18.8:
Heat Flow and Specific Heat
When a cup of hot coffee is placed in a room, its temperature reduces as it is attaining thermal equilibrium with the atmosphere.
The energy that is transferred from the coffee due to the temperature gradient is called heat, and the energy transfer that takes place in the process is called heat flow.
The SI unit of heat is the joule. Another standard unit of heat is the calorie, where one calorie is equal to 4.186 joules.
Further, the quantity of heat required to raise the temperature of mass m of a certain material from T1 to T2 is proportional to mass and change in temperature. The proportionality constant c is called specific heat.
Thus specific heat is the amount of heat required to raise the temperature of a unit mass of a substance by one-degree celsius.
If the molar mass, which is the mass per mole of a substance, is used in the formula for specific heat, it is called the molar heat capacity and is denoted by capital C.
18.8:
Heat Flow and Specific Heat
Heat is a type of energy transfer that is caused by a temperature difference, and it can change the temperature of an object. Since heat is a form of energy, its SI unit is the joule (J). Another common unit of energy often used for heat is the calorie (cal), which is defined as the energy needed to change the temperature of 1 g of water by 1 °C, specifically between 14.5 °C and 15.5 °C, since the energy needed shows a slight temperature dependence. Another commonly used unit is the kilocalorie (kcal), which is the energy needed to change the temperature of 1 kg of water by 1 °C. Since mass is most often specified in kilograms, the kilocalorie is more convenient. Confusingly, food calories are actually kilocalories, a fact not easily determined from package labeling.
Heat transfer is the movement of energy from one place or material to another as a result of a difference in temperature. Heat transfer is fundamental to everyday activities such as home heating and cooking, as well as many industrial processes.
Specific heat depends on the material and its phase. For example, the specific heat of water is 5 times that of glass and 10 times that of iron, which means that it takes 5times as much heat to raise the temperature of the water compared to glass and 10 times as much compared to iron. In fact, water has one of the largest specific heats of any material, which is important for sustaining life on Earth. The value of specific heat must generally be measured because there is no simple way to calculate it precisely.
Suggested Reading
- OpenStax. (2019). University Physics Vol. 2. [Web version]. Retrieved from https://openstax.org/details/books/university-physics-volume-2; pp 19–20.