Purpose

To define work, energy, and power and differentiate between them

You will...

• Define work
• Define energy
• State the unit of measure for energy
• State the unit of measure for work
• Define power
• State the unit of measure for power
• Determine the work done in a system
• Investigate the applications of work, energy, and power in technological problems and solutions

Background Information

Work

Work occurs when energy gets transferred from one system (or object) to another system (or object).  Usually this results in some form of motion in the object receiving the energy.  Or put another way, work is the application of a force resulting in a movement in the direction of the force.  For example, if you push on a block with a force of 50 N (newtons), and the block does not move, no work was done on the block.  You may have worked, but there was no energy transferred to the block and the block did not move, so no work was done on the block.  If, on the other hand, you moved the block, then you did work on the block.

The image below illustrates the principle of work.

Figure Work is force applied over a distance (ie. it requires movement)

Energy

Energy is the ability to do work (that is, to make something move).

Two Types of Energy

Energy can be one of two types

• Potential energy.  Potential energy is the stored energy that an object has because of its position or physical properties. Potential energy is the capacity to do work, or in other words the amount of stored energy available to do work. 
• Kinetic energy.  Kinetic energy is the energy an object has because of its motion.  Work is kinetic energy in action.

Energy comes from many sources and takes a variety of forms.  We will look at those in a future activity. 

Renewable and Non-Renewable Energy

There are two broad ways to obtain energy—from renewable resources and from non-renewable resources.  Generally speaking, renewable energy comes from wind, solar, and bio-fuels (fuels made from plants).  Water can be a renewable source of energy, depending on where it comes from and how it is used.  Non-renewable sources are mainly fossil fuels.

Recently, people are beginning to think of energy in terms of how 'green' it is.  Called the carbon footprint (for its impact on the earth), this is a measure of how much carbon and greenhouse gasses are put into the atmosphere during the production, distribution and use of the energy.  Ethanol, which is a fuel made from plants, has been considered a good replacement for gasoline.  Recent studies, however, show that the production, distribution, and burning of ethanol has a total carbon footprint almost as great as gasoline.

Unit of Measure for Energy

The unit of measure for energy is the joule.  A joule is the amount of energy required to lift a 1 newton weight a distance of 1 meter.

Figure Unit of Energy — 1 joule = energy required to lift 1 newton a distance of 1 meter

Unit of Measure for Work

The unit of measure for work is also the joule.  And in fact, 1 joule of work is done when a weight of 1 newton is lifted a distance of 1 meter.

Figure Unit of Work—1 joule of work = 1 newton lifted 1 meter

The unit of energy and the unit of work are the same—the joule.  As can be seen from the preceding diagrams, when a joule of energy is expended to move a 1 newton weight over a distance of 1 meter, 1 joule of work is done.

Power

Power is a measure of how long it takes to do work, or the rate at which work is done.  More power means the work can be done faster, and less power means it takes longer to do.  For example, suppose you had to move 120 railway cars, and a single locomotive could move 30 cars. If you ignore things like friction, and the time for return trips

• Using 1 locomotive it would take 4 trips (30 cars each trip) to move the cars
• Using 2 locomotives (double the power) it would take 2 trips (60 cars each trip)
• Using 4 locomotives (4 times the power) it would take 1 trip (120 cars)

Figure Power is the rate at which work is done

   

Unit of Measure for Power

Power is measured in watts. 

1 Watt is defined as 1 joule of energy expended in 1 second.  Since energy applied over a distance is work done on the object,

1 watt of power = 1 joule of work completed in 1 second

For More Information

• Work, Energy and Power

When you are ready, go to Your Turn