What Does Energy In Equals Energy Out Mean?

The Law of Conservation of Energy

The law of conservation of energy, also called the first law of thermodynamics, states that energy can neither be created nor destroyed, only transformed from one form to another. This law applies to isolated systems, where no outside energy can enter or leave.

The law of conservation of energy is based on the fundamental observation that energy comes in many different forms that can be converted into one another, such as kinetic energy, potential energy, thermal energy, chemical energy, nuclear energy and more. However, the total quantity of energy remains the same.

For example, when amoving object collides with a stationary object, the kinetic energy of the moving object is converted into heat and sound energy upon impact. The kinetic energy appears to disappear, but it has simply changed form. The total energy before and after the collision remains constant.

The law of conservation of energy states that energy cannot disappear into nothing or appear from nothing. In an isolated system, the amount of energy remains fixed, though it can change forms. This is why this law is considered one of the most fundamental laws in physics, and is a key component of the study of thermodynamics.

Energy In

Energy in refers to the energy entering a system, such as an organism or machine. There are many different forms of energy that can enter a system, including:

Chemical potential energy: The energy stored in the chemical bonds of molecules like food, fuel, and batteries. This is one of the most common forms of energy entering living organisms, as they obtain chemical potential energy by breaking down food.

Heat: The transfer of thermal energy between objects. Heat energy can enter a system through radiation, convection, or conduction. For example, an engine takes in heat from burning fuel.

Motion: The kinetic energy of moving objects entering a system. Wind turbines and water wheels take in the motion of air and water to generate electricity.

Electromagnetic radiation: Energy transmitted by electromagnetic waves, like sunlight. Solar panels take in radiant energy from the sun and convert it into electricity.

Electrical energy: The movement of electrons into a system, like electricity from wall outlets powering appliances and devices.

In summary, there are many forms of energy that can enter a system, providing the capacity to do work. Tracking energy inputs is crucial in applying the law of conservation of energy.

Energy Out

energy can take many forms as it flows through a system
Energy out refers to the energy leaving a system, such as in the form of heat, motion, sound, or light. This is the energy being transferred out or transformed as the system does work. Some examples of energy out include:

  • Heat leaving an engine or a person’s body into the surrounding environment
  • Kinetic energy of an object in motion, like a ball being thrown
  • Sound waves emitted from a speaker
  • Light waves given off by a light bulb

In order for useful work to be done, energy must be transferred out of the system. For example, an electrical motor converts electrical energy into kinetic energy to spin the motor. The kinetic energy flowing out of the motor is what allows it to perform mechanical work.

It’s important to note that while energy is conserved overall, the usable energy in a system tends to decrease over time. This is because when energy is transferred or transformed, some amount is often dissipated or lost as heat due to inefficiencies. The energy is still conserved, but it is no longer available to do useful work.

Equality of Energy In and Out

In an isolated system, the amount of energy going in equals the amount going out. This is known as the law of conservation of energy. Energy cannot be created or destroyed, only converted from one form to another.

For example, when an appliance like a lightbulb is turned on, electrical energy is converted into light and heat energy. The amount of electrical energy going into the lightbulb equals the total amount of light and heat energy coming out of it. No energy is lost or gained in the process.

This law applies to all isolated systems where no external energy is added or removed. The total energy before a process occurs equals the total energy after the process is complete. While energy forms may change within the system as they are converted from one to another, the net energy always stays the same.

Real World Examples

The law of conservation of energy can be seen in many real world examples across the sciences:

Biology: Living organisms require energy to survive and grow. The energy comes into an organism in the form of food and leaves the organism as heat and waste products. The organism cannot produce more energy than it takes in through food.

Physics: In simple machines like levers and pulleys, the amount of mechanical energy put into the system equals the amount of work output. Energy changes form but the total amount remains the same.

Chemistry: In chemical reactions, the total energy of the reactants equals the total energy of the products. Energy can be released or absorbed during a reaction, but the net change in energy is always zero.

Engineering: In electrical circuits, energy enters the system in the form of electricity and leaves the system as light, heat, motion, or another form. A generator cannot produce more electrical energy than the mechanical energy put into turning it.


The law of conservation of energy is not strictly applicable to open systems that can exchange energy with their surroundings. In these systems, accounting must be made for any energy entering or leaving the system boundaries.

For example, living organisms constantly take in energy from food and expel energy through heat and waste. While the law of conservation of energy applies to closed systems, additional factors need to be considered for open systems like biological organisms.

On a cosmic scale, the law does not necessarily apply to the universe as a whole if it is not a closed system. Energy could enter or leave the universe through unknown mechanisms, meaning the total energy in the universe may not remain fixed.

Additionally, the theory of relativity incorporates mechanisms like mass-energy equivalence that require refinements to the simplistic “energy in equals energy out” notion. Converting between mass and energy must be accounted for in energetic calculations at relativistic speeds.

While conservation of energy remains a fundamental physical law, its application is limited to closed systems. Accounting for mass-energy conversions, system boundaries, and unknown cosmic factors is required for rigorous application to real-world open systems.

Practical Applications

Understanding that energy is conserved allows us to better analyze real-world systems and processes. By tracking energy flows and transformations, we can identify inefficiencies and opportunities for improvement. Some key practical applications of the law of conservation of energy include:

Understanding energy flow through systems: We can follow the path of energy through a system, from the initial input through various transformations and eventually exiting the system. This allows us to create energy flow diagrams and Sankey diagrams to visualize where energy is used, stored or lost.

Improving efficiency of energy use and transformations: Knowing that energy is conserved means any energy “losses” are really just energy being transformed into unusable forms like heat. This knowledge helps guide efficiency improvements like insulation to reduce unwanted heat loss.

Evaluating compliance with conservation of energy: When analyzing a process or performing an energy audit, the principle of conservation of energy is used to find and quantify any discrepancies in energy flows. Violating conservation of energy indicates errors in measurements, missing outputs, or a flawed understanding of the process.

Common Misconceptions

Many people confuse the law of conservation of energy with other laws of thermodynamics. The law of conservation of energy states that energy can neither be created nor destroyed in an isolated system. However, the second law of thermodynamics says that some usable energy is always lost to heat in any real process. These two laws are related but distinct.

Another common misconception is the idea that energy can be created or destroyed within a closed system. According to the law of conservation of energy, the total amount of energy in a closed system remains constant. Energy can change forms within the system, but the total quantity of energy is the same at the start and end of any process. Believing that energy can appear out of nothing or disappear into nothing violates this fundamental law of physics.

Importance and Impact

The law of conservation of energy is a fundamental scientific principle that states that energy can neither be created nor destroyed – it can only transform from one form to another. This principle forms the very basis for many scientific and engineering fields and applications.

Some key reasons why the law of conservation of energy is so important and impactful are:

  • It is one of the most fundamental laws of physics and is considered to be true for all isolated or closed systems in the universe. It establishes that there is a fixed quantity of energy in the universe that remains constant, despite energy transformations and transfers.
  • It enables the study of energy and its transformation in quantitative terms in sciences like physics, chemistry, biology, and engineering. Complex energy calculations and problems can be solved by applying this law.
  • It allows the design of machines and processes that transform energy from one form to another with maximal efficiency, enabling most modern technology, from power plants to motors and batteries.
  • It underlies our understanding of thermodynamics and allows mathematical modeling of thermodynamic systems and cycles, with applications in heating, refrigeration, engines, and more.
  • It guides research in cosmology by anchoring our models of the origin and evolution of the universe with a constant total energy since the Big Bang.
  • It informs studies of metabolism and living systems by providing a measurable quantity that is neither created nor destroyed during chemical reactions in organisms.
  • It establishes useful concepts like thermodynamic free energy to determine the maximum amount of work that can be extracted from a system.

In summary, this fundamental law forms the basis for many critical scientific disciplines and technologies that society relies on today. It will likely continue providing a guiding quantitative principle for understanding and harnessing energy transformations for human benefit.


The law of conservation of energy states that within an isolated system, energy can be transformed from one form to another, but the total quantity of energy remains constant. Energy cannot be created or destroyed, only changed from one type to another. This principle is summarized by the statement “energy in equals energy out.”

In real-world systems that are approximately isolated, we find that when all forms of energy are accounted for, the total energy entering the system equals the total energy leaving the system. Examples of these types of systems include chemical reactions, mechanical systems like pendulums, electrical circuits, and thermodynamic processes. The law holds true in each case when all energy inputs and outputs are precisely measured.

While energy changes form, the total amount of energy in an isolated system remains fixed. This law has profoundly impacted science, technology, and industry. Understanding that energy can only change form allows us to track energy flows, improve efficiencies, and solve engineering problems. The concept of energy conservation permeates almost every field of science and is one of the most far-reaching laws in all of physics.

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