Definition of Heat

Heat is the transfer of thermal energy between two thermodynamic systems as a result of the temperature difference between them. When the two systems are in thermal equilibrium, that is, at the same temperature, there is no heat transfer.

Heat transfer is something we experience every day. When we cook our food and when we store it in the fridge, to the “heat” we feel when the outside temperature rises. All of this involves the transfer of thermal energy and temperature changes.

temperature and heat

Although colloquially we talk about temperature and heat as if they were the same, the reality is that they are two very different concepts, but they are related. The temperature of a system is a macroscopic measure of the average kinetic energy of the atoms or molecules that make it up. The more movement the atoms or molecules of the system have, the higher its temperature will be, and vice versa.

On the other hand, heat is the transfer of thermal energy between two systems when there is a temperature difference between them. For example, when we put ice in a drink, the temperature difference between the two causes heat to be transferred from the drink to the ice until both reach thermal equilibrium and heat transfer stops.

We can also tell this difference between heat and temperature by the units in which it is measured. Temperature is typically measured in degrees Celsius (°C) or Kelvin (K), while heat, being energy, is measured in Joules (J) or calories (cal).

forms of heat transfer

It has already been said that heat is a transfer of energy between two systems, but how can it be carried out? There are three ways of transmission:

• Driving: This transfer occurs by direct physical contact between two or more systems. For example, when we cook something, the flame transfers heat to the pan or grill, causing its temperature to rise, and then the pan or grill transfers heat to the food.

• Convection: It is the transfer of heat through the movement of a gas or a liquid. An example of this is when we sit near a campfire, the fire transfers heat through the air.

• Radiation: In this form of transmission, heat is transferred by means of electromagnetic waves. This is the way the Sun manages to heat our planet.

Heat capacity

It is very useful to have a measure of how much the temperature of some substance varies when a certain amount of heat is transferred to it or when it transfers heat to another body. The heat capacity of a substance is precisely that, it is a ratio between the heat transferred or absorbed and the temperature difference resulting from the process. Mathematically, this is written as:

\(C = \frac{Q}{{{\rm{\Delta }}T}} = \frac{Q}{{{T_f} – {T_i}}}\)

Where \(C\) is the heat capacity, \(Q\) is the heat, and \({\rm{\Delta }}T\) is the temperature difference. The latter is equal to the final temperature (\({T_f}\)) minus the initial temperature (\({T_i}\)). This equation can be rewritten as:

\(Q = C{\rm{\Delta }}T\)

This would be the heat transferred or absorbed by a substance when it has a temperature variation taking into account its heat capacity. Although this definition of heat capacity is very useful, the reality is that heat capacity also varies depending on the mass of the substance or material.

Specific heat

Specific heat can be defined as the heat capacity per unit mass of a substance. That is:

\(c = \frac{C}{m} = \frac{Q}{{m{\rm{\Delta }}T}}\)

Where \(c\) is the specific heat and \(m\) is the mass. More specifically, the specific heat is the ratio between the heat transferred or absorbed by a substance and the mass of the substance, as well as the temperature difference resulting from the process. We can also rewrite this equation as:

\(Q = cm{\rm{\Delta }}T\)

This is the heat that a quantity of a certain substance has to transfer or absorb to achieve a certain temperature difference, all taking into account the specific heat of the substance in question.

To give an example, let’s imagine that we have a container with only 1 gr. of water and we want to find the amount of heat that we have to apply to it to make its temperature rise 1°C. The specific heat of water is 4.184 J/kg•K. Using the previous equation and using the appropriate units we obtain that the heat that we have to apply to achieve this is \(Q = 4.184\) K. This last is the definition of calorie, that is, the amount of heat necessary for 1 gr . of water raises its temperature 1°C. We can also say that water has a specific heat of 1 cal/g•K, this is a unit that is also used regularly for specific heat.

Following

References

David Halliday, Robert Resnick & Jearl Walker. (2011). Fundamentals of Physics. United States: John Wiley & Sons, Inc.