PrintIn the first lesson, we saw that energy can be transformed from one form to another, and during this conversion, all the energy that we put into a device comes out. However, all the energy that we put in may not come out in the desired form.
For example, we put electrical energy into a bulb and the bulb produces light (which is the desired form of output from a bulb), but we also get heat from the bulb (undesired form of energy from an electric bulb).
Therefore, energy flow into and out of any energy conversion device can be summarized in the diagram below:
When all forms of energy coming out of an energy conversion device are added up, it will be equal to the energy that is put into a device. Energy output must be equal to the input. This means that energy can not be destroyed or created. It can only change its form.
In the case of an electric bulb, the electrical energy is converted to light and heat.
The amount of electrical energy put into a bulb = the amount of light energy (desirable form) plus the heat energy that comes out of the bulb (undesirable form).
Say you go to the mall with $100, and you come back with only $10. You need to account for the $90 that was spent. After thinking about it, you come up with the following list:
Gas ($15); Sandwich, fries, and a drink ($8); Lost ($5); New clothes ($62)
So you spent $62 on something useful - the clothes - but you spent additional money for other things that were necessary for your trip to the mall.
Self Check
Instructions: Identify the useful energy output(s) and undesirable energy output(s) in the energy conversion devices below. Enter your answers in the fields provided, and click the "Check" button to check your work.
Self Check:
Energy Conversion Devices
For each of the following examples, determine the types of useful energy and undesired energy for the given energy converter.
Example 1: Lawnmower with a chemical energy input. (Hint: How do you know when your neighbor is mowing the lawn?)
Example 2: Car with a chemical energy input. (Hint: Think about mufflers, tires, and generator.)
Example 3: Television with an electrical energy input. (Hint: Have you ever felt the back of your TV after it has been on for a few hours?)
Example 4: Desktop computer with an electrical energy input. (Hint: What’s in your tower and why?)
Answers:
Example 1: The useful energy for a lawnmower is mechanical, while the undesired energy is thermal (heat) and radiation (noise).
Example 2: The useful energy for a car is mechanical, while the undesired energy is thermal or heat (tail pipe).
Example 3: The useful energy for a TV is radiation (light and sound) and the undesirable energy is heat (from circuits).
Example 4: The useful energy for a computer is radiation (light and sound) and the undesirable energy is heat (circuits – electrons moving through system) and mechanical (fan for cooling).