Sunday, November 25, 2018

Ohm's Law

  1 Law  

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

Ohm's Law is expressed by the equation  I = E /  
Spoken as, "  Amps is equal to volts divided by resistance."

So... what does directly and inversely proportional mean?

* Directly Proportional... 
...means that when one quantity increases the other increases
or  when one decreases the other decreases 

** Inversely Proportional,
...means that when one quantity increases the other decreases 
or  when one decreases the other increases 

So, when we say Ohm's Law,
"The  current in a circuit is directly proportional* to the applied  voltage  
and inversely proportional** to the circuit  resistance."
We are saying that as the circuit's  current  goes up  the applied  voltage is also going up 
and at the same time that the circuit's current  is going up the circuit's  resistance is going down 

The  current and  voltage rise and fall at the same time because they are Directly Proportional... 
they both go up at the same time  or they both go down at the same time 

The  current and  resistance rise and fall in opposite directions because they are Inversely Proportional... 
As one goes up the other goes downat the same time .

Does that make sense? 
OK so...  if the circuit's current  is decreasing , what is the circuit's voltage doing?
How about the circuit's  resistance ? What is it doing when the circuit's current  is decreasing ?

The answer is that the circuit's voltage is going down when the current is going down because they are Directly Proportional  and the circuit's  resistance is going up when the current is going down ▼ because they are Inversely Proportional.. .

One more time,
"The  current in a circuit is directly proportional* to the applied  voltage 
and inversely proportional** to the circuit  resistance."

The  current  (called amps represented by the letter    in a circuit)
 is directly proportional or  to the applied  voltage  (represented by the letter  E   and...

the  current is also inversely proportional or  to the circuit  resistance (represented by the letter  )
Expressed as,    I = E / 

Another way to express Ohm's Law is with triangle charts >  4 triangle charts

  4 Quantities P-I-E-R  

P or Watts is not considered a part of "Ohm's Law" but is commonly referenced in Ohm's Law calculations. 

The term "watts" originated in Watt's law which is defined as,  
Electrical energy, expressed as "P" measured in watts 
The Watt's Law formula,  P = I x E  is spoken as "watts (P) is equal to amps (I) times volts (E)"

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

  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  =    (P/R)  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 =  (P/R)   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 2x 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.

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.

*Basic Electricity US Navy Dover Publications ISBN 0-486-20973-3 shown below

  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 constantly changing...
,,, the values calculated with Ohms law would also be changing.

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. 2017 NFPA 70 NEC 310-15. 

The effect of rising temperatures increasing resistance is noticed during a summer heatwave. 
As the summer air temperatures rise, 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 sometimes creates an electrical blackout.

During an electrical blackout the news journalists will 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.



via GIPHY  Air Resistance


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Tony R. Kuphaldt   
A free series of textbooks on the subjects of electricity and electronics