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Lesson 15: The Law of Conservation of Energy


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Download here: Ontario Curriculum Expectations

Energy is the ability to do work. The more energy you have, the more work you can do.

Characteristics of Energy

Energy has the following five characteristics:

  1. Energy comes in many forms that are interchangeable.

There are four different forms of potential energy and five different forms of kinetic energy. When you eat an apple and then go for a run, you have taken the chemical energy that was stored in the molecules of the apple and converted it into motion, or kinetic energy.

  1. Energy is transferred from one object to another whenever work is done.

When a bulldozer pushes a pile of dirt across a construction site it is doing work on the dirt. In order for this to occur, the chemical energy in the gasoline is transferred into the motion, or kinetic energy, of the bulldozer and the dirt.

  1. Energy can be stored and used at a later time to do work.

Stored energy is called potential energy.

  1. Energy is measured in Joules (J).
As a matter of interest, a Joule (J) is equal to a Newton·metre (N·m).This relationship can be derived from the work equation:equation where the units are Joules (J).
  1. The total amount of energy is always conserved in a closed system.

system is like a part of the universe. It may be as large as the universe itself, or smaller than a sample in a test tube. If you can prevent any energy from entering or exiting the system, it can be called a closed system. It is like sealing off a section of the universe. Although no system can be completely closed, if you are careful you can approximate a closed system.

The Law of Conservation of Energy

The law of conservation of energy states that:

Energy cannot be created or destroyed; it can only be transformed from one form of energy into another. The total amount of energy in the universe remains the same.

This means that the chemical energy stored in a molecule of gasoline does not get “used up” by a car’s engine. Instead it is converted into kinetic energy as the car starts moving, sound energy when you can hear the car’s engine, electrical energy which you use to power the radio, lights, and dashboard, and thermal energy which causes the car’s engine to heat up, along with friction from the air and the road.

Thermal energy is often considered to be waste energy because it is very difficult to convert thermal energy into any other form of energy efficiently. During almost all energy conversions, some of the system’s energy will be lost in the form of thermal energy.

The multimedia activity Conservation of Energy will allow you to investigate several situations where energy is converted from one form into another.