Day 97

1st Law of Thermodynamics

Thermodynamics is the study of the quantitative relationships between heat and other forms of energy.

The internal energy of a substance is the total kinetic and potential energies of the particles of the substance. (NOTE: Remember that the average kinetic energy of the particles of a substance is the temperature of the substance. Therefore, as temperature increases, internal energy increases and vice versa.)

The process by which energy is exchanged between object (in thermal contact) because of differences in their temperature is called heat. (Internal energy that moves is heat.) Substances in thermal contact seek thermal equilibrium, leading to this process.

This leads to the zeroth law of thermodynamics (law of equilibrium):

If objects A and B are separately in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other.

This statement, while seemingly obvious, makes it possible to define temperature. Thus two objects in thermal equilibrium are at the same temperature.

The First Law of Thermodynamics is a special case of the Law of Conservation of Energy and encompasses changes in internal energy (the total available potential and kinetic energy of the particles of a substance, formerly called thermal energy). This law states that the quantity of energy supplied to any isolated system in the form of heat is equal to the work done by the system plus the change in internal energy of the system. In equation form:

U =  = Q + W

(U = internal energy, Q = change in energy transferred by heat, W = work done)

This means that when heat is converted to other forms of energy or other forms of energy are converted to heat, there is no loss of energy, satisfying the law of conservation of energy.

The total internal energy of an isolated system remains constant. If the system is not isolated, there is a change in the total internal energy, but as a whole, the energy in the universe always remains constant.

A process in which no heat is added or removed is called an adiabatic process. This is the case in an isolated system. In this case, the work done would equal zero.

Q=0, work can be done to or by the system – it would result in a change in U, the internal energy.

Systems are often not isolated, however.

In a cyclic process (originates and ends in the same state), the final U = initial U, so the change in energy must again be zero. This means that the energy added to the system must equal the negative of the work done on the system in the course of one cycle. Heat engines (heat energy is converted to mechanical energy such as internal combustion engines) are an example of cyclic processes. A cyclic process occurs when you have a gas confined in a cylinder by a piston and the gas continually exchanges energy by heat and work with its surroundings. (This is an example of adiabatic expansion because, during one cycle, there is no time for heat to be transferred.)

An isothermal process is one in which the temperature is kept constant. Again, this is an idealized process. The work done on an ideal gas in this type of system would have to equal the negative of the energy added by heat. In this case, the engine supplies work to the outside world. The change in internal energy is equal to zero.

NOTE: On a microscopic scale, no practical distinction exists between the results of energy transfer by heat and work!

When pressure is kept constant, the system undergoes an isobaric process. Calculus is required to determine the exact work done in this case. The work done and the energy transferred by heat are both nonzero. So change in internal energy = heat transferred + work done.

An isovolumetric process is one in which the volume is kept constant. Work done is equal to zero because there is no change in volume. So change in internal energy = heat transferred.

The work done on a system depends on the process by which the system goes from the initial to the final state.


When applying the First Law of Thermodynamics to human metabolism, you find that, on the average, energy (Q) flows out of the body and work (W) is done by the body on its surroundings and both Q/∆t and W/Δt change in time are negative. This means that ∆u/Δt would be negative and the internal energy and body temperature of a human would decrease with time. This would happen if a human were a closed system with no way of ingesting matter or replenishing its internal energy store. However, all animals are open systems and add internal energy to themselves through the processes of eating and breathing.

Read over the different kinds of processes again and look at the different examples on the websites.

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