What is Heat? - Definition & Explanation

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  • 0:00 Heat & Temperature
  • 1:00 Thermal Expansion
  • 1:56 Thermodynamics
  • 3:27 Heat Transfer
  • 4:53 Specific Heat Capacity…
  • 7:04 Lesson Summary
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Lesson Transcript
Instructor: Thomas Zesiger

Thomas has taught electronics and communications engineering, math, and physics and has a master's degree in electrical engineering.

In this lesson, you will learn the definition of heat and find out how it differs from temperature. You will also explore practical applications of heat such as expansion, thermodynamics, heat transfer, specific heat, phase transitions, and heat engines.

Heat and Temperature

Heat is the form of energy that is transferred between two substances at different temperatures. The direction of energy flow is from the substance of higher temperature to the substance of lower temperature. Heat is measured in units of energy, usually calories or joules. Heat and temperature are often used interchangeably, but this is incorrect. Temperature is the measure of hotness or coldness of matter. Stated another way, temperature is the average kinetic energy per molecule of a substance. Temperature is measured in degrees on the Celsius (C) or Fahrenheit (F) scale, or in kelvins (K). In simplest terms, temperature is how hot or cold an object is, while heat is the energy that flows from a hotter object to a cooler one. For example, the temperature of a cup of coffee may feel hot if you put your hand around it. It is hot because heat from the coffee is transferred to the cup.

Thermal Expansion

Thermal expansion is a phenomenon that takes place in solids, liquids, and gases. Almost all substances expand when their temperatures increase, unless they are constrained in some manner. Examples include the heating of air in a hot air balloon, which makes the balloon expand and rise, and mercury in a thermometer, which rises in response to heat. Metal rods are used in a variety of applications also. For example, metal rods or strips that are used as expansion joints at the ends of bridge sections account for the expansion of steel bridges in hotter weather. The amount of expansion that occurs and how we predict it depends on the substance. For example, a solid metal rod generally expands linearly and increases in length, while liquids and gases experience an increase in volume. In all three cases, thermal expansion occurs in response to an increase in temperature and useful devices take advantage of this concept.


Thermodynamics is the study of heat and its transformation to mechanical energy. There are four laws of thermodynamics, but we only concentrate on the two principal laws here: the first law and the second law.

The first law says that the change in internal energy of a substance equals the work done on it plus the heat transferred to it. Mathematically, we use the equation:

delta U = work + Q

Internal energy is the sum of the kinetic and potential energies of all the atoms and molecules within a substance. The significance of the first law of thermodynamics is that there are two ways to increase the temperature of a substance:

1) By exposing it to another substance that has a higher temperature and

2) By doing certain kinds of work on the substance

Friction and compression of gases are two examples of ways to increase temperature by the work method. Pistons in internal combustion engines take advantage of this concept. Air is compressed in a cylinder by the piston, which raises the temperature to almost twenty-seven times the temperature of the uncompressed state.

The second law says that heat cannot be transferred from a colder body to a hotter body without work being done by an outside agent. Stated another way, no device can be built that will repeatedly extract heat from a source and deliver mechanical energy without ejecting some heat to a lower-temperature reservoir. The perfect example is the heat engine, which is discussed later in this lesson.

Heat Transfer

Heat transfer occurs by three mechanisms: conduction, convection, and radiation.

  • Conduction is the transfer of heat between atoms and molecules in direct contact
  • Convection is the transfer of heat by movement of the heated substance itself, such as by currents in a fluid
  • Radiation is the transfer of heat by way of electromagnetic waves

An example of conduction is heating a pot of water on an electric stove. The bottom of the pan is in contact with the hot stove top. Heat flows from the burner to the bottom of the pan and even up the sides and possibly to the handle. Conduction also occurs between the pan and the water, which are also in contact with each other. An example of convection is a forced-air heating system. Warm air is blown and mixed with cooler air to cause heating of the cooler air by the warmer air. An example of heat transfer by radiation is the sun or a hot fire. The sun radiates electromagnetic waves that heat the earth as does a fire that heats your hands or body when you move near. Combinations of these mechanisms are also used. An example is the heating of a home that uses all three principles. Using insulation in the house actually reduces the conduction of heat from hot surfaces inside the house to the colder surfaces outside. A forced-air system along with allowing sunlight into the house can be used inside as well.

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