The Concept of Energy
Energy in physics is also called work. Work is a force applied through some distance. A force is something like the weight of an object, which is the force of gravity acting on the matter in an object. Energy takes two basic forms, kinetic and potential. Kinetic energy is the energy of motion. A bullet in flight possesses a certain amount of kinetic energy because it is traveling at some velocity relative to its environment. A bullet in a gun magazine possesses a certain amount of potential energy in the chemical explosive that is contained inside the shell. Energy can be converted from one form to another, but it is neither created nor destroyed. This is stated in the principle of the conservation of energy.
Mechanical energy is expressed mathematically as force multiplied by distance. If you lift a weight of one hundred pounds two feet in the air you have used 100 x 2 = 200 foot-pounds of energy. If you just hold the 100 pounds steady in one position you have used 100 x 0 = 0 energy. Even though it may be as difficult to hold the weight steady as it is to lift the weight, you are not imparting any energy to the weight by holding it still. Once you lift the weight, you can recover the energy you put into the weight by letting it drop those two feet. The chemical energy your body put into the weight is converted to potential energy due to the gravitation of the earth acting on the weight. Notice that not only is the energy given by the force applied through the distance lifted, but it is also given by the force multiplied by the distance dropped.
Another form of energy is electrical energy. Electricity has the equivalent of a pressure, which is its voltage. Voltage is also known as electromotive force. Electricity consists of electrons under the influence of some electromotive force. Electrons are everywhere, as constituents of all atoms. All ordinary matter contains electrons. The electrons in metals are free to move from one atom to another because they are not tightly bound to any given atom. This is a property of metals that allows them to conduct electricity. The number of electrons that are moved under the influence of an applied voltage is known as the current. Current is measured in Amperes. One Ampere is a current that consists of one Coulomb of electrons moving some distance for one second of time. One Coulomb of charge is a certain large number of electrons, 6.24150962915265×1018 electrons to be exact. Electrical power is given as voltage multiplied by current. P = V x I, where I is the current in Amperes, V is the voltage, and P is power. A voltage of 120 volts and a current of ten amps gives a power of 1200 Watts. Power is energy divided by time. This means that the given conditions will yield 1200 Joules of energy per second. One Joule of energy is the equivalent of lifting one kilogram ten centimeters in earth’s gravitational acceleration of ten meters per second squared.
If energy seems complicated it is largely because not only are there different ways of measuring it, there are also different kinds of energy. Take chemical energy for instance. The chemical energy of combustion for some fuel is usually given in calories. This is because the energy of combustion is measured in a calorimeter. A calorie is the amount of energy necessary to raise one gram of water one degree Centigrade in temperature. A chemist will burn a known amount of some chemical in a calorimeter and measure how much the temperature of the water in the calorimeter rises. Calories can be converted directly into Joules or foot-pounds of energy through simple multiplication.
Perhaps the most famous equation involving energy is Einstein’s famous mass-energy equivalency formula, E = MC^2. This formula, derived from general relativity, states that energy is equivalent to the mass of an object multiplied by the speed of light squared. This means that there is an incredible amount of energy contained in even the tiniest quantity of matter. Nuclear reactions tap this source of energy by splitting or fusing atomic nuclei into new nuclei that have less mass than the original reactants. Nuclear energy has a downside in that it usually generates radioactive atoms which are a source of radioactive waste.
Energy is present in all matter and is all around us. Our bodies use energy constantly in the form of chemical energy contained in food. Our transportation, manufacturing, farming, heating, water, and all systems of modern society run on energy of one form or another. Without energy, the universe would be a cold dead dull place indeed.
