Definition Of Energy And Power

To understand renewable energy and the environment, the definitions of energy and power are needed. Work is the force on an object moved through some distance. Work is equal to force times distance:

W = F * D, Joule (J) = Newton (N) * meter (m) (2.1)

A number of symbols will be used, and problems can be solved using personal computers, spreadsheets, and calculators. Examples are supplied for illustration and understanding.

Many people have a mental block as soon as they see mathematical symbols, but everybody uses symbols. Ask any person what piano means and he or she understand the symbol, but to a South Seas islander, a piano is "a big black box, you hit him in teeth and he cries." By the same token, Equation 2.1 can be understood as a shorthand notation for the words and concepts written above it.

To move objects, do work, and change position of objects requires energy, so energy and work are measured by the same units. Some units of energy are Joule, calorie, British thermal unit (BTU), kilowatt-hour (kWh), and even barrels of oil.

Calorie = amount of energy required to raise 1 g of water 1°C BTU = amount of energy required to raise 1 lb of water 1°F Some conversion factors for energy are: 1 calorie = 4.12 J

1 calorie = kilocalorie = 1,000 calories, the unit used in nutrition 1 BTU = 1,055 J

1 barrel of oil (42 gallons) = 6.12 * 109 J = 1.7 * 103 kWh 1 metric ton of coal = 2.5 * 107 BTU = 2.2 * 1010 J 1 cubic foot of natural gas = 1,000 BTU 1 therm = 105 BTU = 100 ft3 of natural gas 1 quad = 1015 BTU 1 kWh = 3.6 * 106 J = 3.4 * 103 BTU 1 kW = 1.33 horsepower

Natural gas is sold by the mcf (which is 1,000 cubic feet) and it has an energy content of around 106 BTU. You need to be careful when comparing energy from coal with other sources, because 1 metric ton = 1,000 kg = 2,200 lb, 1 ton or long ton = 2,400 lb, and 1 short ton = 2,000 lb. Metric tons will be used unless noted. Also, different types of coal have different energy contents. A barrel of oil (160 L, 42 gallons) is refined to around 166 L (44 gallons) of components, of which 72 L (19 gallons) is gasoline.

Objects in motion can do work; therefore, they possess energy, kinetic energy (KE):

where m is the mass of the object and v is its speed.

EXAMPLE 2.1

A car with a mass of 1,000 kg moving at 15 m/s has kinetic energy.

KE = 0.5 * 1,000 * 15 * 15 = 112,500 J = 1.1 * 105 J to two significant figures

Because objects interact, for example, by gravity or electromagnetics, then due to their relative position they can do work or have energy, potential energy (PE). To raise a 1 kg mass, 2 m high, requires 20 J of energy. Then at that upper level, the object has 20 J of potential energy. Energy from fossil fuels is chemical energy, which is the potential energy due to the electromagnetic interaction.

Power is the rate of energy use or production and is equal to energy divided by time.

If either power or energy is known, then the other quantity can be calculated for any time period. Always remember, a kilowatt (kW) is a measure of power and a kilowatt-hour (kWh) is a measure of energy.

EXAMPLE 2.2

A 5 kW electric motor that runs for 2 h consumes 10 kWh of energy.

EXAMPLE 2.3

Ten 100-watt lightbulbs that are left on all day will consume 24 kWh of energy.

Heat is another form of energy, thermal energy. Heat is just the internal kinetic energy (random motion of the atoms) of a body. Rub your hands together and they get warmer. As you heat your home, you are increasing the speed of the particles of air and other materials in the home. Heat and temperature are different. Heat is energy, and temperature is the potential for transfer of heat from a hot place to a cold place. Do not equate temperature to heat (energy).

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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