The conservation of energy

Thermodynamics is based on three principles:

  • temperature (or the zero principle)
  • the first principle
  • the second principle

If we say that temperature is the zero principle, it is because this notion has often been used without being really defined.

The first principle, which we are going to see today, is neither more nor less than the conservation of energy. It may seem trivial, but it allows us to see properties that are not so obvious.

To apply the first principle of thermodynamics, we start by studying a system and determining the parameters that define its state. We can take the example of the system of a thermal engine with pistons whose gas we study. Thus, the parameters that can vary are: temperature, pressure and volume. The transformations will thus exchange energy with the outside. In the case of the engine, it will exchange a mechanical work thanks to the pistons. Thus, the gas will exert on the piston a driving force on the wheels of the car.

The various forms of energy

Energy comes in many forms and it is often possible to convert one energy into another. The goal of thermodynamics is to optimize this conversion and to obtain a maximum of this energy in a reusable form.

Mechanical energy

The different forms of mechanical energy are :

  • kinetic energy : due to a movement
  • gravitational potential energy : due to its position in a gravitational field (ex.: hydroelectric dam)
  • elastic potential energy (e.g. a compressed or extended spring)

Electrical energy

It can be seen as a special case of mechanical energy, because it is interpreted as resulting from the action of electrostatic forces. It is the movement / transfers induced by differences in electrical potential.

It is an energy difficult to store, it passes through inductances or capacitors, but this stores only a small amount of energy. It is then necessary to use batteries, but it is then necessary to use chemistry.

The chemical potential energy

The energy is stored by the chemical bond in the molecules. It is the main source of energy. It is used in batteries and in the combustion of coal, wood, oil or gasoline. Combustion converts chemical potential energy into heat.

Electromagnetic energy

This energy is moved by electromagnetic waves. For example, filament bulbs convert the heat produced by passing an electric current into light (electromagnetic energy).

Energy conversions

All conversions between different forms of energy are possible, but the efficiency is not always 100%. During a transformation where there is an exchange of heat, it is not possible to convert the entire amount of heat into a macroscopically ordered form of energy.

Final form \ Initial formDisordered microscopic energyMechanicalElectricalChemicalElectromagnetic

Disordered microscopic energy

-Friction force Efficiency = 100%Joule effect Efficiency = 100%Reaction
chemical yield 100%
Absorption by a black body = Efficiency close to 100%
MechanicalThermal engines (engines of very large boats) Max efficiency = 55%-Electric motor Efficiency close to 100%.Micromotors, biology (muscles: efficiency of the order of 10%)No simple direct conversion except for solar sails in space (about 10% efficiency)
ElectricalThermoelectric effect Very low efficiency (a few %)
Alternator, dynamo Efficiency close to 100%
Batteries and cells Efficiency above 90% for some batteries
Photovoltaic cell Efficiency ≈ 20% (even over 45% for some technologies).
Displacement of chemical equilibria
Shift of chemical equilibrium due to pressure
Reaction induced by photons (example: photosynthesis)
Hot body radiation in the visible range Efficiency = 2-3%.
No simple direct conversion
Antenna, diodes Efficiency 10 to 20 % for diodes

Statement of the first principle

Every system has an associated state function U, called internal energy, which characterizes the stored energy and which depends only on the state of the system. During any transformation, the variation of U is equal to the energy received by the system, which can be translated as:

ΔU = Q + W, where Q is the amount of heat received by the system, W is the sum of work received, and ΔU = Ufinal - Uinitial.
U is a state function, i.e. only the final state counts, we are not interested in the intermediate transformations that served to arrive at this state.

For example, for thermal machines that we will study later in the context of heat pumps, the systems undergo cycles and go through the same states. When the system returns to the initial state, its internal energy must be identical. So after a complete cycle, the internal energy must be zero.

The thermal capacities

  • Cp : is the coefficient which relates the quantity of heat exchanged to the temperature increase, the pressure being invariant.
  • Cv : is the coefficient which relates the quantity of heat exchanged to the temperature increase, the volume being invariant.

The enthalpy

From the internal energy, we can form other state functions.

The enthalpy function, generally noted H, is H = U + PV.
Since pressure P, volume V and internal energy U are state functions, H is also a state function.

This function is very useful for isobaric transformations (at constant pressure), because its variation is identified with the amount of heat exchanged.

It remains only: ΔH = Q


All these notions (I passed some of them in order not to weigh down the text, if you want more precision send me an email or Wikipedia is your best friend!) are essential for the study of thermodynamic systems.

Tomorrow, we will discuss the second principle of thermodynamics.

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