Energy Conversion and Conservation: Laws, Examples, and Calculations

Energy conversion and conservation

Energy Conversion Energy can be converted from one form to another. The process of changing energy from one form to another is energy conversion. Energy conversions are constantly taking place all around you, often without you noticing. Light bulbs convert electrical energy into thermal energy and electromagnetic energy. In some cases, energy is converted from one form into another in a series of steps. Friction between the match and the matchbox converts some of the match’s kinetic energy into thermal energy. The thermal energy triggers a chemical reaction on the match tip, releasing some of the match’s stored chemical energy. The stored chemical energy is then converted into thermal energy and electromagnetic energy in the flame.

Conservation of Energy

When energy changes from one form to another, the total energy remains unchanged even though many energy conversions may occur. The law of conservation of energy states that energy cannot be created or destroyed. According to the law of conservation of energy, energy can be converted from one form to another. However, in a closed system, the amount of energy present at the beginning of a process is the same as the amount of energy at the end. (In a closed system, nothing can enter or leave.) The work done by friction changes kinetic energy into thermal energy. When the energy lost to frictional forces is accounted for, energy is conserved overall. Recall that friction within machinery reduces efficiency. Friction is a major cause of energy consumption in cars and factories. All moving parts are subject to friction. You can reduce friction but you can’t avoid it. Friction is everywhere.

Energy Conversions

One of the most common energy conversions is between potential energy and kinetic energy. The gravitational potential energy of an object is converted to the kinetic energy of motion as the object falls. Conversions between kinetic and potential energy can happen in both directions, that is, from kinetic to potential energy, or from potential to kinetic energy.

Energy Conversion in Pendulums

A pendulum consists of a weight swinging back and forth from a rope or string. Pendulums were used in the first truly accurate clocks. The Dutch scientist Christiaan Huygens (1629–1695) made the first pendulum clock in 1656. Kinetic energy and potential energy undergo constant conversion as a pendulum swings. At the highest point in its swing, the pendulum is momentarily motionless as it changes direction. At this point, the weight at the end of the pendulum has zero kinetic energy and maximum potential energy. As the pendulum swings downward, potential energy is converted to kinetic energy. At the bottom of the swing, the pendulum has maximum kinetic energy and zero potential energy. The pendulum then moves upward again, repeating the process. Eventually, frictional forces slow down the pendulum.

Energy Conversion in the Pole Vault

The pole vault is a difficult track and field event that requires a combination of speed, strength, timing, and energy conversion. In order to start the jump with as much kinetic energy as possible, the pole-vaulter sprints down the runway as fast as he can. At the end of his sprint, he plants the end of a long pole at the base of the high bar and propels himself into the air. The pole-vaulter’s kinetic energy is partially converted into elastic potential energy as the pole bends. The pole springs back into shape, propelling the pole-vaulter upward, hopefully high enough to clear the bar. As the pole-vaulter soars, his kinetic energy decreases while he gains gravitational potential energy. Once the highest point has been reached, his gravitational potential energy begins to convert back to kinetic energy. The pole-vaulter picks up speed as he falls back to the ground.

Energy Conversion Calculations

When friction is small enough to be ignored, and no mechanical energy is added to a system, then the system’s mechanical energy does not change. Recall that mechanical energy is the total kinetic and potential energy of an object. Mechanical energy = KE + PE. You can apply the law of conservation of energy to any mechanical process. A mechanical process can be any action, such as the motion of a pendulum, water falling in a waterfall, or a diver propelled by a diving board. In all of these processes, if friction can be neglected, the mechanical energy at the beginning equals the mechanical energy at the end. That is, total mechanical energy remains constant. This equality can be stated as follows. (KE  PE)beginning +(KE  PE)end

Energy and Mass

Physicist Albert Einstein (1879–1955), developed his special theory of relativity in 1905. This theory included the now-famous equation E = mc2. In Einstein’s equation, E is energy, m is mass, and c is the speed of light. This seemingly ordinary equation has surprising consequences. Einstein’s equation, E = mc2, says that energy and mass are equivalent and can be converted into each other. In other words, energy is released as matter is destroyed, and matter can be created from energy.