles of matter. When particles of matter move more quickly, they have more kinetic energy, so their temperature is higher. With a higher temperature, matter feels warmer. When particles move more slowly, they have less kinetic energy on average, so their temperature is lower. With a lower temperature, matter feels cooler.

How Thermometers Measure Temperature

Many thermometers measure temperature with a liquid that expands when it gets warmer and contracts when it gets cooler. Look at the common household thermometer pictured in the Figure below. The red liquid rises or falls in the glass tube as the temperature changes. Temperature is read off the scale at the height of the liquid in the tube.

Illustration of a thermometer

Q: Why does the liquid in the thermometer expand and contract when temperature changes?

A: When the temperature is higher, particles of the liquid have greater kinetic energy, so they move about more and spread apart. This causes the liquid to expand. The opposite happens when the temperature is lower and particles of liquid have less kinetic energy. The particles move less and crowd closer together, causing the liquid to contract.

Temperature Scales

The thermometer pictured in the Figure above measures temperature on two different scales: Celsius (C) and Fahrenheit (F). Although some scientists use the Celsius scale, the SI scale for measuring temperature is the Kelvin scale. If you live in the U.S., you are probably most familiar with the Fahrenheit scale. The Table below compares all three temperature scales. Each scale uses as reference points the freezing and boiling points of water. Notice that temperatures on the Kelvin scale are not given in degrees (°).

Scale Freezing Point of Water Boiling Point of Water
Kelvin 273 K 373 K
Celsius 0 °C 100 °C
Fahrenheit 32 °F 212 °F

Because all three temperature scales are frequently used, it’s useful to know how to convert temperatures from one scale to another. It’s easy to convert temperatures between the Kelvin and Celsius scales. Each 1-degree change on the Kelvin scale is equal to a 1-degree change on the Celsius scale. Therefore, to convert a temperature from Celsius to Kelvin, just add 273 to the Celsius temperature. For example, 10 °C equals 283 Kelvin.

Q: How would you convert a temperature from Kelvin to Celsius?

A: You would subtract 273 from the Kelvin temperature. For example, a temperature of 300 Kevin equals 27 °C.

Converting between Celsius and Fahrenheit is more complicated. The following conversion factors can be used:

CelsiusFahrenheit: (°C × 1.8) + 32 = °F
FahrenheitCelsius: (°F - 32) ÷ 1.8 = °C

For example, to convert 10 °C to Fahrenheit, use the first conversion factor:

(10 °C × 1.8) + 32 = 50 °F

Q: The weather forecaster predicts a high temperature today of 86 °F. What will the temperature be in Celsius?

A: To convert 86 °F to Celsius, use the second conversion factor:

(86 °F – 32) ÷ 1.8 = 30 °C


  • Temperature is the average kinetic energy of particles of matter.
  • A thermometer can measure temperature with a liquid that expands when it gets warmer and contracts when it gets cooler
  • The SI scale for measuring temperature is the Kelvin scale. Celsius and Fahrenheit temperature scales are also commonly used. You can use conversion factors to convert temperatures between the different scales.


  1. What is temperature?
  2. Explain how the thermometer pictured in this article measures temperature.
  3. Assume that the temperature outside is 293 Kelvin but you’re familiar only with the Fahrenheit scale. Do you need to wear a hat and gloves when you go outside? To find out, convert the Kelvin temperature to Fahrenheit. (Hint: Convert the Kelvin temperature to Celsius first.)


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Kelvin scale SI scale for measuring temperature in which the freezing point of water is 273 K and the boiling point of water is 373 K.
temperature Average kinetic energy of particles of matter.

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