Monday, December 1, 2014

Ohm's Law

  1 Law  

"The  current in a circuit is directly proportional to the applied  voltage  and inversely proportional to the 
circuit  resistance."

...or you could say...
The  current (called amps) represented by the letter ( I )  in a circuit,
 is directly proportional to the applied  voltage represented by the letter ( E ) ...
(when one increases the other increases, and when one decreases the other decreases )

... and the  current is also inversely proportional to the circuit  resistance represented by the letter ( R )
(when one increases the other decreases and when one decreases the other increases ) 

Ohm's Law is expressed by the equation  I = E / R  
Spoken as, "  Amps is equal to volts divided by resistance."
Another way to express Ohm's Law is with triangle charts >  4 triangle charts




  4 Quantities  

E   represents electromotive force, EMF and is measured in voltage or volts or V.
I   represents the intensity of electrical current and is measured in amperage or amps or A.
R   represents resistance and is measured in ohms or the omega symbol; Ω
P   represents power or electrical energy and is measured in wattage or watts or W. 



  12 formulas  

  This equation; E = I x R = P / I =  (PxR) represents the first 3 formulas; 
Formula 1)  E = I x R  Volts (represented by the letter E) is equal to amps (letter I ) times resistance (R). 
Formula 2)  E =  P / I  Volts is equal to watts (P) divided by amps. 
Formula 3)  E =  (PxR)    Volts is equal to the square root of the answer to watts times resistance. 

  This equation; I = E / R = P / E =  (PxR)  represents;
Formula 4)  I = E / R  Amps is equal to volts divided by resistance. 
Formula 5)  I = P / E  Amps is equal to watts divided by volts.
Formula 6)  I =  (PxR)   Amps is equal to the square root of the answer to; watts divided by resistance.

  This equation; P = I x E = E 2 / R = I 2 x R  represents; 
Formula 7)  P = I x E  Watts is equal to amps times volts. 
Formula 8)  P =  2 / R  Watts is equal to volts squared divided by resistance. 
Formula 9)  P =   I 2 x R  Watts is equal to amps squared times resistance.

  This equation; R = E / I = P /  I 2 = 2 /  P  represents;
Formula 10)  R = E / I  Resistance is equal to volts divided by amps.
Formula 11)  R = P /  I 2  Resistance is equal to watts divided by the answer to; amps squared.
Formula 12)  R = 2 /  P  Resistance is equal to volts squared divided by watts.

If you know the value of any 2 quantities, you can find the third by using one of the above 12 formulas.

Example;
You want to find the watts (P) and you already know the amperage (I) is 5 amps and the voltage (E) is 10 volts. Look through the "P" formulas above for the one that contains I and E. Use formula 7:  P = I x E.
Now replace the letters with your values; P (watts) = 5 (amps) x 10 (volts) or P = 5 x 10.
Now solve for P; 5 x 10 = 50, P = 50 watts.



  1 Flaw   

Missing, in the formulas, is temperature and it's effect on resistance. 
As the temperature increases the resistance also increases. Because temperature is not constant but changing with time it also changes the values calculated with Ohms law.

It is as though you solved this equation; E = I x R   finding 10 (E) volts = 5 (I) amps x 2 (R) ohms.
But then a change in temperature creates more resistance and when you are not looking it erases your 2 ohms and changes it to 3 ohms leaving you with an faulty calculation. 

To electricians, the National Electrical Code addresses this flaw by adding temperature "correction factors" to the allowable conductor ampacity tables beginning with table 310-16. 

















Ohm's flaw as explained by Professor Walter Lewin of MIT beginning at 21:00 on the video counter.
Don't let the technical talk distract you. Take notice of the effect temperature has 
on the resistance especially during his demonstration with the oscilloscope as the heat 
from the lamp increases.

Changing air temperature also has an effect on resistance as seen during a summer heatwave. 
As the summer air temperature rises it increases the resistance in the cross country, overhead power lines. 
This added resistance is like a "heat monster" consuming electricity and wasting it with no purpose.
The additional electrical demand overloads the utility and creates a blackout.

During an electrical blackout there is mention of the added electrical demand of air conditioners but many fail to note the added electrical demand of the "heat monster" .. that invisible electrical consumer that may have also been unseen by Professor Ohm.


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  Links  

 Using facial muscles for resistance to electrical current;
  



 Khan Academy   Free online math lessons 
 http://www.khanacademy.org/math/arithmetic


Tony R. Kuphaldt   
A free series of textbooks on the subjects of electricity and electronics 
http://www.ibiblio.org/kuphaldt/electricCircuits













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


http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html


                 




http://phet.colorado.edu/en/simulation/ohms-law

                                      
http://www.kpsec.freeuk.com/ohmslaw.htm


http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/ohmslaw.htm

http://www.wisc-online.com/Objects/ViewObject.aspx?ID=dce11904
http://www.teachersdomain.org/resource/hew06.sci.phys.maf.ohmslaw/

Text based square root symbol thanks to http://www.scientificpsychic.com/etc/square-root.html

More from this author; http://120v.blogspot.com/
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