Think very carefully about commonly used electrical devices in your home such as a light bulb

What type of energy conversions are taking place?

Inputted: electrical energy

Outputted: Light Heat

If you think about the purpose of a light bulb, it is designed to produce light in order to illuminate a room

Is the heat energy being generated as well of any use to us?

Recently, the Ontario government has been pushing homeowners to change their old incandescent bulbs to the new compact flourescent bulbs (CFB)

And incandescent light bulb and a flourescent light bulb both produce the same amount of light – which means that the USEFUL ENERGY OUTPUT generated by the bulbs is about the same

However, an incandesncent light bulb (IB) has a power rating of 60 W, whereas a CFB has a power rating of 15 W

For about ¼ of the electrical energy input, a CFB produces the same amount of light as a IB

Which means that it is 4 times more efficient This suggests that compared to the CFB, the

IB wastes about 75% of its energy – it gets converted to heat energy

You should realize this since no one wants to touch an IB that has been on for an extended period of time

CFB’s are more efficient when left on – they produce less heat energy which is not useful in the regular functioning of a light bulb

Since they also last longer – CFB’s are a good choice for replacing IB’s since they use less energy and they don’t have to be replaced as often

A machine is a device that converts energy or force into a form that can be used to complete a job that the machine is designed for

A light bulb turns electrical energy into light energy

But what about other machines? Are they as efficient?

Your stove takes electrical energy and converts it to similar forms

But what type of energy, when discussing a stove, is useful?

Remember that the type of device will determine what is considered to be USEFUL ENERGY OUTPUT

In a stove, heat is the USEFUL ENERGY OUTPUT whereas the light produced would be wasted energy

Whereas in a light bulb, this is opposite

There are many interactions going on in any machine at the molecular or subatomic level that we can’t see

Remember that most machines create movement: in a stove or a bulb, electrons are forced to move through resistors

These small particles carry the inputted energy and in order to convert it to another type, it has to undergo some interaction that saps energy from it

Electrons slam into the molecules of resistors, increasing the motion of these particles and thus creating energy in the form of heat

In mechanical devices like cars, moving metal parts rub up against each other creating heat through friction

That most cars are less than 50% efficient?

Most of the gasoline you pump into a car (chemical energy) is converted to mechanical energy through combustion

This creates motion in metal parts such as pistons inside an engine

This generates a great deal of heat – think about how hot the hood of your car gets after a long drive

http://www.grc.nasa.gov/WWW/K-12/airplane/engopt.html

The amount of energy put into the machine to complete a job is the useful energy input

For example, for both the light bulb and stove, this is the amount of electrical energy inputted as defined by E = Vit

It is important to note that for all machines, useful energy input is always GREATER than useful energy output

Since there is no such thing as 100% efficient machine, the following must be true:

Useful energy input > Useful energy output

Why is it impossible to have a machine that has an efficiency?

Efficiency values of over 100% violate the Law of Conservation of energy

It essentially means that the work being completed by the machine is being completed for free – since you are getting more out of the machine than you are putting in

UEI = useful energy input in J UEO = useful energy output in J

Efficiency = UEO x 100% UEI

Where: UEI > UEO for a real machine