Energy Can Change Forms But is Conserved

Forms of energy. There are many different forms of energy from different sources: e.g., radiation from the sun, (kinetic) energy from the wind, thermal energy from heat sources, and (kinetic) energy from moving water. Remember that energy is related to motion.

Energy can convert (or change) from one form to another. For example:

This allows us to use the energy from sources like light, wind, and kinetic energy of water to generate electricity (movement of electrons along a wire or other material) that we use for lots of things! Some devices that use electricity are shown below.

These devices in turn convert electricity to other forms of energy, including light, sound, and air movement.

When energy is converted to a different form, the total amount of energy must remain the same. This is a very important scientific principle called Conservation of Energy. According to the principle of conservation of energy, the total amount of energy always stays the same, including when energy is converted to a different form!

In the following pages, we'll first talk in more detail about how energy is converted from one form to another. After that, we'll talk about how energy is conserved when it changes forms...

Examples of energy changing forms:

• Radiation from the sun (which is really the kinetic energy of photons) can be converted into thermal energy (the total amount of kinetic energy of the particles — atoms and/or molecules — that make up the object) when it hits an object. 
  • Example 1: Some of the light hitting icicles is absorbed by the molecules that make up the icicles, making the molecules move faster. So, the thermal energy of the ice increases. Because the molecules that make up the ice move around faster when they absorb some of the light, the temperature of the ice will also increase. This is why the icicles melt.
  
  
  • Example 2: Infrared radiation from the sun hits CO₂ molecules in the Earth's atmosphere. This radiation energy changes to vibrational motion of CO₂ molecules (a form of Kinetic Energy). Soon, it converts back to infrared radiation, which may then be converted to thermal energy.
  


• The kinetic energy of objects can be converted into thermal energy.
  • Example 3: When you rub your hands together quickly, the (kinetic) energy of the movement of your hands is converted into the energy of motion of the molecules that make up your hands. As a result, you can feel your hands warm up because the thermal energy of your hands increases!

Energy is conserved! In all of these examples, the total amount of energy before the conversion is equal to the total amount of energy afterward.

• Total energy before conversion = Total energy after conversion

Check your understanding...

A solar oven (like the one shown below) converts energy from the sun's radiation into thermal energy. Radiation from the sun passes through the transparent plastic wrap covering the oven. The radiation causes molecules of the air, food, and anything else in the "oven" to move faster. This results in the temperature of the oven increasing.

Application question: Can you see any ways to improve this oven to make its temperature even higher?

Energy is conserved! (This is important!) Anytime that energy changes forms, the total amount of energy before the conversion is equal to the total amount of energy afterward.

Conversion of Kinetic and Potential Energy. Another way energy changes forms is between kinetic energy and potential energy. But the total amount of energy must remain the same at all times: this is the principle of Conservation of Energy

• Example 4: A roller coaster is at rest at the top of a hill (Time 1). At this time, it has no kinetic energy (because it is not moving). But it has potential energy because the earth's gravitational field is pulling down on it. The roller coaster will start to accelerate down the hill. As it does that, it will gain kinetic energy because its speed increases. But, it will lose potential energy because it's getting closer to the bottom of the hill (and so, there is less potential for gravity to pull it down). 
• When it reaches the very bottom of the hill (Time 2), it only has kinetic energy. It has no potential energy due to gravity because gravity does not have the potential to pull it down any farther.
• The opposite happens between Time 2 and when the coaster climbs to the top of the next hill (Time 3). Because it is slowing down between Time 2 and 3, its kinetic energy decreases. But because it's getting higher and higher, its potential energy increases! (The higher the coaster gets, the more potential it has for faster speed when it comes back down.)
• This is also an example of Conservation of Energy. According to this principle, energy is neither created nor destroyed. But it can change forms. In other words, the total amount of energy [of a system mdash in this case, the roller coaster] does not change in time. So, we can look at the roller coaster at different points in time and know that the total energy is always the same:
  • Total energy (Time 1) = Total energy (Time 2) = Total energy (Time 3) = Sum (PE + KE) at any time.  

The potential energy of the roller coaster at Time 1 is (Select one) greater than less than equal to its potential energy at Time 2.

The potential energy of the roller coaster at Time 2 is (Select one) greater than less than equal to its potential energy at Time 3.

And, the potential energy of the roller coaster at Time 3 is (Select one) greater than less than equal to its potential energy at Time 4.

The kinetic energy of the roller coaster at Time 1 is (Select one) greater than less than equal to its kinetic energy at Time 2.

The kinetic energy of the roller coaster at Time 2 is (Select one) greater than less than equal to its kinetic energy at Time 3.

And, the kinetic energy of the roller coaster at Time 3 is (Select one) greater than less than equal to its potential energy at Time 4.

The total energy of the roller coaster (kinetic + potential energy) at Time 1 is (Select one) greater than less than equal to its kinetic energy at Time 2 (it has no potential energy at Time 2).

The total kinetic energy of the roller coaster (it has no potential energy) at Time 2 is (Select one) greater than less than equal to the sum of its kinetic and potential energy at Time 3.

And, the total energy of the roller coaster (kinetic + potential energy) at Time 3 is (Select one) greater than less than equal to its kinetic energy at Time 4 (it's at the bottom of the hill and has no potential energy).

The Conservation of Energy principle is shown in the animation below. The total amount of energy, which is represented by the sum of the heights of the red bars, is always the same. Energy just converts from kinetic to potential energy and from potential to kinetic energy.

Check your understanding...

• Conversion of kinetic energy of electrons into light. The energy of electrons in atoms can be converted into light energy and vice versa.
  • Light is released when electrons move to a lower energy circle. Sometimes, electrons gain energy in an atom (e.g., if they absorb light at the right energy or frequency) and move to a higher Energy circle temporarily. But these electrons will quickly move back to the lower Energy circle. When this happens, they lose energy.
  • But, the total amount of energy during this process must remain the same. So, that "lost" energy must go somewhere else. One way that this energy is released is in the form of light (or electromagnetic radiation). This process is represented in the picture below.
  
  • Burning materials like wood results in electrons moving from higher to lower energy circles and releasing light. We call the light produced by electrons as they move to lower energy levels in atoms during chemical reactions "fire."

We just saw how the energy of electrons can be converted into light energy. But the reverse process can also happen: Light energy can be converted into kinetic energy of electrons. Light may hit electrons in certain elements (such as the metals cesium and potassium) and molecules (such as chlorophyll molecules), causing the electrons to escape from the atoms or molecules. As represented below, the amount of kinetic energy of the escaped electrons is related to the energy of the light the electrons absorb. Electrons that absorb higher-energy blue or violet light will have more kinetic energy than electrons that absorb lower-energy green light. Red light, which has even less energy than green light, doesn't have enough energy to allow electrons to escape from the metal.

In both cases (when electron's energy is converted into light energy OR  light energy is converted into electron energy), the total amount of energy is the same before and after the conversion.

• Conversion of kinetic energy of electrons into kinetic energy of molecules (ATP). Another important way that energy may be conserved happens when electrons move to lower energy levels in molecules that break apart during chemical reactions. Because energy must be conserved, one or more of the resulting molecules may have a greater kinetic energy. This happens with a molecule called ATP, which is a very important molecule because it is a source of energy in animals and plant cells. ATP stands for "Adenosine Tri-Phosphate." It is a fairly complex molecule, so we show a simplified picture of its main parts below: Adenosine and three Phosphate (P) molecules.

• Each Phosphate molecule has one phosphorus atom that is bonded with four oxygen atoms. The electrons that bond the Phosphate molecules are at high energy levels. The Phosphate on the end may be pulled off of the rest of the molecule (now called Adenosine Di-Phosphate, since there are only 2 Phosphates left). When this happens, the electrons in the freed Phosphate move to lower energy levels. This loss in energy is converted to energy of motion (or kinetic energy) of the freed Phosphate molecule.
• (For more on ATP, click here.)