Basic Physics

# Definition of heat, mechanical equivalent of heat, equation of heat

1. Definition of heat

If objects that have temperature differences touch each other, there will be heat transfer from high-temperature objects to low-temperature objects. Heat transfer stops after touching objects reach the same temperature. For example, if we mix hot water with cold water, usually heat moves from hot water to cold water. When we put hot iron into cold water, heat moves from iron to water. Heat will stop flowing after iron and water reach the same temperature. When a nurse attaches a thermometer to your body, heat moves from

your body goes to the thermometer. Heat transfer stops after your body and thermometer have reached the same temperature. If the thermometer used is a mercury thermometer, when your body and thermometer reach the same temperature, the surface of the mercury stops moving. The number shown on the surface of mercury is your body temperature at that time.

Naturally, heat automatically moves from high-temperature objects to

low-temperature objects. Heat transfer tends to equalize the temperature of objects that touch each other. In the 18th century, physicists suspected that heat flow was the movement of a fluid, a type of fluid that was not visible (fluid is a substance that can flow. Fluid includes liquid and gaseous materials. Water (liquid) includes fluid because it can flow. Air also includes fluid because it can flow). The fluid is called caloric. The theory of caloric is not used anymore because based on the results of the experiment, the existence of this caloric cannot be proven. In the 19th century, an English physicist named James Prescott Joule (1818-1889) learned how to heat water in a container using a stirring wheel.

Based on the results of the experiment, Joule made a comparison with heated water using fire. When a flame and a container containing water in contact, heat moves from fire (high temperature) to water (low temperature). After making comparisons between rising water temperature due to contact with fire and rising water temperature due to work done by the stirrer, Joule concluded that heat is the energy that moves from a high-temperature object to a low-temperature object. Heat is not energy (heat is not a certain type of energy, such as kinetic energy, potential energy, chemical energy, etc.). Heat is energy that moves due to temperature differences. So when heat flows from high-temperature objects to low-temperature objects, energy moves from high-temperature objects to low-temperature objects. Energy transfer stops after touching objects reach the same temperature.

2. Unit of heat and the mechanical equivalent of heat

Heat units are calories (abbreviated as cal). Calories are the amount of heat needed to raise the temperature of 1 gram of water by 1 Co (precisely from 14.5 oC to 15.5 oC). The amount of heat needed varies for different water temperatures. For the same amount of heat, a water temperature increase of 1 Co only occurs between temperatures of 14.5 oC to 15.5 oC. Heat units are often used, especially to state the value of food energy is kilocalories (kcal). 1 kcal = 1000 calories. Other names from 1 kcal = 1 Calorie (big K letter). The heating unit for the British system is Btu (British thermal unit). 1 Btu = the amount of heat needed to raise the temperature of 1 pound of water by 1 Fo (Exactly from 63 oF to 64 oF).

Heat is related to energy, so we need to know the relationship between heat units and energy units. Based on experiments conducted by Joule and other scientists, it is known that a work of 4.186 Joule is equivalent to 1 calorie of heat.

1 calorie = 4.186 Joule

1 kcal = 1000 calories = 4186 Joule

1 Btu = 778 ft.lb = 252 calories = 1055 Joule

(1 calorie = 4.186 Joule and 1 kcal = 4186 called the mechanical equivalent of heat)

3. The equation of heat

If objects that have temperature differences touch each other, naturally heat moves from high-temperature objects to low-temperature objects. Heat transfer stops after both objects are in thermal equilibrium. Heat transfer causes changes in temperature of objects that touch one another. Based on this brief explanation, can conclude that heat (Q) has a relationship with an object and changes in temperature (ΔT) of the object, object’s mass (m) and type of object.

The relationship between heat (Q) and temperature change (ΔT)

The more heat, the greater the temperature change. It can be concluded that the amount of heat (Q) is proportional to the change in temperature of the object.

Q α ΔT —– Comparison 1

The relationship between heat (Q) and mass of objects (m)

The amount of heat (Q) is proportional to the change in temperature of the object. The more water, the more heat is needed to increase the temperature of the water.

Q α m —– Comparison 2

The relationship between heat (Q) and type of object (c)

The heat needed to raise the temperature of an object is proportional to the kind of object.

Q α c —– Comparison 3

The three comparisons above are put together into an equation:

Q α m c ΔT

Q = m c ΔT

This is the equation of heat.

Description: Q = heat (Joule), m = mass of object (kg), c = specific heat (J / kg Co or K)