Law of Conservation of Energy

Core Concepts

Sometimes referred to as the First Law of Thermodynamics, we explore how the Law of Conservation of Energy is defined, how it can be visualized, and how it can be applied in real-life examples of energy conversion.

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What is the Law of Conservation of Energy?

The Law of Conservation of Energy states that energy is neither created nor destroyed, but is a fixed total. This law is also sometimes referred to as the first law of thermodynamics. The first law applies to isolated systems, the universe being a prime example. Energy may take on many forms, but the total energy within a system is constant.

How do we visualize the Law of Conservation of Energy?

The law of conservation of energy is observable in everyday occurrences. We define energy as the ability to do work. Though energy is not a concrete object we can visualize, it exists in various forms everywhere! In fact, mass is a form of stored energy. Let’s explore the different forms of energy and the exchange between these forms.

Forms of energy

Mechanical energy (kinetic and potential energy)
Thermal energy (heat)
Chemical energy
Nuclear energy
Electrical energy

For an introduction to energy (Kinetic, potential, and thermal), check out our Intro to Energy Article!

Processes to explore

Turning on a lightbulb

Electrical energy is produced from various natural sources. For example, wind and running water provide kinetic energy which can be converted to electrical energy. Once the conversion is complete, the electrical energy is transferred where needed. Some electrical energy may be “lost” as heat during this transfer. When a light bulb is switched on, the stored electrical energy converts to light and heat!

Fun fact: LED light bulbs were created because earlier forms converted electrical energy to light and a great amount of heat. Because energy is conserved, the same amount of energy was “lost” to heat. LEDs reduce the amount of energy required to produce light!

boiling pot law of conservation of energy — Source: Wikimedia Commons

Boiling water

Just as with turning on a light bulb, boiling water requires electrical energy. A stove acts as the connection between the stored energy and the water. When the stove is turned on, heat energy is produced. The pot conducts heat, which allows the water to transform the heat into kinetic energy. As the water molecules begin to move more freely, their kinetic energy is converted back into heat.

speaker law of conservation of energy — Source: Wikimedia Commons

Playing music from a speaker

Speakers are powered by batteries, which store chemical energy. When speakers are turned on, the energy stored in the batteries is converted to electrical energy. Then, the electrical energy is converted to sound waves, which is a form of kinetic energy.

Note: If we were to tangibly measure the energy at the beginning of the processes versus the end, we would find that the energy levels are not exactly equal. This is because the described processes are not isolated systems! Energy that cannot be accounted for when the processes are complete is generally “lost” as heat or sound. However, if we were to measure the energy on a universal level, we would see that the total energy is the same at the beginning and the end.

Thermodynamics Application

In thermochemistry, the equation ∆U = q + w measures the change in internal energy of an isolated system. Isolated systems are split into two parts when using this equation – the “system” and the “surroundings.” Heat (q) transfer to the system is equal to the heat transfer from the surroundings; this statement is also true for work (w).

As previously noted, isolated systems experience zero energy change (∆U_universe = 0). Therefore, any energy lost by the system is gained by the surroundings, and vice versa. Also, we know that mass is a form of stored energy. As a result, isolated systems also experience zero mass gain or loss.

While energy and mass are constant, it is important to note that working conditions are not. Conditions such as pressure, temperature, and volume will affect the work done and heat transferred. However, no matter the conditions, the internal energy change is still equal to zero.