The law of conservation of energy is one of the basic laws of physics and therefore governs the microscopic motion of individual atoms in a chemical reaction. The law of conservation energy states:
In SI units, energy has units of Joules. 1 Joule = 1 kgms.
Some forms of energy:
Since everything has a microscopic origin, the last three are really special cases of potential and kinetic energies, however, the classification is useful.
The kinetic energy of an object of mass , moving with a velocity is given by
Potential energy is a little less straightforward. Since it is an energy of location with respect to some reference point, the potential energy of an object depends on the specific situation. An example is gravity. An object of mass at a height above the Earth's surface has a gravitational potential energy
In general, the potential energy may not be such a simple function of location. In this case, one needs a potential energy curve to describe the potential energy as a function of some coordinate describing the object's location. Consider the example of a mass attached to a spring moving in one spatial dimension. Let represent the mass's position along the -axis, and let represent its equilibrium position. As the mass stretches the spring (), its potential energy increases, and as it compresses the spring compresses (), its potential energy increases as well. The potential energy can be described by a potential energy function that is symmetric about , as represented in the figure below:
Notice, also, that the mass actually moves under the action of a force, which also changes as a function of . In fact, the force exerted by the spring on the mass can be determined from the potential energy curve via
Consider the case of a diatomic molecule:
The distance between the atoms A and B is . The chemical forces that hold the molecule together come from a potential energy curve that depends only on the distance between them. Such a curve might look like:
The presence of a deep ``well'' indicates a particular distance of lowest potential energy. This corresponds to the bond length of the molecule and is, therefore, the most likely value of the separation of the two atoms (think of a ball placed in a gully of this shape - if placed at the bottom of the well, the force on it would be 0, hence it would not move, but would remain there forever unless disturbed).
According to Newton's second law of motion, force is the action that causes a change in the motion of a particle or sets it in motion from rest. Consider a particle of mass moving in one dimension so that its position is . If a force acts on the particle, it will be set in motion, so that is no longer a fixed number but a function of time . At any instant in time, the particle will have a velocity defined to be the rate of change of with respect to time
Since motion generally occurs in three rather than one dimension, position, velocity, acceleration, and force are all vector quantities. The position is usually denoted , and has three components . Similarly velocity has components , accleration , and . In vector notation, Newton's second law reads
Work: Consider again a particle of mass moving in one spatial dimension. If a force is needed to move the particle from position to position , then mechanical work has been done on the particle. Since, as we have seen, can be a function of , the general definition of work is the area under the force function between and , i.e. the integral