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What is Temperature? - Definition & Measurement

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  • 0:03 Temperature
  • 0:44 Kinetic Energy of Particles
  • 2:33 Measuring Temperature
  • 5:32 Thermal & Kinetic Energy
  • 7:45 Example
  • 8:24 Lesson Summary
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Lesson Transcript
Instructor: Elena Cox
In the most intuitive terms, temperature refers to how hot or cold something is. Learn more about temperature as a measure of the average kinetic energy of the particles in an object and test your knowledge with quiz questions.

Temperature Defined

Temperature is a measure of how hot or cold something is; specifically, a measure of the average kinetic energy of the particles in an object, which is a type of energy associated with motion. But how hot is hot, and how cold is cold? The terms hot and cold are not very scientific terms. If we really want to specify how hot or cold something is, we must use temperature. For instance, how hot is melted iron? To answer that question, a physical scientist would measure the temperature of the liquid metal. Using temperature instead of words, like hot or cold, reduces confusion.

Temperature Depends on the Kinetic Energy of Particles

All matter is made of particles - atoms or molecules - that are in constant motion. Because the particles are in motion, they have kinetic energy. The faster the particles are moving, the more kinetic energy they have. What does temperature have to do with kinetic energy? Well, as described in this figure, the more kinetic energy the particles of an object have, the higher is the temperature of the object.

gas particles

Temperature is an average measure. Particles of matter are constantly moving, but they don't all move at the same speed and in the same direction all the time. As we can see in this figure, the motion of the particles is random. The particles of matter in an object move in different directions, and some particles move faster than others. As a result, some particles have more kinetic energy than others. So what determines an object's temperature? An object's temperature is the best approximation of the kinetic energy of the particles. When we measure an object's temperature, we measure the average kinetic energy of the particles in the object.

The higher the temperature, the faster the molecules of the substance move, on the average. Dyes will spread more rapidly through hot water than cold water. This is because of the increased motion of the molecules. Temperature does not have to do with the number of molecules involved. Under given conditions, the temperatures of 10-ml and 100-ml samples of boiling water are equal. This means that the average kinetic energy of the molecules is the same for the two different quantities of water.

The temperature of tea in the mug and in the teapot is the same.
teapot

In this image, there is more tea in the teapot than in the mug, but the temperature of the tea in the mug is the same as the temperature of the tea in the teapot.

Measuring Temperature

Since molecules are so small, you must use an indirect method to measure the kinetic energy of the molecules of a substance. As heat is added to a substance, the molecules move more rapidly. This increased motion causes a small increase in the volume, or amount of space, taken up by most materials. There are devices that use the expansion of a substance to give an indirect measure of temperature. Such devices are called thermometers.

There are many types of thermometers. Many thermometers are thin glass tubes filled with a liquid. Mercury and alcohol are often used in thermometers because they remain liquids over a large temperature range. A change in temperature causes a small change in the volume of the liquid. However, this effect is magnified when the liquid expands in the very thin tube of the thermometer.

Thermometer
yellow

Some thermometers involve the use of bimetal strips. In such thermometers, strips made of two different metals are bonded or glued together. Because the metals expand at different rates, the combined strip bends in a certain direction when it is heated. When it cools, it bends in the opposite direction. The figure below shows a bimetal strip used as a thermostat. A thermostat is a device used to control heating and cooling systems.

Thermostat
car thermometer

Some thermometers, often used on the outside of aquariums, contain liquid crystals that change color based on temperature. As temperature increases, the molecules of the liquid crystal bump into each other more and more. This causes a change in the structure of the crystals, which in turn affects their color. These thermometers are able to accurately determine the temperature between 65 F and 85 F.

Baby thermometer
baby thermometer

A thermometer with no marks, or graduations, would not be very useful to you. A thermometer is calibrated by marking two fixed points. The space between these fixed points is broken up into divisions called degrees. Degrees are used to indicate temperature. There are three types of temperature scales commonly used today: Celsius, Fahrenheit, and Kelvin. We are used to expressing temperature with degrees Fahrenheit (F). Scientists often use degrees Celsius (C), but the Kelvin (K) is the SI unit for temperature.

Thermometers can measure temperature because of thermal expansion. Thermal expansion is the increase in volume of a substance due to an increase in temperature. As a substance gets hotter, its particles move faster. The particles themselves do not expand; they just spread out so that the entire substance expands. Different substances expand by different amounts for a given temperature change. When you insert a thermometer into a hot substance, the liquid inside the thermometer expands and rises. You measure the temperature of a substance by measuring the expansion of the liquid in the thermometer.

Comparison of Fahrenheit and Celsius
comparing f and c

Thermal and Kinetic Energy

Atoms are always in motion. Imagine you had a microscope powerful enough to see individual molecules in a compound (or atoms in the case of an element). You would see that the molecules are in constant motion, even in a solid object. In a solid, the molecules are not fixed in place, but act like they are connected by springs as shown here:

Molecules in solid
molecules in solid

Each molecule stays in the same average place, but constantly jiggles back and forth in all directions. As you might guess, the 'jiggling' means motion, and motion means energy. This 'jiggling' is caused by thermal energy, which is a kind of kinetic energy.

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