Energy is defined as the ability to do work, such as moving an object or transferring heat. Understanding energy begins with the Law of Conservation of Energy, which states that energy can neither be created nor destroyed. This means that in any isolated system, the total amount of energy remains constant over time. All physical and biological processes rely on energy existing in various forms and shifting between them.
The Major Forms of Energy
Energy is generally categorized as either potential or kinetic, though it manifests in many specific forms. Potential energy is stored energy that an object possesses due to its position or its internal structure, while kinetic energy is the energy of motion. These two broad categories encompass all other types of energy that are routinely observed and utilized.
Chemical energy is a form of potential energy stored in the bonds between atoms and molecules, such as the energy held in food or fossil fuels. Thermal energy, or heat, is the kinetic energy associated with the random movement of atoms and molecules within a substance. Electrical energy involves the movement of charged particles, typically electrons, and radiant energy is electromagnetic energy that travels in waves, including visible light and X-rays.
The Principle of Energy Transformation
Transformation is the process by which energy changes from one specific form into another. This shift is the only way energy can be utilized, ensuring the total energy before and after the change is precisely the same. The energy that is input into a system must be accounted for in the output, even if it is in a different form than what was intended.
Many processes involve a sequence of changes, referred to as an energy chain or pathway. For instance, a simple process might start with potential energy stored at a height, which converts into kinetic energy as an object falls, and then into thermal energy upon impact. Devices designed to facilitate these conversions, such as a battery or a dam, are referred to as transducers.
How Energy Changes in Common Systems
A car engine provides a clear example of a transformation chain, starting with chemical energy stored in gasoline. When the fuel ignites in the engine’s cylinders, this chemical energy converts into thermal energy, which rapidly expands the gases. The expanding gases then push against the pistons, converting the thermal energy into the mechanical motion of the crankshaft.
A simple incandescent light bulb demonstrates the conversion of electrical energy into light and heat. As electric current flows through the thin tungsten filament, its electrical energy encounters resistance, causing the filament to heat up to thousands of degrees Celsius. The filament then emits radiant energy, or light, along with a substantial amount of thermal energy.
In a hydroelectric dam, the process begins with gravitational potential energy, which is stored in the mass of water held back at a height by the dam. When the water is released, its potential energy converts to the motion of the falling water, becoming kinetic energy. This moving water then strikes and spins a turbine, which is connected to a generator that converts the mechanical kinetic energy into electrical energy.
Accounting for Energy Loss
Although the total amount of energy is conserved during transformation, the amount of energy that is successfully converted into the desired form is never 100% efficient. Some energy will always be converted into a less useful form, typically waste heat. This unavoidable outcome results from mechanisms like friction, air resistance, and electrical resistance.
Friction in mechanical systems, for example, converts organized kinetic energy into disorganized thermal energy that dissipates into the environment. Similarly, electrical resistance in wires and devices converts a portion of the electrical energy into heat. This “lost” energy still counts toward the total energy conserved in the system, but since it is dispersed as low-grade heat, it is generally considered unusable for performing further work.