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
- 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 =
- 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
- 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
- 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
- Heat energy moves in three ways:
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
- 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.