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by Ron Kurtus (revised 7 November 2014)

To heat an object or system means to transfer thermal energy from an object of higher temperature to the lower temperature object. This process of energy transfer is called heating the object. Also, feeling heat means sensing the increase in temperature.

Note: Many sources state that "heat is the transfer of energy." Unfortunately, that wording can be confusing and counterintuitive. Ask your teacher what definition is used in your class.

An object does not possess "heat". The appropriate term is thermal energy. This thermal energy may be transferred by conduction, convection, or radiation.


Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion

Process of heating

Heating an object means you are transferring thermal energy from a higher temperature object in order to raise another object's temperature.

An object feels warm or hot if its temperature is higher than your skin. To say something is hot means its temperature is relatively high.

Cooling an object is when you are transferring thermal energy from the object from an another object that is at a lower temperature. You could say you are removing thermal energy from your object.

An object feels cool or cold if its temperature is lower than your skin. To say something is cold means its temperature is relatively low.

Whether heating or cooling, the end result is that the two objects become the same temperature after a period of time. This is called thermal equilibrium.

Heat transfer

Heat energy is transferred from an object of high temperature to one of lower temperature by conduction, convention and radiation. This process is usually called heat transfer or heat flow, although it is the thermal energy that is really being transferred.

(See Heat Transfer for more information on that subject.)


Conduction is when materials are in physical contact and kinetic energy is transferred through collisions of their particles, according to the Kinetic Theory of Matter.



Convection is the movement of thermal energy from one area to another in a liquid or gas.


Radiation is when warm or hot matter emits electromagnetic radiation—especially infrared—that is then absorbed by an object at a distance. The absorption heats the second object.

Units of heat

The amount of heat or thermal energy transferred from one object to another can be measured in joules, which is the unit of energy. But more often, you see heat measured in calories. A calorie (cal) is the amount of energy required to raise the temperature of 1 gram of of water by 1° C.

The relationship between joules and calories is: 1 cal = 4.186 J.


A kilocalorie (kcal) equals 1000 calories. Transferring 1 kcal of heat to 1 kilogram of water will increase its temperature 1° C. A kilocalorie is also called a Calorie (with a capital "C") by those dealing with food and diets. When you hear that some food has 200 Cal, that means it has the potential of transferring 200 kilocalories of heat energy to the body.


In the United States, some use the BTU (British Thermal Unit) as a unit of heat transfer. A BTU is defined as the quantity of energy necessary to raise the temperature of 1 lb. of water 1° Fahrenheit. Often the BTU is used to indicate the heat capacity of a home furnace.


To heat an object means to transfer thermal energy from the higher to lower temperature object. Heat transfer between objects is done by conduction, convection and radiation. The standard unit of heat measurement is the calorie.

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