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# Heat is the Transfer of Thermal Energy

by Ron Kurtus (revised 24 November 2008)

Heat is defined as the transfer of thermal energy. When you heat an object, you are transferring thermal energy to it from an another object that is at a higher temperature. Heat is the amount of thermal energy that is transferred between the two objects due to a temperature difference. Heat transfer between objects is done by conduction, convection and radiation. The standard unit of heat measurement is the calorie.

Questions you may have include:

• What is heat?
• How does heat get from one object to another?
• What is the measure of heat?

This lesson will answer those questions.

Useful tool: Metric-English Conversion

## Heat is energy in transit

Heating an object is when you are transferring thermal energy to the object from to another object that is at a higher temperature. Heat is often defined as energy in transit or the the flow of energy. Thermal energy is the energy itself.

Thermal energy is the amount of internal kinetic energy and potential energy of an object. It is also simply called internal energy. Temperature is a measure of the average kinetic energy of the particles in an object.

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

Thermal 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. Heat is the amount transferred.

### Conduction

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

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 heat 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.

## Summary

Heat is the amount of thermal energy that is transferred between the two objects due to a temperature difference. Heat transfer between objects is done by conduction, convection and radiation. The standard unit of heat measurement is the calorie.

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