Law of Conservation of Energy
The law of conservation of energy is one of the basic laws of physics along with the conservation of mass and the conservation of momentum. The law of conservation of energy states that energy can change from one form into another, but it cannot be created or destroyed. Or the general definition is:
The total energy of an isolated system remains constant over time.
Energy can be defined as the capacity for doing work. It may exist in a variety of forms and may be transformed from one type of energy to another in hundreds of ways.
For example, burning gasoline to power cars is an energy conversion process we rely on. The chemical energy in gasoline is converted to thermal energy, which is then converted to mechanical energy that makes the car move. The mechanical energy has been converted to kinetic energy. When we use the brakes to stop a car, that kinetic energy is converted by friction back to heat, or thermal energy.
A consequence of the law of conservation of energy is that a perpetual motion machine of the first kind, which produces work without the input of energy, cannot exist.
The concept of energy conservation is widely used in many fields. In this article the following fields are discussed:
- Conservation of Mechanical Energy
- Conservation of Energy in Fluid Mechanics
- Conservation of Energy in Thermodynamics
- Conservation of Energy in Electrical Circuits
- Conservation of Energy in Chemical Reactions
- Conservation of Energy in Special Relativity Theory
- Conservation of Energy in Nuclear Reactions
Law of Conservation of Energy in Classical Physics
Conservation of Mechanical Energy
First the principle of the Conservation of Mechanical Energy was stated:
The total mechanical energy (defined as the sum of its potential and kinetic energies) of a particle being acted on by only conservative forces is constant.
Emech = U + K
The potential energy, U, depends on the position of an object subjected to a conservative force.
It is defined as the object’s ability to do work and is increased as the object is moved in the opposite direction of the direction of the force.
The potential energy associated with a system consisting of Earth and a nearby particle is gravitational potential energy.
The kinetic energy, K, depends on the speed of an object and is the ability of a moving object to do work on other objects when it collides with them.
K = ½ mv2
The above mentioned definition (Emech = U + K) assumes that the system is free of friction and other non-conservative forces. The difference between a conservative and a non-conservative force is that when a conservative force moves an object from one point to another, the work done by the conservative force is independent of the path.
In any real situation, frictional forces and other non-conservative forces are present, but in many cases their effects on the system are so small that the principle of conservation of mechanical energy can be used as a fair approximation. For example the frictional force is a non-conservative force, because it acts to reduce the mechanical energy in a system.
Note that non-conservative forces do not always reduce the mechanical energy. A non-conservative force changes the mechanical energy, there are forces that increase the total mechanical energy, like the force provided by a motor or engine, is also a non-conservative force.
The Law of Conservation of Energy – Nonconservative Forces
We now take into account nonconservative forces such as friction, since they are important in real situations. For example, consider again the pendulum, but this time let us include air resistance. The pendulum will slow down, because of friction. In this, and in other natural processes, the mechanical energy (sum of the kinetic and potential energies) does not remain constant but decreases. Because frictional forces reduce the mechanical energy (but not the total energy), they are called nonconservative forces (or dissipative forces). But in the nineteenth-century it was demonstrated the total energy is conserved in any process. In case of pendulum its initial kinetic energy is all transformed into thermal energy.
For each type of force, conservative or nonconservative, it has always been found possible to define a type of energy that corresponds to the work done by such a force. And it has been found experimentally that the total energy E always remains constant. The general law of conservation of energy can be stated as follows:
The total energy E of a system (the sum of its mechanical energy and its internal energies, including thermal energy) can change only by amounts of energy that are transferred to or from the system.