Basics


Work and Energy

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/s2) requires a force given by:

force = mass * g

= 10 kg * 9.81 m/s2

= 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.

Ch. Elster
Aug 26 14:27:03 EDT 2020