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Back Energy Mechanics Physics Contents Index Home  Knowing the difference between heat and temperature is important. It can lead to a clearer understanding of energy. Above is a picture of an ice cube melting in a small dish. The ice, water, dish, and are experience heat exchanges and temperature changes. In this section we will define both heat and temperature and hopefully reach an understanding of how they are related, but not identical ideas. This page covers some introductions about heat and temperature. Directly below, and at the end of this page, are some links to further material: Motion of Gas Molecules, Heat, and Temperature Changes of Phase, Heat, and Temperature Heat is not temperature. Often the concepts of heat and temperature are thought to be the same, but they are not. Heat ≠ Temperature Perhaps the reason the two are usually and incorrectly thought to be the same is because as human beings on Earth our everyday experience leads us to notice that when you add heat to something, say like putting a pot of water on the stove, then the temperature of that something goes up. More heat, more temperature - they must be the same, right? Turns out, though, this is not true. Initial Definitions Temperature is a number. That number is related to energy, but it is not energy itself. Temperature is a number that is related to the average kinetic energy of the molecules of a substance. Read that last sentence carefully. It does not say that temperature is kinetic energy, nor does it state exactly what is the relation between temperature and kinetic energy. Here is the relation: If temperature is measured in Kelvin degrees, then the value of temperature is directly proportional to the average kinetic energy of the molecules of a substance. Note that temperature is not energy, it is a number proportional to a type of energy. Heat, on the other hand, is actual energy measured in Joules or other energy units. Heat is a measurement of some of the energy in a substance. When you add heat to a substance, you are adding energy to the substance. This added heat (energy) is usually expressed as an increase in the kinetic energies of the molecules of the substance. If the heat (energy) is used to change the state of the substance, say by melting it, then the added energy is used to break the bonds between the molecules rather than changing their kinetic energy. Again, About Temperature So, temperature is not energy. It is, though, a number that relates to a type of energy possessed by the molecules of a substance. Temperature directly relates to the kinetic energy of the molecules. Temperature can be measured in a variety of units. If you measure it in degrees Kelvin, then the temperature value is directly proportional to the average kinetic energy of the molecules in the substance. Notice we did not say that temperature is the kinetic energy. We said it is a number, if in degrees Kelvin, that is proportional to the average kinetic energy of the molecules of a substance. That means if you double the Kelvin temperature of a substance, you double the average kinetic energy of its molecules. When the average kinetic energy of the molecules goes up (a rise in temperature), the average speed of the molecules increases. And lower average kinetic energy of the molecules means they have lower speed. However, a change in average kinetic energy is not directly proportional to a change in average speed. More About Heat Heat is energy. When you add heat to a substance, you are adding energy. When heat (energy) goes into a substance one of two things can happen:
So, when heat comes into a substance, energy comes into a substance. That energy can be used to increase the kinetic energy of the molecules, which means an increase in their temperature which means an increase in their speed. Or at certain temperatures the added heat could be used to break the bonds between the molecules causing a change in state that is not accompanied by a change in temperature. Animations and Further Explanations: Motion of Gas Molecules, Heat, and Temperature Changes of Phase, Heat, and Temperature Back Energy Mechanics Physics Contents Index Home
In order to continue enjoying our site, we ask that you confirm your identity as a human. Thank you very much for your cooperation. Kinetic energy is the energy of motion. Any object that is moving possesses kinetic energy. Baseball involves a great deal of kinetic energy. The pitcher throws a ball, imparting kinetic energy to the ball. When the batter swings, the motion of swinging creates kinetic energy in the bat. The collision of the bat with the ball changes the direction and speed of the ball, with the idea of kinetic energy being involved again.
As stated in the kinetic-molecular theory, the temperature of a substance is related to the average kinetic energy of the particles of that substance. When a substance is heated, some of the absorbed energy is stored within the particles, while some of the energy increases the motion of the particles. This is registered as an increase in the temperature of the substance.
At any given temperature, not all of the particles of a sample of matter have the same kinetic energy. Instead, the particles display a wide range of kinetic energies. Most of the particles have a kinetic energy near the middle of the range. However, a small number of particles have kinetic energies a great deal lower or a great deal higher than the average (see figure below). The blue curve in the figure above is for a sample of matter at a relatively low temperature, while the red curve is for a sample at a relatively high temperature. In both cases, most of the particles have intermediate kinetic energies, close to the average. Notice that as the temperature increases, the range of kinetic energies increases and the distribution curve "flattens out". At a given temperature, the particles of any substance have the same average kinetic energy.
As a sample of matter is continually cooled, the average kinetic energy of its particles decreases. Eventually, one would expect the particles to stop moving completely. Absolute zero is the temperature at which the motion of particles theoretically ceases. Absolute zero has never been attained in the laboratory, but temperatures on the order of \(1 \times 10^{-10} \: \text{K}\) have been achieved. The Kelvin temperature scale is the scale that is based on molecular motion, and so absolute zero is also called \(0 \: \text{K}\). The Kelvin temperature of a substance is directly proportional to the average kinetic energy of the particles of the substance. For example, the particles in a sample of hydrogen gas at \(200 \: \text{K}\) have twice the average kinetic energy as the particles in a hydrogen sample at \(100 \: \text{K}\). Summary
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