2. Energy, Power, and Environment.**
Electric energy is produced through an energy conversion process. In the process energy is converted from a form such as potential energy, kinetic energy, or chemical energy to electricity. This module, Energy, Power & Environment, covers the basic concepts of energy, power, and work. The common methods of converting energy to electric power are discussed, as well as many of the environmental effects of energy conversion. This module requires a basic understanding of physics. This module aims to identify different energy sources, methods of storage and conversion, and to articulate the relationship between electric energy and the environment. The study questions and solutions for this module are included in the text of the sections.
2.1 Work, Energy and Power
Work U is defined as the product of the force F applied to a mass to move the mass a linear displacement l .
U = Fl
If the displacement is not in the direction of F , then the work performed is the product of the displacement and the component of the force along the displacement. If a is the angle that F makes with l , then
U = Fl cos(a)
Work is measured in Joules (J). One joule is the work done by a force of one Newton moving a body one meter.
1 J = 1 N · m
Energy is the capacity of a body to do work. Energy is expressed with the same unit as work (J=N· m). There are several types of energy. We will be focusing on electrical, mechnical, and thermal energy. All three types have typical units of expression and each will be described. Electric energy is typically expressed with a different unit, the watt-second (W · s), where
1 W · s = 1 J
More commonly, electric energy is expressed in terms of kilowatt-hours (kWh), where
1 kWh = 3.6 x 106 J .
You have probably seen this expressed on your electricity bill. Recently, I saw a 1000kWh bill!
Two of the most important forms of mechanical energy are kinetic energy and potential energy. These are especially important for electric energy generation, as will be discussed later. A body possesses kinetic energy through its motion. An object of mass M (kilograms), moving with a velocity u (meters per second), has a kinetic energy
A body possesses potential energy through its position to an accelerating field. Gravitational potential energy results from the body's position in a gravitational field. An object of mass M (kilograms) at a height h (meters) has a potential energy due to the acceleration of gravity g (meters per second per second) given by
PE = Mgh J
Thermal energy is measured in calories (cal). One calorie is the amount of heat required to raise the temperature of one gram of water at 15°C one degree Celsius. It has been found experimentally that
1 cal = 4.186 J
A common unit of thermal energy is the British thermal unit (Btu), which is equivalent to:
1 Btu = 1.055 x 103J = 0.252 x 103 cal
Power is the time rate at which work is performed, or the time rate of change of energy. Therefore, instantaneous power p may be expressed as
where U represents work and w represents energy. The unit of power is the watt (W). One watt is equivalent to
1 W = 1 J/s
Power engineering commonly uses the kilowatt (103) and the megawatt (106) multiples of the watt. The power ratings of electric motors are generally expressed in terms of horsepower (hp), where
1 hp = 745.7 W
It is important to be able to convert between the common units for work and power and the International System (SI) of units. The example problem should help both with conversion and concept clarification.
The average person needs 2000 calories of energy per day to live. The typical candy bar contains 230 calories. How many candy bars does a person need to eat per day to live? What is the daily power consumption of a person in megawatts?
**This chapter of the education module was written by Jaime DeLaRee Lopez, full citation.