Energy causes things to happen around us. Look out the window. During the day, the sun gives out light and heat energy. At night, street lamps use electrical energy to light our way. When a car drives by, it is being powered by gasoline, a type of stored energy. The food we eat contains energy. We use that energy to work and play.
Energy Is the Ability to Do Work.
Energy can be found in a number of different forms. It can be chemical energy, electrical energy, heat (thermal energy), light (radiant energy), mechanical energy, and nuclear energy.
Stored and Moving Energy
Energy makes everything happen and can be divided into two types: Stored energy is called potential energy. Moving energy is called kinetic energy.
With a pencil, try this example to know the two types of energy. Put the pencil at the edge of the desk and push it off to the floor. The moving pencil uses kinetic energy. Now, pick up the pencil and put it back on the desk. You used your own energy to lift and move the pencil. Moving it higher than the floor adds energy to it. As it rests on the desk, the pencil has potential energy. The higher it is, the further it could fall. That means the pencil has more potential energy.
How Do We Measure Energy? Energy is measured in many ways. One of the basic measuring blocks is called a Btu. This stands for British thermal unit and was invented by, of course, the English. Btu is the amount of heat energy it takes to raise the temperature of one pound of water by one degree Fahrenheit, at sea level.
One Btu equals about one blue-tip kitchen match.
One thousand Btus roughly equals: One average candy bar or 4/5 of a peanut butter and jelly sandwich. It takes about 2,000 Btus to make a pot of coffee.
Energy also can be measured in joules. Joules sounds exactly like the word jewels, as in diamonds and emeralds. A thousand joules is equal to a British thermal unit.
1,000 joules = 1 Btu
So, it would take 2 million joules to make a pot of coffee.
The term "joule" is named after an English scientist James Prescott Joule who lived from 1818 to 1889. He discovered that heat is a type of energy. One joule is the amount of energy needed to lift something weighing one pound to a height of nine inches. So, if you lifted a five-pound sack of sugar from the floor to the top of a counter (27 inches), you would use about 15 joules of energy.
Around the world, scientists measure energy in joules rather than Btus. It's much like people around the world using the metric system of meters and kilograms, instead of the English system of feet and pounds.
Like in the metric system, you can have kilojoules -- "kilo" means 1,000.
1,000 joules = 1 kilojoule = 1 Btu
A piece of buttered toast contains about 315 kilojoules (315,000 joules) of energy. With that energy you could:
Jog for 6 minutes
Bicycle for 10 minutes
Walk briskly for 15 minutes
Sleep for 1-1/2 hours
Run a car for 7 seconds at 80 kilometers per hour (about 50 miles per hour)
Light a 60-watt light bulb for 1-1/2 hours
Or lift that sack of sugar from the floor to the counter 21,000 times!
Energy can be transformed into another sort of energy. But it cannot be created AND it cannot be destroyed. Energy has always existed in one form or another. Here are some changes in energy from one form to another.
Stored energy in a flashlight's batteries becomes light energy when the flashlight is turned on. Food is stored energy. It is stored as a chemical with potential energy. When your body uses that stored energy to do work, it becomes kinetic energy. If you overeat, the energy in food is not "burned" but is stored as potential energy in fat cells. When you talk on the phone, your voice is transformed into electrical energy, which passes over wires (or is transmitted through the air). The phone on the other end changes the electrical energy into sound energy through the speaker. A car uses stored chemical energy in gasoline to move. The engine changes the chemical energy into heat and kinetic energy to power the car. A toaster changes electrical energy into heat and light energy. (If you look into the toaster, you'll see the glowing wires.) A television changes electrical energy into light and sound energy.
Heat is a form of energy. We use it for a lot of things, like warming our homes and cooking our food. Heat energy moves in three ways:
Conduction occurs when energy is passed directly from one item to another. If you stirred a pan of soup on the stove with a metal spoon, the spoon will heat up. The heat is being conducted from the hot area of the soup to the colder area of spoon.
Metals are excellent conductors of heat energy. Wood or plastics are not. These "bad" conductors are called insulators. That's why a pan is usually made of metal while the handle is made of a strong plastic.
Convection is the movement of gases or liquids from a cooler spot to a warmer spot. If a soup pan is made of glass, we could see the movement of convection currents in the pan. The warmer soup moves up from the heated area at the bottom of the pan to the top where it is cooler. The cooler soup then moves to take the warmer soup's place. The movement is in a circular pattern within the pan.
The wind we feel outside is often the result of convection currents. You can understand this by the winds you feel near an ocean. Warm air is lighter than cold air and so it rises. During the daytime, cool air over water moves to replace the air rising up as the land warms the air over it. During the nighttime, the directions change -- the surface of the water is sometimes warmer and the land is cooler.
Radiation is the final form of movement of heat energy. The sun's light and heat cannot reach us by conduction or convection because space is almost completely empty. There is nothing to transfer the energy from the sun to the earth.
The sun's rays travel in straight lines called heat rays. When it moves that way, it is called radiation. When sunlight hits the earth, its radiation is absorbed or reflected. Darker surfaces absorb more of the radiation and lighter surfaces reflect the radiation. So you would be cooler if you wear light or white clothes in the summer.