Energy is the source of all change on the planet Earth.
Earth is a complex of active physical and biological systems which interact and influence one another and change in response to energy in its various forms.
There are various sources and forms of energy on Earth; some exert their influence naturally and others do so through human intervention. Energy cannot be destroyed but it can be (and constantly is) converted from one form to another.

1. What is the most important source of energy for Earth?
The Sun. Energy radiating from our closest star - the Sun - provides the natural power that fuels most of the physical and biological processes important to life. These processes include the movement of Earth's atmosphere which distributes energy, produces global weather, and oceanic circulation. Sunlight energy also sustains plant and animal life. About half of the Sun's energy is absorbed or reflected back into space as it passes through our atmosphere.

2. Is the Sun our only natural source of energy ?
No. Another natural source is the gravitational force which pulls matter toward the center of Earth. The pull is strongest on or near Earth's surface; it causes flows of air and water, and falls of earth and rock.
Another natural source of energy is geothermal energy which is heat contained beneath the Earth's crust and includes heat generated within the Earth by radioactive decay of unstable atomic isotopes.
A third natural source of energy is the energy contained in the oceans and rivers. Electrical energy can be generated from the energy released by flowing water.

3. What are the different forms of energy?
From the point of view of a physical geographer they are: solar radiant energy, heat energy, gravitational energy, kinetic energy, and chemical energy. These are the forms of energy that are most significant to natural processes on Earth's surface.
From the point of view of a physicist for whom the definition of energy is the capacity or power available for doing work (causing change) the different forms may be described as being electrical, mechanical, thermal, or nuclear.

4. What is Solar Energy?
Solar energy is radiant energy that travels from the Sun to Earth (a distance of about 150 million kilometers) and reaches Earth in about eight-and-a-half minutes at a speed of about 300,000 km per second (186,000 miles per second). Most of the solar radiant energy reaching Earth is visible light (wavelengths of energy to which the eyes of animals are sensitive). Some of the energy is longer-wavelength thermal energy that is felt as heat. Shorter-wavelength ultraviolet solar energy is what causes sunburn.

5. What is Heat Energy?
This form of energy is produced by the random motion of atoms and molecules of substances. The more vigorous the movement of its atoms and molecules, the hotter a substance becomes. The movement requires the input of energy - which can be in another form (other than heat) such as longer-wavelength solar energy.
Heat energy can be transmitted from one body/substance to another by conduction, convection, or radiation.
Heat energy can be used to perform mechanical work.

6. What is the relationship between heat energy and temperature?
Temperature is a measure of average molecular motion. Heat energy is the amount of energy in a substance. For example, a hot drink at 122 degrees Fahrenheit (50 degrees Celsius) is hotter (has a higher temperature) than a bathtub of warm water at 95 degrees Fahrenheit (35 degrees Celsius) but there is more heat energy in the warm bathtub water because of its larger volume.

7. What is Gravitational Energy?
Gravitational energy is the potential energy an object (or a substance) has because of its elevation or distance from Earth's surface. Gravitational energy is proportional to altitude and mass; a large boulder at the top of a mountain will hit the ground at the foot of the mountain with more force than will a smaller boulder further down the mountainside.

8. What is Kinetic Energy?
This is the energy of motion. The higher the speed at which something is moving, the more kinetic energy it has. For a given speed, the amount of kinetic energy is proportional to the mass of an object. If equal volumes of air and water are moving at the same speed, the water will have much more kinetic energy because of its much greater mass. This is why ocean waves can do more damage than winds even when the winds have more velocity.

9. What is Chemical Energy?
This is energy stored in the electrical bonds that hold together the atoms and molecules of all substances. When substances react chemically, their energy is either released, absorbed, or converted to other forms of energy. For example, when striking a match, the chemical energy stored in the match-head is transformed into heat and light energy. In plants, radiant solar energy is used in photosynthesis and is stored in the resulting carbohydrate molecules.

10. Is Chemical Energy the Same as Atomic Energy.
No. Atomic energy is the energy released by the splitting of an atom as in a nuclear reactor. Stars, like our Sun, are natural nuclear reactors in that hydrogen is continuously converted to helium with resulting violent outputting of energy.

11. What are some examples of Energy Transformation?
a) When you are out in the sunshine (sunlight) you feel warm because the solar radiant energy excites the molecules of your skin and so creates heat there. b) When you do physical exercise the chemical energy stored in the carbohydrate, fat, and other molecules in the cells of your body is converted to energy of motion. c) When coasting downhill on your bicycle the potential energy you possess because of your distance from the center of Earth becomes converted to kinetic energy and your speed increases until you reach the bottom of the hill.

12. What is the relationship between change (meaning 'work') and energy?
Change - or work - in all systems and processes in both the physical and biological worlds relies on the availability of energy. Heat energy within the Earth creates change through the activity of volcanoes and distortion of the Earth's crust. Chemical energy creates change by being the basis of metabolism in plant and animal life. Energy stored in dead vegetable matter ultimately (as fossil fuel) is a source of electrical power which is necessary for work done by electrical equipment of all kinds.

13. Is energy involved in changes of physical state?
Yes. When water, for example, changes its physical state from one in which the molecules are tightly packed together to one in which they are loosely packed (as from ice to water) energy is necessary to effect the change. The amount of energy needed for one gram of ice to change to water is 80 calories (335 joules). To change one gram of solid ice to water vapor requires 680 calories (2843 joules) of energy.

14. How is energy involved when water changes from its gaseous (water vapor) state to its liquid state?
In this situation loosely packed molecules of water vapor release energy as they become more closely packed together to become liquid water. This is why a burn from steam from boiling water in a tea kettle can be more severe than a burn from the hot water in the kettle. As it touches the skin the hot steam not only heats up the skin because it is hot but, in addition, as it changes from steam to hot water its molecules release extra heat energy this increasing the severity of the burn.

15. How can energy be measured?
In animals and plants the amount of stored energy can be measured using a 'bomb calorimeter'. In this device a sample of plant or animal material can be converted to heat energy which is measured as an increase in temperature. This increase in temperature is converted to an energy measurement. (Energy is measured in joules or in calories.)

16. What units are used in the measurement of energy?
For example, the amount of energy needed to move one kilogram of mass a distance of one meter in one second-per-second (one second squared) is called a Newton (named for Isaac Newton an English scientist 1642-1727). Energy is also commonly measured in units called gram calories. One gram-calorie - usually spoken of simply as a calorie with a lower case 'c', is the amount of energy needed to raise the temperature of one gram of water from 14.5 degrees Celsius to 15.5 degrees Celsius.

17. Are food Calories the same as other calories?
No. The term Calorie used in relation to food energy means a kilocalorie and one Calorie (upper case 'C') or kilocalorie is equal to 1,000 gram-calories. In many parts of the world (but not yet in the US) a different unit is now used for food energy. This unit is the 'joule'.

18. Is food energy always measured in calories?
No. The calorie (gram-calorie) and Calorie (kilocalorie) are being replaced by the joule and kilojoule. Here are some equivalents:
1 Calorie (kilocalorie) = 1,000 calories
1 Calorie (kilocalorie) = 4184 joules
1 Calorie (kilocalorie) = 4.184 kilojoules
1 kilojoule = 0.239 kilocalories (Calories).

19. Does all of the Sun's radiated energy reach Earth?
No. The Sun is a spherical object and it radiates energy equally in all directions. Earth intercepts only a tiny fraction of the total radiation emitted by the Sun. However, this tiny fraction is actually an enormous amount of energy. The amount of solar energy intercepted by Earth in one minute is approximately equal to the total electrical energy artificially generated on Earth in an entire year.

20. Does Earth receive the same amount of solar energy all the time?
No. Earth intercepts the greatest amount of solar radiation in January when its orbit brings it closest to the Sun; and the least amount in July when its orbit takes it farthest from the Sun. This difference amounts to about 7 percent.

21. Do all parts of Earth receive the same amount of solar radiation (daylight)?
No. The Earth's axis tilts it at an angle as it orbits the sun so that the duration of daylight and darkness varies seasonally everywhere except at the equator. This tilt of the Earth's axis affects not only the duration of daylight but also the intensity of the solar radiation. When the Sun appears directly overhead it's intensity is greatest.

22. What happens to the solar radiation as it passes through Earth's atmosphere?
The atmosphere is made up of gases, particles and clouds. Solar radiation on its way to Earth's surface is affected by these atmospheric components. Some of the radiation (mostly long wavelength) is absorbed by atmospheric gas molecules. Some of the shorter wavelength radiation is scattered by the molecules, particles and the cloud water droplets. Some of the solar radiation is reflected back into space.

23. Do biological processes rely upon a supply of energy?
Yes. Both plants and animals are absolutely dependent upon energy for growth and movement and for the maintenance of basic function and structure. If energy intake drops below the amount required for maintenance, an organism consumes its own energy stores until those are exhausted and the organism dies.

24. Does all life on Earth depend upon radiant energy from the Sun?
With the exception of certain bacteria and algae which get their energy by the oxidation of inorganic compounds, and the animals that get their energy from these particular bacteria and algae, all life on Earth depends on radiant energy from the Sun.

25. How is solar (radiant) energy made available to living organisms?
Animals cannot use radiant energy but plants can. Chlorophyll molecules in green plants and algae absorb radiant energy (sunlight) and use this energy in the manufacture of glucose (a sugar). The radiant energy is now stored in the glucose molecule as chemical energy and is available to the plant for its growth and to animals when they eat plants.

26. Are plants the only source of energy for animal life?
Yes. Some animals (herbivores or vegetarians) eat plants and derive their energy from the chemical energy stored in the plants' complex carbohydrate molecules. Other animals (predators and scavengers) eat animals, or the remains of animals, who have eaten plants; so, ultimately all animal energy is derived from the radiant energy absorbed by chlorophyll molecules in green plants.

27. Which animals are herbivores and which are predators or scavengers?
Cattle, some insects, and some birds are examples of plant eaters - herbivores or vegetarians. Cats, wolves, spiders, amphibians and reptiles are examples of meat eaters - carnivores or predators. Examples of scavengers - animals that eat the dead remains of other animals - are lobsters and vultures.

28. What is a food chain, and what does a food chain have to do with energy?
A food chain has everything to do with energy. It is a term used by ecologists for the transfer of energy from its usable source in plants (see Day 25) through a sequence of living organisms.
This sequence starts with a plant-eating animal which is the source of food for the animal(s) above it in the food chain.
a) Grasses manufacture complex carbohydrate molecules using solar energy (sunlight);
b) herbivorous animals eat plants in order to obtain that food energy; and
c) carnivorous animals eat herbivores for their food energy.

29. Is photosynthesis the only way that solar energy can be captured in a usable form?
No. Although photosynthesis is so far the only way that solar energy is made available for the maintenance of organic life, scientists have invented solar cells that capture solar energy and transform it to electrical energy. This electrical energy can be used in machines to enable them to do work (cause change).

30. What is a Btu?
A Btu (British thermal unit) is a universal heat energy unit used to describe the amount of heat necessary to raise the temperature of one pound of water from 60 degrees to 61 degrees Fahrenheit (at sea level).
One Btu is equivalent to 1054.5 joules (see also Number 18).
A joule, named for James Joule, an English physicist (1818-1889), is a another unit of energy equal to a very specific amount of work done when a specific amount of force displaces a specific distance.

31. Do we need to conserve energy?
Yes. Although there is virtually an unlimited amount of solar energy available to Earth, the sources of that energy converted to a usable form are limited. These sources include plant life - humans are gradually covering more and more of the Earth with concrete thus reducing the space for plant life; and oil, coal and gas which exist in specific and limited amounts throughout the world.

[page created September 1999]

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