There are many ways to detect heat. The method chosen often depends on what heat source we are trying to measure. For example, the way we detect the heat in the air is different from how we detect heat from a fire or heat from objects in deep space.

We have all felt various levels of heat. Our skin is a good detector of heat and we interpret the average molecular motion within an object as a feeling that the object is hot or cold. However our skin does not always give us consistent measurements of heat energy.


Get three bowls of water - one very cold, one lukewarm - at room temperature, and one very warm - from the faucet (not too hot to avoid burns). Place one hand in the hot water and one in the ice water for 5 seconds. then place both hands in the lukewarm water. You will notice that the water feels warm to the hand that was in the cold water and cool to the hand that was in the hot water. Our skin gives us information about the temperature difference between our skin temperature and what it is touching but is not accurate for measuring the temperature itself.

For this we need special instruments which can accurately measure temperature, like a thermometer. Thermometers, and other temperature measuring devices, are used to get a quantitative measure of the average motion of the molecules in a substance. They interpret this average molecular motion as a certain number of degrees which we call the temperature.

We have all used thermometers to measure the level of heat but sometimes we need to measure heat in places where you can't put a thermometer. For example, in space, in molten metals and in hot fires. To make measurements in these situation we need instruments which can measure heat without touching the heat source. These instruments measure the heat radiation emitted by the heat source. Examples of these types of devices are infrared cameras and detectors.

An infrared array detector used to measure the heat energy from objects in space.

An outdoor thermometer used to measure the average thermal energy in the air.

A thermal infrared camera used to image heat. .

Heat is measured in quantities called joules (pronounced the same as jewels) in the metric system and in British Thermal Units (BTU) in the English system. Heat can also be measured in calories.

The unit Joule is named after the English physicist James Prescott Joule (1818 - 1889). He discovered that heat is a type of energy.

Joule's experiment was ground breaking because he demonstrated that we can heat water without using fire. He put water in a glass with a thermometer to monitor the increase in heat. Then he added a paddle system and turned it vigorously. After a while he realized that the water temperature had increased. Infact he repeated this experiment many times with different systems and always reached the conclusion that 4.19 Joule of work was required to raise the temperature of 1 gram of water by 1 degree Celsius.

A BTU is the amount of heat needed to raise temperature of one pound of water by one degree Fahrenheit.

1 BTU = 1,000 joules

A calorie is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius.

1 calorie (cal) = 4.186 Joules


A candy bar has 150 kilocalories per serving and has two servings per bar. How many Joules does it have?

How many calories do we need to heat 2 grams of water from 20 degrees Celsius to 22 degrees Celcius?

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