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This topic comprises 2 pages: 1 2
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Topic: Ohms Law Explanation
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Tim Reed
Better Projection Pays
Posts: 5246
From: Northampton, PA
Registered: Sep 1999
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posted 11-29-2007 05:33 PM
Ohm's Law is the fundamental relationship between the properties that cause electron flow in an electronic circuit.
Here are the basic forms of the relationship:
E=IR
I=E/R
R=E/I
Where:
E=Electromotive Force (i.e., voltage, measured in Volts)
I=Intensity (i.e., current, measured in Amperes)
R=Resistance, measured in Ohms
Voltage (E) is the pressure that causes a current (I) to flow through a resistance (R). A common analogy is that of water flowing through a pipe, with the exception that electrons do not continue to flow if the circuit is open. A pressure (E) causes a certain amount of current (I) to flow through a resistance (R) - based on the amount of that resistance and the value of the applied voltage. If the resistance is great enough (i.e., infinity, or an open circuit), the current flow stops (even though the applied voltage may be constant). The less the resistance, as indicated by the formula, the more current will flow. As the resistance decreases near zero (or, down to the resistance of just the wire making up the circuit), maximum current will flow -- depending on the value of the applied voltage (E).
Current flow will increase if either the voltage is increased, or the resistance is decreased.
Current flow will decrease if either the voltage is decreased, or the resistance is increased.
If any two values of voltage, current, or resistance are known, you can use the Ohm's Law formula to find the missing value.
Hope that's not too obscure.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 11-29-2007 05:40 PM
Okay...here we go.
Roger gave you a mix of good and bad info...Ohms law is one of the absolutes in electricity thus it applies to both AC and DC circuits...simply replace the "R" with a "Z" and you get impedance. The real big absolute in electricity is Power in = Power Out...but you didn't ask about power here so lets get into Ohms Law.
What Roger showed in the equations and example are true.
Ohm's law gives you way to express the RELATIONSHIP of current, voltage and resistance (impedance) in any circuit. Providing you know basic algebra, if you know any two pieces of the puzzel, you can find the 3rd. As Roger stated, with AC things get more complicated since you have time variances introduced from the reactive components (inductances and capacitances). Since to work with the AC variants one needs an understanding of Calculus (both derivative and integration), it is highly unlikely that information would be on any test for a projection position...even if in management.
So, as far as Ohms law is for you, the DC version is all that might be asked (or AC with hypothetical non-reactive parts for even wire has inductive and capacitive properties).
So I=E/R (E=Electromotive Force...aka Voltage so it is also written I=V/R). I is current and R = resistance and denotes no reactive components.
Again, presuming you know basic math, don't be confused by alternative forms...V=R*I, R=V/I...it is all the same thing. All you might possibly be asked to do is "Plug and Chug" as we used to say.
It is also possible that "Power" might come up, I guess and be mixed with Ohms Law. Power equalling V*I.
Using some basic algebra you can use that power equation and substitute R*I for V as shown with Ohms law so you get R*I*I = P (Power, in watts). That is, P=I^2*R. Likewise, you could have used the V/R instead of I so you would have had V*(V/R)=P...or P=(V^2)/R.
So there it is in all its likely forms. The rest is your ability to deal with algebra and word problems.
Steve
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