Category: 1. Conservation of energy

The law of conservation of energy states that energy can neither be created nor destroyed – only converted from one form of energy to another. This means that a system always has the same amount of energy, unless it’s added from the outside. This is particularly confusing in the case of non-conservative forces, where energy is converted from mechanical energy into thermal energy, but the overall energy does remain the same. The only way to use energy is to transform energy from one form to another.

The amount of energy in any system, then, is determined by the following equation:

• is the total internal energy of a system.
• is the initial internal energy of a system.
• is the work done by or on the system.
• is the heat added to, or removed from, the system.

It is also possible to determine the change in internal energy of the system using the equation:

This is also a statement of the first law of thermodynamics.

While these equations are extremely powerful, they can make it hard to see the power of the statement. The takeaway message is that energy cannot be created from nothing. Society has to get energy from somewhere, although there are many sneaky places to get it from (some sources are primary fuels and some sources are primary energy flows).

Early in the 20^th century, Einstein figured out that even mass is a form of energy (this is called mass-energy equivalence). The amount of mass directly relates to the amount of energy, as determined by the most famous formula in physics:

• is the amount of energy in an object or system.
• is the mass of the object or system.
• is the speed of light, roughly .

We often say motor produces work. But in reality, the motor does not give us any work if electrical energy is not supplied to it. Similarly, electricity is generated by losing some mechanical or thermal energy. The net change of the internal energy of the system and surroundings is zero.

The change of internal energy is independent of the path or way of transformation but heat change and work done are dependent on the path of transformation. For example, when the element zinc reacts with copper sulfate in solution, a considerable quantity of heat is produced. But if the same redox reaction takes place in the voltaic cell practically no heat evolved and the internal energy converted into electric work.

If oxidation of the mixture of gasoline or burnt gasoline is in the air the large quantity of heat is produced. But if the same change is carried out in the engine, the combustion pushes out the piston and performs work. Here engine produces a Lasser quantity of heat than the oxidizing process. Therefore the magnitude and work done very with the condition of the experiment in thermodynamics energy conservation.

In an isolated system, neither energy nor matter can be transfer to or from it. Therefore, q = 0 for isolated system. Hence from the energy conservation principle, w = – dU. Therefore, the isolated system uses internal energy for doing work.

First law of thermodynamics for cyclic process

Internal energy changes in the cyclic process, dU = 0. Therefore, according to the first law, q = w. Hence from the energy conservation principle, heat is completely converted into work for the cyclic process.

Work done in isothermal process

Internal energy change in the isothermal process for the ideal gas, dU = nc_vdT = 0. Therefore, q = w. Hence, thermodynamics heat is completely converted into work for the ideal gas in the isothermal process.

Energy change in isochoric process

Volume change in isochoric process, dV = 0. From the energy conservation principle, q = dU = nc_vdT. Therefore, energy change to an isochoric process only increases the internal energy or temperature of the system.

Let q amount of heat supplied to the system containing one mole gas in a cylinder fitted with a frictionless weightless moveable piston. At constant pressure, the gas molecules expand from volume V₁ to V_2, and the temperature changes from T₁ to T₂. According to the 1st law of thermodynamics, q = dU + w, where dU = change of internal energy. If the work is restricted to pressure volume work or mechanical work then w = PdV. Hence, q = dU + PdV.

Heat change in thermodynamics for ideal or real gases derived from the ist law of thermodynamics and ideal gas law or Van der Waals equation. All ideal and real gases obey these energy conservation formulas.

Law of conservation of energy

Law of Conservation of energy or conservation of energy principle in thermodynamics states that the total heat or energy of our universe must remain constant. The relation between heat and work is the origin of the first law of conservation of energy. The first law of energy conservation in thermodynamics states that energy can neither be created nor be destroyed but may be transferred from one form to another. In other words, when one form of energy disappears, an exact and same amount of other form appears to maintain the net amount of energy in our universe. Hence a definite quantity of electrical energy is given to be equivalent quantity of heat or mechanical work.

In chemistry or physics, the conservation of energy principle provides the calculation and formula for specific heat change and transfer in cyclic, isothermal, isochoric, and isolated processes in terms of internal energy and work done by the system.