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.
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.
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.
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.
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.
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.
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.
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:
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.
Thermal energy is proportional to temperature; when the temperature goes up, the energy of motion increases. That means the molecules 'jiggle' around more vigorously. The higher the temperature, the more thermal energy molecules have and the faster they move around. Temperature measures a particular kind of kinetic energy per molecule.
When the molecules of a substance absorb energy, they tend to move faster. A molecule moves more rapidly if it has more kinetic energy. If you throw a rock, the rock gets more kinetic energy, but the temperature of the rock does not go up. How can temperature measure kinetic energy then? The answer is the difference between random motion of the molecules and average motion of the object. For a collection of many molecules (like a rock), the kinetic energy has two parts. The kinetic energy of the thrown rock comes from the average motion of the whole collection, the whole rock. This kinetic energy is not what temperature measures.
Each molecule in the rock is also 'jiggling' back and forth independently of the other molecules in the rock. This 'jiggling' motion is random motion. On average, there are as many molecules moving one way as there are moving the opposite way. Temperature measures the kinetic energy of the random motion. Temperature is not affected by any kinetic energy associated with average motion. That is why throwing a rock does not make it hotter.
Example of Temperature
Let's learn more about temperature and thermal expansion with an example. Have you ever gone across a highway bridge in a car? You probably heard and felt a 'thuh-thunk' every couple of seconds as you went over the bridge. That sound occurs when the car goes over small gaps called expansion joints.
The concrete segments of a bridge can expand on hot days. When the temperature drops, the segments contract. These joints keep the bridge from buckling as a result of thermal expansion. Recall that thermal expansion is the increase in volume of a substance due to an increase in temperature.
Heat and temperature are related, but not the same. Temperature is a measure of the average kinetic energy of the particles of a substance. The higher the temperature of an object, the higher is its kinetic energy. Kinetic energy is a type of energy associated with motion. Temperature is measured with thermometers, which are devices that use the expansion of a substance to give an indirect measure of temperature. The units that are used to measure temperature are called degrees. When temperature increases in an object, the molecules inside it start to move faster, and this causes the object to expand. Thermal expansion is the increase in volume of a substance due to an increase in temperature.
Once you've completed this lesson, you should be able to:
- Define temperature
- Explain the relationship between temperature and kinetic energy
- Describe how temperature is measured
- Recall how thermal expansion occurs
To unlock this lesson you must be a Study.com Member.
Create your account
How much kinetic energy are in the molecules of a substance determines its temperature. Cold is not a measurable quantity, but heat is therefore if something is cold we could say it lacks enough heat. Let's do an activity where we take multiple measurements of water as it is heating up to its boiling point.
We are going to measure the temperature of water from room temperature to its boiling point. Be sure to have a responsible adult supervisor you while doing this activity as water being heated can cause burns if it splashes or spills on you.
- Container of water in a pot or beaker
- Enough water to half-way fill the pot or beaker
- Stove or Bunsen burner
- Striker if using Bunsen burner
- Ring stand with platform is the beaker is being used
- Paper and pen
- Put the water into the pot or beaker.
- Put the beaker on the ring stand platform and the Bunser burner underneath it or put the pot on the stove.
- Attach the thermometer to the pan or in the beaker so that it is secure.
- Record the temperature
- Light the Bunsen burner or turn on the stove to high.
- Take temperature measurements every 1 minute.
- What is happening to the molecules of water when the mass of water is being heated?
- Plot the temperature versus time data. Is the graph linear or quadratic?
- The molecules are absorbing heat energy and therefore are moving faster and faster.
- The graph should be linear.
Register to view this lesson
Unlock Your Education
See for yourself why 30 million people use Study.com
Become a Study.com member and start learning now.Become a Member
Already a member? Log InBack