CHEMystery: Energy: Energy

Energy

Return to the Energy Page.

Energy is measured in a couple of different ways.

One of the basic measuring blocks is called a Btu. This stands for British thermal unit.

Btu is defined as 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 roughly equals:

One average candy bar
4/5 of a peanut butter and jelly sandwich
One blue-tip kitchen match

It takes, for example, about 2,000 Btus to make a pot of coffee.

Energy can also be measured in joules. Joules sounds the same way as the word jewels, like diamonds and emeralds. It takes 1,000 joules to equal a British thermal unit. So:

         1,000 joules = 1 Btu

So, it would take 2 million joules to make a pot of coffee.

Joule is named after an English physicist named James Prescott Joule who lived from 1818 to 1889. He discovered that this a type of energy.

One joule is the amount of energy needed to lift one pound about nine inches.

Around the world, scientists measure energy in joules rather than Btus. It's much like people around the world using the metric system, 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 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 about 50 miles per hour
Light a 60 watt light bulb for 1-1/2 hours

According to the Law of Conservation of Matter and Energy, Energy can only change its form, It cannot be created nor can it be destroyed.

Here are some changes in energy from one form to another.

Stored energy in a flashlight's batteries becomes light energy when turned on.
Food contains energy stored as chemical potential energy. Your body uses the stored energy to do work, kinetic energy.
If you overeat, the food's energy is stored as potential energy in fat.
When you talk on the phone, your voice is changed to electrical energy. The phone on the other end changes the electrical energy into sound energy.
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 energy.
A television changes electrical energy into light and sound energy.

Heat 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
Convection
Radiation

Conduction is 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. Other things like wood or plastics are not good conductors of heat energy. These "bad" conductors are called insulators. That's why a pan is usually made of metal and 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 the soup pan above was 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 (see picture above).

Wind is often caused by convection currents. During the daytime, cool air from over water moves to replace the warm air over land that rises. During the nighttime, the directions changes and the water is 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 convention 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 like that, it is called radiation.

When the sun light hits the earth, its radiation is absorbed or reflected. Darker surfaces absorb more of the radiation and lighter surfaces reflect the radiation. So, if you wear light or white clothes outside during the summer, you be cooler.