Wednesday, January 28, 2009

Energy, Power, and Time by Lee Bond N7KC

January 28, 2009 Educational Radio Net, PSRG

For the 36th session of the Educational Radio Net, I have chosen to review basic and important concepts that cannot be avoided when dealing with electrical equipment including radios. There are many but the really important ideas wheel around voltage, current, resistance, energy, power, and time. Probably the least understood are the relationships of energy, power, and time so let's start with these three.

Energy and work are equivalent. If you perform any task that requires physical exertion then you have expended energy in some form. Move a heavy box from the floor to a table top and you have done 'work' and expended energy in the process. Another example might be your car on a level street. Think of a level, and straight, street 1/4 mile long. Simply starting the engine requires electrical energy to turn the starter which does work on the flywheel which then rotates the crank, etc., and the engine starts. Moving the car toward the street end requires energy expenditure and, finally, stopping the car requires energy expenditure as well. So, the starter uses electrical energy to start the car followed by burning fuel to run the heat engine followed by braking which produces heat in the brakes to stop the car. The car starts from a standstill and stops at a standstill on a level street so the net energy is zero. This means that all of the energy used to accelerate the car is exactly equal to the energy dissipated in stopping the car using the brakes.

Lets look at another example using an antenna tower. Jim and Joe weigh the same but Jim is in very good condition. Both men start from the ground and climb the tower to the 100 foot level. Jim makes the top in one minute but Joe requires two minutes to make the same climb. Both Jim and Joe perform exactly the same physical work climbing the tower but Joe takes longer. Jim is the more 'powerful' climber since he expended the same energy as Joe in one half the time. By definition, power is the rate that energy is expended. The word 'rate' always denotes something per time like miles per hour, pounds per second, furlongs per fortnight, feet per second, or joules per second.

Electrical energy is measured in joules so electrical power is expressed by joules per second. You will not be surprised to find out that one joule per second is one watt. A tiny bit of algebraic manipulation shows that one joule is one watt-second so power times time equals energy.

Enter the kilowatt-hour meter on your house. Notice that power given by kilowatts multiplied by time given by hours represents the energy consumed in the house. So you end up paying the power company for the total work performed to maintain your home over one billing cycle. KwH meters are average indicators in that they cannot register the rate that power is delivered to the home. Some industrial energy meters also measure 'demand' or how fast the energy is delivered to the plant in addition to total energy used over one billing cycle.

Common knowledge tells us that electrical power can be computed by multiplying voltage and current, or multiplying current squared and resistance, or by voltage squared divided by resistance. All three of these operations yield watts and now we know that there is a time connection since all three operations are really representing energy per second.

In summary, easy to understand examples in the physical world relating work, power, and time can be extended to the electrical world as well. Knowing definitions and being able to manipulate variables such as current, voltage, and resistance will certainly increase your enjoyment of radio phenomena many fold.

This concludes the set up discussion of energy, power, and time. Are there any questions with regard to tonight's discussion?

This is N7KC for the Wednesday night Educational Radio Net.

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