POTENTIAL AND KINETIC ENERGY

What is Energy?

Energy is the ability to do work, usually against some force. When you climbed the stairs to this class, you had to do work against gravity to get up here. That required an expenditure of energy.

But, you only borrowed this energy. You can get it back if you jump back down to the atrium floor (not recommended).

Energy comes in many forms, kinetic, potential (stored), heat, etc. Energy is always conserved. It is not created or destroyed but is just transformed from one form to another.

Kinetic and Potential Energy:

Throwing a ball into the air represents a situation in which the total energy is fixed and there is a continous transformation from kinetic energy to potential energy.

• When the ball is on the ground there and not moving there is no potential energy or kinetic energy

• when the the ball is tossed into the air it will reach a maximum height which is determined by how much kinetic energy it has (air resistance is important however)

• when the ball reaches its maximum height its velocity is 0 and all of the energy in the system is potential energy

• as the ball falls to the ground that potential energy is converted to kinetic energy

• Some amount of energy is loss due to air friction which then heats up the air (just a tiny bit).

Most Energy loss is via heat. This is generally not released. Heat loss is usually irrecoverable. This is a principle feature of the field of thermodynamics that we will discuss later.

Thermodynamics Primer:

THERMODYNAMIC LAWS

You can not subvert or change these laws:

The Zeroth Law (0): Systems are in equilibrium when they are at the same temperature.

• System is in it lowest energy state
• No more energy can be extracted from it
• All systems of different temperature will tend to equilibrium when they are no longer thermally isolated.

The First Law (1): Energy is Conserved in a closed system:

• The net flow of energy across some system is equal to the change in energy of that system
• We usually consider work and heat flow as the two kinds of energy

The Second Law (2): The Law of Entropy:

• It is not possible to extract heat-energy from a reservoir and perform work without creating waste heat that does no work.
• The amount of disorder increases
• Things tend toward a state of randomness(this is not the same as chaos)
• You can not go from a disordered system to an ordered system without inputting more energy (this is the most important attribute of the second law)

  Example:

  We can do this
  
  100% of electrical energy converted to heat by pushing a current through a resistive element.

  We can not reverse that process to do this:
  

  In the course of doing the original work we have increased the disorder to the system by heating it. In no way can we recover work of that this disordered system without putting energy into the system.

To decrease local entropy requires work (energy).

• Your dorm room - it takes a lot of work to decrease the amount of disorder

• Iron Ore is originally concentrated in mountains - has a low disorder. Eventually it becomes mined and ends up distributed in the nations landfills.

• This combination of letters is readable and hence of low order but
  ticmiainflteriraaladecolwrerdoohsobntoftrsdbne

  is not and would require a lot of work to rearrange into the previous sentence

• Rich fossil fuel deposits are stored in a state of low order. when we liberate all of that so that the individual atoms become randomly distributed, where will society find the energy to maintain local order?

But what is heat? Heat is infrared radiation.

Common Types of Energy
Type of Energy	What it is
Kinetic Energy (KE) Gravitational Potential Energy (GPE) Chemical Potential Energy (CPE) Elastic Potential Energy (ELPE) Thermal Energy (H) Electromagnetic Radiation (ER)	energy associated with motion energy associated with position in a gravitational field energy associated with the bonding of molecules in a chemical "spring energy" - a form of stored energy heat dissipation release of photons

The concept of conservation of energy will be reinforced with some classroom demonstrations today. The most important concept is to observe different systems and identify the potential sources of energy loss.

Conservation of Energy is crucial to understanding atomic spectra and how its possible to determine that different elements are inside of stars. This will be discussed in the context of photon emission and absorption on Thursday.