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.

Expert Contributor
Christianlly Cena

Christianlly has taught college physics and facilitated laboratory courses. He has a master's degree in Physics and is pursuing his doctorate study.

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

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Additional Activities

First Law of Thermodynamics True or False Activity

In this activity, you will check your knowledge regarding the law of conservation of energy as presented from the lesson.

Directions

Determine whether the following statements are true or false. To do this, print or copy this page on a blank paper and underline or round the answer.

1. The law of conservation of energy states that energy can either be created or destroyed

True | False

2. A closed system is a system that has external interactions, such as energy transfers into or out of the system boundary.

True | False

3. According to the first law of thermodynamics, when heat enters or leaves a system, the system gains or loses an amount of energy equal to the amount of heat transferred.

True | False

4. A measure of energy transfer that occurs when an object is moved over a distance by an external force is known as heat.

True | False

5. The transformation of chemical energy into kinetic and heat energy by a car engine is a manifestation of the first law of thermodynamics.

True | False

6. An open system could not create or destroy energy.

True | False

7. Energy is a conserved quantity associated with the capacity of doing work.

True | False

8. In this modern era, it is already possible to create or build a perpetual motion machine.

True | False

9. Friction is a dissipative force that resists the relative motion of two bodies in contact and causes the loss of energy.

True | False

10. An example of the first law of thermodynamics is when a gas stove transforms chemical energy from natural gas into heat energy.

True | False


Answer Key

1. False, because the correct statement is, The law of conservation of energy states that energy can neither be created nor destroyed.

2. False, because the correct statement is, An open system is a system that has external interactions, such as energy transfers into or out of the system boundary.

3. True

4. False, because the correct statement is, A measure of energy transfer that occurs when an object is moved over a distance by an external force is known as work.

5. True

6. True

7. True

8. False, because the correct statement is, In this modern era, it is still impossible to create or build a perpetual motion machine.

9. True

10. True

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