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First Law of Thermodynamics: Law of Conservation of Energy

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  • 0:07 First Law of Thermodynamics
  • 4:04 Application of the First Law
  • 6:35 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

This video describes the first law of thermodynamics. Several examples of the application of this law are discussed. The significance of the law and its practical application are discussed as well.

First Law of Thermodynamics: Law of Conservation of Energy

Have you ever wondered what happens to wood as it burns? It seems as if the wood may disappear into thin air. While burning wood appears to create energy and destroy the wood, neither is created or destroyed. Rather, energy and matter are changing from one form to another. Wood contains what we call chemical potential energy, which is energy stored in the bonds that hold the chemicals together. This stored energy is released in the form of heat and light when the wood is burned.

Wood also contains matter, which is anything that has mass and takes up space (volume). The matter within the wood is transformed into different matter, including ash and soot, as it burns. The total amount of energy and matter in the wood before burning is equal to the energy and matter of the ash, soot, heat, and light after burning. In other words, energy and matter are conserved both during and after the wood is burned.

When burning wood, energy and matter are conserved and converted into different forms.
Energy and Matter Are Conserved

This phenomenon of conservation is explained by what we call the first law of thermodynamics, sometimes referred to as the law of energy conservation. The law states energy cannot be created or destroyed. Energy can be described as the ability to do work, where work is the movement of matter when a force is applied to it. With the example of burning wood, the energy we see in the form of fire is not created out of nothing but rather comes from the energy that is stored in the wood. Likewise, the wood is not destroyed but rather is converted into ash and soot.

In order to better understand the law of energy conservation, we need to consider the fact that it applies to systems. A system is simply a collection of component parts that make up a whole. Burning wood is a system that includes the wood, heat, ash, and soot. The universe is the largest system that we know of, and it includes all matter and all energy, including the burning wood that we're talking about. There are other examples of small systems. For example, you can consider your body as a system. When you're cooking, you can consider a pot of water on the stove as a system as well.

Now that we have a good understanding of systems, let's consider the difference between an open and a closed system and discuss the law of energy conservation as it applies to each. A closed system is a system in which no matter or energy is allowed to enter or leave. The first law of thermodynamics tells us that the amount of energy within any closed system is constant - it doesn't change.

An open system, on the other hand, allows stuff to come in and go out, like burning wood in a fireplace. Here, you can add wood to the fireplace and light it with a match from, say, your pocket. Heat, ashes, and soot can leave the fireplace as the fire burns. In other words, energy and mass can enter and leave a system as long as they come from a system or leave to go to another system. It's important to note, however, that the total mass and energy in our universe remains constant.

Since most systems are not closed, the law of energy conservation can be rephrased to say that the change in the internal energy of the system is equal to the difference between the amount of energy coming in minus the amount of energy going out. In other words, the amount of energy in a system can change, but only if it comes from another system or goes to another system.

At any rate, systems, whether they're open or closed, do not create or destroy energy. Rather, energy can enter from one system and leave to another. Energy that enters a system must either be stored there or leave. A system cannot expend more energy than it contains without receiving additional energy from an external source.

Application of the First Law of Thermodynamics

Now that we understand that energy is conserved within a system, let's consider some practical applications of the law. In other words, what does it do for me? How does understanding the law of conservation help us out?

The reason machines need a constant input of energy to work is that some energy is lost to friction.
Energy Lost to Friction

Have you ever heard of a perpetual motion machine? Such a machine would continue to work without any input of energy. No such machine has ever been built, and according to the energy conservation law, such a machine never will be. Every machine requires the continual input of energy in order to keep working. The type of energy providing the input can vary, including the sun for solar energy, wind to move a windmill, water flowing over the dam, the breakdown of chemicals like gasoline to run our automobiles, or the breakdown of chemicals in the food we eat to pedal a bike. A really good bike can coast on level ground for a long time, but it's eventually going to come to a stop unless someone pedals the bike.

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