7.8 efficiency
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7.8 Efficiency. Think very carefully about commonly used electrical devices in your home such as a light bulb What type of energy conversions are taking place?. Is complete energy conservation possible?. Inputted: electrical energy Outputted: Light Heat. LIGHT BULB. - PowerPoint PPT PresentationTRANSCRIPT
Think very carefully about commonly used electrical devices in your home such as a light bulb
What type of energy conversions are taking place?
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