In everyday conversation we speak of someone having lots of energy, of a
song sounding energetic, or an athlete being energized. In science, the term
"energy" has a precise definition that is somewhat differernt from the
ordinary meaning. To see what scientists mean when they talk about energy,
we must first introduce the concept of work
. Work
is done whenever a force
is exerted over a distance. when you pick up a book, for example, your
muscles apply a force equal to the weight of the book over a distance of
30 cm or so. You did work. The mathematical definition of work is:
In words:
Work is equal to the force that is exerted times the distance over which it
is exerted In equation form:
work (in Joules) = force (in Newtons) * distance (in meters)
where a joule is the unit of work. In symbols:
W = F * d
In practical terms, even a small force can do a lot of work if it is
exerted over a long distance. In the entire system of units, when force
is measured in newton
, work is measured in newton . meter
.
This unit is given the name joule
. One joule (j)
is defined
as the amount of work done when you exert a force of one newton through a
distance of one meter:1 joule of work = 1 newton of force x 1 meter of distance.
Example:
How much work do you do if you lift a 10 Kg suitcase, 1m off the ground? Reasoning:
We must first calculate the force exerted by a 10 Kg suitcase before we
can determine work. To lift a 10 Kg mass against the acceleration of
gravity (9.81 m/s 2
) requires a force
given by:
force = mass * g
= 10 kg * 9.81 m/s 2
= 98.1 N
Solution:
:
Then, from the equation for work,
work = force * distance
= 98.1 N * 1 m
= 98.1 J
Energy
is defined as the ability to do work. If a system's
capable of exerting a force over distance, then it possesses energy.
The amount of a system's energy, which can be recorded in J is a measure of
how much work the system might do. When you run out of energy. You can not
do any work.
Power
provides a measure of both the amount of work that is
done (or, equivalently, the amount of energy ecxpended), and the time it
takes to do it. In order to complete a physical task quickly, you must
generate more power than if that some amount of work is done more slowly.
Power
is defined as the rate at which work is done, or at which energy
is expended.
In words:
Power is the amount of work done, divided by the time it takes to do it. In equation form:
power (in watts) = work (in joules) / time (in seconds)
Where the watt (W) is the unit of power.
In symbols
:
P = W / t
If you do more work, or do a task in a shorter time, you use more power. Also, because the work performed equals the energy expended. power (in watts) = energy (in joules) / time (in seconds)
The watt
is defined as: 1 watt of power = 1 joule of energy / 1 second of time
For example, the rating of a light bulb, 60, 75, or 100 watts, is a measure of the rate of energy that the light bulb consumes when it is operating. The unit of 1000 watts is called a kilowatt
, a commonly used measurement of electrical power.The equation defining power may be rewritten as follows: energy (in joules) = power (in watts) x time (in seconds)
This equation is very important, because it allows you (and the electric company) how much energy you consume (and how much you have to pay for).
Important terms
Quantity |
Definition |
Units |
Force |
mass * distance |
N |
Work |
force * distance |
J |
Energy |
ability to do work |
J |
Energy |
power * time |
J |
Power |
work / time = energy / time |
W |
Work is the same as energy.
Another unit to measure energy is the Calorie
1 Wh = 1 Calorie
1kWh = 1000 Calories = 1kcal
Consider
the energy per gram
of different substances. Consider
the cost of energy
of different energy carriers.