EUES - What is Energy? (Lesson)

What is Energy?

Energy Basics

Energy exists in many forms, such as heat, light, chemical energy, and electrical energy. Energy is the ability to bring about change or to do work, and thermodynamics is the study of the energy and work of a system. Like the conservation of matter, energy is also conserved as explained by the First Law of Thermodynamics. Energy can be changed from one form to another, but it cannot be created or destroyed. Therefore, the total amount of energy and matter on Earth and in the Universe remains constant; it is merely changing from one form to another.

The Second Law of Thermodynamics states that in all energy exchanges (or work done) if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state. OK, if energy is neither created nor destroyed, why is there less potential energy? This energy change means that there is the same amount of energy, but it is just less usable. This is known as entropy. In biological systems, a large percentage of the fuel (food) we use ends up as unusable heat, the same is true for all energy exchanges and entropy is this increasing disorder of less usable energy.

Kinetic Energy

The law of conservation of energy (First Law of Thermodynamics) says that energy cannot be created or destroyed. This means that even though energy changes form, the total amount of energy always stays the same. How does energy get converted from one type to another when you kick a soccer ball? When your body breaks down the food you eat, it stores the energy from the food as chemical energy. But some of this stored energy has to be released to make your leg muscles move.

The chemical energy is converted to another form of energy called kinetic energy. Kinetic energy is the energy of anything in motion. Your muscles move your leg, your foot kicks the ball, and the ball gains kinetic energy from the kick. So you can think of the action of kicking the ball as a story of energy-changing forms.

Potential Energy

Potential energy is energy that is stored. Potential energy has the potential to do work or the potential to be converted into other forms of energy. If a ball is sitting on the very edge at the top of the hill, it is not moving, but it has a lot of potential energy.

Fuel

If you read a book beneath a lit lamp, that lamp has energy from electricity. The energy to make the electricity comes from fuel. Fuel contains energy that can be released and then captured for use in a variety of ways.  A fuel is any material that can release energy in a chemical change.

What are some examples of fuel, and what are they used for?

• Food is fuel for your body.
• Sunlight is the energy plants need to make food (fuel) by photosynthesis.
• Gasoline is fuel for cars.
• Hydrogen is fuel for the Sun

For a fuel to be useful, its energy must be released in a way that can be controlled. Controlling the release of energy makes it possible for the energy to be used to do work.

Heat

When fuel is used for its energy, it is usually burned (combustion), and most of the energy is released as heat. The heat may then be used to do work. Think of a person striking a match to set some small twigs on fire. After the twigs burn for a while, they get hot enough to make some larger sticks burn. The fire keeps getting hotter, and soon it is hot enough to burn whole logs. Pretty soon the fire is roaring, and a pot of water placed on the fire starts to boil. Some of the liquid water evaporates.

What is the source of energy for boiling and evaporating the water? Although some chemical energy from the match was put into starting the fire, the heat to boil and evaporate the water comes from the energy that was stored in the wood. The wood is the fuel for the fire.

Nonrenewable Resources

Fossil fuels, coal, oil, and natural gas, are the most common example of nonrenewable energy resources. Fossil fuels are formed from fossils, the partially decomposed remains of once-living plants and animals. These fossils took millions of years to form. When fossil fuels are burned for energy, they release pollutants into the atmosphere. Fossil fuels also release carbon dioxide and other greenhouse gases, which are causing global temperatures to rise.

Renewable Resources

Renewable energy resources include solar, water, wind, biomass, and geothermal. These resources are either virtually limitless like the Sun, which will continue to shine for billions of years, or will be replaced faster than we can use them. Amounts of falling water or wind will change over the course of time, but they are quite abundant. Biomass energy, like wood for a fire, can be replaced quickly.

The use of renewable resources may also cause problems. Some are expensive, while some, such as trees, have other uses. Some cause environmental problems. As technology improves and more people use renewable energy, the prices may come down. At the same time, as we use up fossil fuels such as coal, oil, and natural gas, these nonrenewable resources will become more expensive. At some point, even if renewable energy costs are high, nonrenewable energy will be even more expensive. Ultimately, we will have to use renewable sources.

Important Things to Consider about Energy Resources

With both renewable and nonrenewable resources, there are at least two important things to consider. One is that we have to have a practical way to turn the resource into a useful form of energy. The other is that we have to consider what happens when we turn the resource into energy.

For example, if we get much less energy from burning fuel than we put into making it, then that fuel is probably not a practical energy resource. On the other hand, if another fuel gives us large amounts of energy but creates large amounts of pollution, that fuel also may not be the best choice for an energy resource.

Electrical Grids

No matter what the source, once it is generated electricity has to move from place to place. It does so by an electrical grid. Many communities have electrical grids that were built decades ago. These grids are inefficient and have high failure rates.

The electrical grids of the future are likely to be smart grids. Smart grids start with electricity production from one or more power generation sources. The electricity is streamed through multiple networks out to millions of consumers. Smart meters are placed with the consumers. They supply information on the state of the electrical system. Operators know within minutes if the power goes out, rather than having to wait for phone calls from consumers. Smart meters measure consumption and assist consumers in using power when it is more economical, even turning on or off appliances in homes or workplaces to smooth demand. Smart grids are essential for integrating renewable energy sources, such as solar and wind, into the network because they have highs and lows in their supply.

Today we rely on electricity more than ever, but the resources that currently supply our power are finite. The race is on to harness more renewable resources, but getting all that clean energy from production sites to homes and businesses is proving to be a major challenge.

Units

Calculations are an important part of this module and are critical to a full understanding of energy and energy efficiency. You will be required to know the main units, but will always be provided the conversions. The only exception that you are responsible for is the prefixes for metric units shown below. Further, as you will be working with numbers that can be quite large and small, you will be expected to read and calculate problems using scientific notation. 

Prefix	Symbol	Multiplier	Exponent
tera	T	1,000,000,000,000	1012
giga	G	1,000,000,000	109
mega	M	1,000,000	106
kilo	k	1,000	103
hecto	h	100	102
deca	da	10	101
base		1	100
deci	d	0.1	10-1
centi	c	0.01	10-2
milli	m	0.001	10-3
micro	µ	0.000001	10-6
nano	n	0.000000001	10-9

Energy Units

UNIT	CONVERSIONS	DESCRIPTIONS
Joule (J)	1 J= 0.00094782 or 9.48 x 10-4 Btu = 0.00000028 or 2.8 x 10-7 kWh	A small unit of energy but used often in scientific measurements and calculations
Btu (British Thermal Unit)	1 Btu = 252 cal = 1055 J	A unit of heat energy and the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit
kWh (kilowatt-hours)	1 kWh = 1,000 J/s x 3,600 s = 3.6 x 106 J	A unit of electrical energy is most often used in the US by utility companies.
calorie (cal)	1 cal = 4.186 joules 1 Cal = 4.186 x 103 J	One calorie is the amount of heat energy required to raise the temperature of one gram of water by one Celsius degree. Dietary calories are actually kilocalories (Cal) and are always written with a capital C
Therm (thermal units)	1 therm = 105 Btu / 1,030 Btu/ft3 = 97.1 ft3 ≈ 100 ft3	Gas companies in the U.S. often measure sales in "thermal units" or therms. One therm is defined as 100,000 Btu, and natural gas at normal temperature and pressure has a heat value of 1,030 Btu/ft3.

Power

Power is used to describe energy flow or the rate of doing work (or using energy). Power is measured in joules/second. In the SI system, the unit of power is the watt (W).

1 watt (W) = 1 joule/second (J/s)

 

Electric Power Plants

Electric utility power plants are rated in terms of their capacity to deliver electric power. For instance, a large coal-fired or nuclear plant might be rated at 1,000 MW (megawatts). This is the "output" capacity of the plant, not the energy input. The input energy is usually measured in terms of the heating value for the fuel; Btus for coal, for instance. If the plant operates at, say, 40 percent efficiency, then the energy input required for such a plant can be computed as follows:

Coal has a heat value of 25 x 10⁶ Btu/ton:

 

 

Operating at full capacity 24 hours a day the plant uses:

 

 

Self-Assessment: Practice Questions

It is time to complete some practice questions, so you can apply this information. It is an expectation to complete calculations of this type for this course and they are commonplace on the AP exam. Use the information above and in the questions to complete this problem set, then check your answers below. 

1. Given that 1 kcal of heat is required to increase the temperature of 1 kg of water by 1°C:

a. How many kcals would be required to heat 100 kg of water by 20°C for a bath?
b. How many joules is this?
c. How many Btus?
d. If your water heater can supply 40 kBtu/h, how long will it take to heat this water?

2. With moderate winds, a modern large wind turbine can generate about 250 kW of electricity, whereas a large nuclear power plant can generate 1,000 MW.

How many wind turbines would be required to give the same output as one nuclear power plant?

3. A typical home in the northern U.S. might require 120 MBtu of heat for the average winter.

a. If this heat were supplied by a natural gas furnace operating at 60 percent efficiency, how many cubic feet of gas would need to be purchased?
b. At a cost of $0.90/100 cf, what would it cost to heat this house for one season?
c. If a new 80 percent efficient furnace could be installed at a cost of $4,000, how long would it take to pay back the cost of this furnace assuming gas prices remained the same?

4. Batteries are usually rated in terms of ampere-hours, indicating the current that the cell is capable of delivering for a specified time. A typical D-cell flashlight battery, for instance, might be rated at 3 ampere-hours. The total electrical energy available from such a battery is found by multiplying the ampere-hour rating by the battery voltage. Thus this same 1.5 volt D cell could deliver 4.5 watt-hours of electrical energy. Convert this energy to kWh and compare the cost of electrical energy derived in this manner to that of standard "grid-based" electricity. Assume that the battery costs $1.00 and that electricity from the power company is available at $0.10/kWh.

5. The table below gives prices and heat energy content for various fuels that are commonly used for home heating. Fuel prices are given as a per-unit cost for fuel delivered to the home. Complete the table by filling in the last two columns and thereby compare the cost of home heating by these various methods. In your computations, assume that the home requires 120 MBtu of heat for a season and that gas- or oil-fired furnaces operate at 80 percent efficiency. Assume that electrical heating is 100 percent efficient.

Click here to check your answers. Links to an external site.

 

RESOURCES IN THIS MODULE ARE OPEN EDUCATIONAL RESOURCES (OER) OR CREATED BY GAVS UNLESS OTHERWISE NOTED. SOME IMAGES USED UNDER SUBSCRIPTION.