Potential Energy, Kinetic Energy & Internal Energy

Energy is the subject matter of a scary-sounding college physics course titled “Thermodynamics.” Separating the term into its two Greek roots, thermo and dynamics, tells us it is all about moving (dynamics) heat (thermo). Heat, of course, is the primary subject of this plengdut.com post. 

Specifically, we are interested in our homes losing less heat in the winter, gaining less heat in the summer, generating and removing heat more efficiently, and paying less for it. Let us begin, however, with a wider look at the many different forms of energy. 

Potential Energy 

Potential energy is the energy possessed by a mass due to gravity and the height of the mass above a reference plane, usually the ground. The formula is: 

PE = mgh 
where: 
PE = potential energy in Joules 
m = mass in kilograms (1 kg = 2.2 lb.) 
g = acceleration of gravity, 9.8 meters/sec2
h = height in meters (1 meter = 3.28 ft.) 

By definition, 1 Joule = 1 kg-m2/sec2

In the English units Americans are more used to: 
1,000 J  = 0.948 Btu 
1 Btu = 1,055 J 

Kinetic Energy 

Kinetic energy is the energy possessed by a mass due to its velocity. The amount of kinetic energy a mass possesses is calculated with the formula: 

KE = ½mv2 
where:
 KE  = kinetic energy in Joules 
m  = mass in kilograms 
v  = velocity in meters/sec 

Note that kinetic energy is also measured in Joules.

Potential Energy vs. Kinetic Energy 

Question (see illustration energy): 

Which has more energy:  (A) a mass weighing 10 lb. (4.54 kg) sitting on a limb  30 ft. (9.14 m) above the ground, or  (B) the same mass sliding across ice at 30 mph (13.41 m/sec)?  (A) has potential energy, due to its height above ground, but has zero kinetic energy because it is not moving. Its energy is:  PE = mgh  = 4.54 kg × 9.8 m/sec2  × 9.14 m  = 406.7 kg-m2/sec2 = 406.7 J  (B) has kinetic energy, due to its velocity across the ice, but has zero potential energy because it is at ground level. Its energy is:  KE = ½mv 2  = 0.5 × 4.54 kg × (13.41 m/sec) 2  = 408.2 kg-m2/sec2 = 408.2 J
Which has more energy: 
  1. a mass weighing 10 lb. (4.54 kg) sitting on a limb  30 ft. (9.14 m) above the ground, or 
  2. the same mass sliding across ice at 30 mph (13.41 m/sec)? 
  3. has potential energy, due to its height above ground, but has zero kinetic energy because it is not moving. Its energy is:

 PE = mgh 
= 4.54 kg × 9.8 m/sec2  × 9.14 m 
= 406.7 kg-m2/sec2
= 406.7 J 

(2) has kinetic energy, due to its velocity across the ice, but has zero potential energy because it is at ground level. Its energy is: 
KE = ½mv 2 
= 0.5 × 4.54 kg × (13.41 m/sec) 2 
= 408.2 kg-m2/sec2
= 408.2 J

Internal Energy 

The third form of energy, internal, is energy in a mass. 

Sensible heat is internal energy created by the rotation and vibration of atoms and molecules. It is “sensible” because its transfer results in a temperature change that can be sensed by a thermometer. As we will see, internal energy can be transferred within a mass or between masses by three processes: conduction, convection, and radiation. The direction of transfer is always from higher to lower temperature. 

In addition, molecules consist of atoms bound by interatomic forces. Molecules are broken apart and rearranged into different molecules during chemical reactions. Even molecules are bound together, this time by intermolecular forces. These forces are: 
  • weak or nonexistent in gases 
  • moderate in liquids 
  • strong in solids, particularly crystals 

Interatomic and intermolecular forces result in two additional types of internal energy: 

Latent heat is the energy required to either break or form intermolecular bonds when a substance changes phase. It is “latent” because it happens without a change in temperature. Most substances exist in three phases or states—sold, liquid, or gas—depending on their temperature. Water provides the example we are most familiar with:
  1. When freezing or melting, water changes phase from liquid to solid or vice-versa. During this phase change, energy is removed or added to the water without a temperature change. The amount of energy removed or added is its latent heat of fusion. 
  2. When condensing or evaporating, water changes phase from gas to liquid or vice-versa. During this phase change, energy is also removed or added without a change in temperature. The amount of this energy is water’s latent heat of vaporization. 
Chemical energy is the energy of interatomic and intermolecular bonds released during chemical reactions. Combustion, a chemical reaction, releases internal energy in the form of heat.

Three Laws of Thermodynamics 

The three principles, or laws, of thermodynamics are simple and readily understood. 

First Law 

We have already seen that a substance (or a system of substances) can possess three types of energy: potential, kinetic, and internal. The First Law says that, while energy can change from one form to another, it can be neither created nor destroyed. The First Law is also called the Law of Energy Conservation. That is not to say, however, that energy cannot be added to or removed from a system. 

Second Law 

The Second Law says that, while energy can be neither created nor destroyed (First Law), the quality (ability to perform work) of the energy in a system naturally deteriorates over time. 

A spring-driven clock provides a good example. When wound, the clock spring possesses potential energy. As the clock runs, the spring unwinds, converting potential energy to kinetic energy. And while the gears turn, friction converts their kinetic energy into internal (heat) energy.

High-quality energy is said to have low entropy (disorder), while lower-quality energy has higher entropy. For this reason, the Second Law is also known as the Law of Increased Entropy. 

Third Law 

The Third Law has little application in the nonscientific world. It states that the entropy, or disorder, of a system approaches zero as its temperature approaches Absolute Zero (-273.15°C or -459.7°F). Another definition of Absolute Zero is the temperature (a measure of the concentration of internal energy of motion) at which all motion (and therefore all kinetic energy) ceases.

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