In the chapter on Work, Energy, and Energy Resources, we learned that doing work on an object—applying force through a distance—transfers energy, often in the form of mechanical energy. In Temperature, Kinetic Theory, and the Gas Laws, we saw that temperature is proportional to the average kinetic energy of atoms or molecules. A system at higher temperature has greater internal energy due to this microscopic motion.

When two objects at different temperatures are brought into contact, energy spontaneously flows from the hotter object to the cooler one. No mechanical work is done—no force acts through a distance—but energy is transferred simply because of the temperature difference. This transfer of energy is defined as heat.

Heat is therefore the energy transferred due solely to a temperature difference between systems. Once the two objects reach the same temperature—known as thermal equilibrium—heat transfer ceases.

This leads to a common misconception: heat is not the same as temperature. People often say things like “the heat was unbearable” when they really mean the temperature was high. Heat refers to the transfer of energy, not the thermal state of an object.

Units of Heat

Since heat is energy, it is measured in joules (J) in the SI system. Another commonly used unit is the calorie (cal), defined as the energy required to raise the temperature of 1 gram of water by 1ºC, typically from 14.5ºC to 15.5ºC. A more practical unit in biology and nutrition is the kilocalorie (kcal), which is the energy required to raise the temperature of 1 kilogram of water by 1ºC. This is the same unit used in food labels, where 1 Calorie (with a capital C) = 1 kilocalorie = 1000 calories.

Mechanical Equivalent of Heat

In the 1800s, James Prescott Joule demonstrated that work—like turning a paddle in water—can also increase temperature. He showed that work and heat are both forms of energy transfer and can produce identical effects. This led to the concept of the mechanical equivalent of heat, the amount of mechanical work needed to produce the same energy transfer as heat.

This equivalence tells us that energy is conserved regardless of the transfer method—work or heat—and both can increase a system’s internal energy.

Joule’s insight helped establish the principle of conservation of energy, a foundational concept in all physical sciences. Importantly, it showed that internal energy can be increased not only by adding heat but also by performing work. However, while a system has a well-defined internal energy, it does not have a specific “heat content.” Heat only exists during transfer.

Check Your Understanding

Summary

• Heat and work are the two primary methods of energy transfer.
• Heat is the energy transferred due to a temperature difference between objects.
• Units of heat include the joule (J) and kilocalorie (kcal).
• 1 kcal is defined as the energy required to raise 1.00 kg of water by 1.00ºC.
• The mechanical equivalent of heat is given by [latex]1.00 \ \text{kcal} = 4186 \ \text{J}[/latex].