Heat energy, like other forms of energy, is measured in joules (J). Temperature is measured in degrees Fahrenheit (°F), Celsius (°C) or Kelvin (K). In Fahrenheit, water freezes at 32° and boils at 212°. The Celsius scale is based on the boiling and freezing points of water, so these are 100°C and 0°C respectively. Kelvin units are the same as Celsius degrees but they start from the lowest temperature possible. On this scale, water freezes at 273K.
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.
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 situations 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.
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.
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. Infect 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
Picture Credit : Google