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Author Topic: E=IR, Eureka!  (Read 3828 times)
NK6Q
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Posts: 202




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« on: October 20, 2010, 05:25:09 PM »

OK, flame me for being dense on electronic theory.  There are still some areas that I find hard getting my mind wrapped around.  I've had E=IR memorized and used it since I was 14 and, yeah, I know the concept, but only today did I get a different understanding on what it means:

I was out back washing out my cats' litterbox, using a regular garden hose, but without a nozzle.  Like most folks in such a situation, I used my thumb to partly block the opening to create a stronger stream of water.  In my intro to electronics books, many times water flow is used as an analogy to current flow.  Let's see, I'm adding resistance to the flow of water; the same quantity of water is coming out of the hose but the force is stronger.  If the quantity of water is I, and I'm increasing R by blocking off part of the hose, the water pressure increases.  Holy Cow!  E is directly proportional to R!  The more I block off the flow of water the stronger the force of the water coming through!

"Well, duh", you say, "of course it is, you dunce!"  Yes, but I now get it.  I conceptualize it!  I never really thought exactly how or why resistance would increase voltage.  Now I'm going to coil up my hose and try inductance!

Bill in Pasadena
playing with water
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K7KBN
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Posts: 2764




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« Reply #1 on: October 20, 2010, 08:00:31 PM »

"Playing" with water is exactly the way Annie Sullivan broke through the wall and was able to teach Helen Keller the meaning of that one word: "water".  Then, with that obstacle out of the way, Keller was able to keep on learning everything she could.  Sometimes an epiphany strikes with an unbelievable "DUH!" -- but you'll never forget the relationship among E, I and R.

73
Pat K7KBN
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73
Pat K7KBN
CWO4 USNR Ret.
K9YLI
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Posts: 850




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« Reply #2 on: October 21, 2010, 07:31:45 AM »

in your example  maybe two things actually  occured.

one  the  pressure went up,  this was to attempt to keep the same 'volumn' of water flowing ,(current)

second, with more  'resistance'  the   volumn of water decreased,
as in more resistance , less current for the same  voltage.

so actually  the   80# of water pressure  at the beginning of the hose didnt change(voltage),,  the added resistance actually reduced the   flow rate((volumn per time) (current)).

there is no free lunch.   so adding resistance did not increase the  actuall pressure, just reduced the flow rate in   gallons per minute..

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NK6Q
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Posts: 202




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« Reply #3 on: October 21, 2010, 07:40:21 AM »

Well, right.  However, then it's I=E/R.  Resistance increases, water pressure stays the same, so the flow rate is reduced.  Flow rate of water relates to coulombs of electrons, so Ohm's law still corresponds to the water hose analogy.  Hey, and water always seeks ground!! :-)
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WB2EOD
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Posts: 214




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« Reply #4 on: October 22, 2010, 11:32:18 AM »

"Twinkle twinkle, little star"
"E is equal to IR"
"Up above the world so high"
"P is equal to EI"

-High School Electronics Shop Class 1970-
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AA4PB
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Posts: 12683




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« Reply #5 on: October 22, 2010, 11:40:07 AM »

The Indian sees the Eagle over the Rabbit (I = E/R)
The Rabbit sees the Eagle over the Indian (R = E/I)
The Eagle sees the Indian and the Rabbit on the same level (E = I*R)

Navy - 1963

We won't go into the one for resistor color codes  Cheesy
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K3RMX
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Posts: 31




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« Reply #6 on: October 22, 2010, 03:41:57 PM »

I recall the resistor code (mnemonic?) having something to do with bad boys and young girls.  Is that the one you're talking about?

steve
k3rmx
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WB2EOD
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Posts: 214




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« Reply #7 on: October 22, 2010, 03:48:30 PM »

another creative memory device from High School.  Yes it had to do with bad boys young girls and Violet
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WB2EOD
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« Reply #8 on: October 22, 2010, 03:57:42 PM »


Better Be Right Or Your Great Big Venture Goes Wrong
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K7KBN
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Posts: 2764




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« Reply #9 on: October 22, 2010, 05:54:43 PM »

"Twinkle twinkle, little star"
"E is equal to IR"
"Up above the world so high"
"P is equal to EI"

-High School Electronics Shop Class 1970-

I learned it as:

Twinkle, twinkle little star,
Power equals I squared R.

Little star up in the sky,
Power equals E times I.

73
Pat K7KBN
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73
Pat K7KBN
CWO4 USNR Ret.
NK6Q
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Posts: 202




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« Reply #10 on: October 24, 2010, 06:27:02 PM »

another creative memory device from High School.  Yes it had to do with bad boys young girls and Violet

Something about "Violet goes willingly".  I forget the rest (not!)
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AC5UP
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Posts: 3825




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« Reply #11 on: October 24, 2010, 08:39:52 PM »

Dr. Ohm, the optometrist, has a small eye chart on the wall...
Looks like this:

E
I R


If a person wanted to know E, it would be I * R (cover E and the only thing showing is IR).
If a person needed I it would be E / R (cover I and E over R is showing).

Anyone want to guess how to solve for R ?

BTW: Bad Boys Run Over Your Grass But Pretty Girls Won't....................  Grin
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KN7F
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Posts: 5


WWW

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« Reply #12 on: November 01, 2010, 12:14:04 PM »

Look at it from the point of view of impedance matching.
No impedance, lots of current but little pressure, not much work (power).
Increasing impedance reduces current but increases pressure, more work (pressure * current).
Increase impedance too much and you get lots of pressure but little current, not much work.
You get the most power at half the maximum pressure and half the maximum current.

Mostly water analogies hold up pretty well but not always.

Have fun
Steve
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VE7DQ
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Posts: 175




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« Reply #13 on: November 01, 2010, 07:53:11 PM »

AC5UP, that's not the way I remember being taught the colour code in high school! 

"Violet Gives Willingly" sounds familiar... so to speak.   Grin
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K7KBN
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Posts: 2764




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« Reply #14 on: November 02, 2010, 01:54:54 PM »

Then there was the way some of us were taught to remember the formulae for finding sine, cosine and tangent .... Grin
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73
Pat K7KBN
CWO4 USNR Ret.
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