O HM’S L AW AND R ESISTANCE
Resistance is one of the basic principles of Ohm’s law, and can be found in virtually any device used to conduct electricity.
Georg Simon Ohm was a German physicist who conducted some very early experiments in electricity. His discovery of the relationship between current, voltage, and resistance is the basic law of current flow, and the formula that connects these three measurements is named in his honor.
Resistors are one of the basic building blocks of electrical circuits. Resistance occurs in all materials, but resistors are discrete components manufactured to create an exact amount of intended resistance in a circuit. Resistors are made of a mixture of clay and carbon, so they are part conductor part insulator. Because of this, they conduct electricity, but only with a set amount of resistance added.
The value of the resistance is carefully controlled. Most resistors have four color bands. The first band reveals the first digit of the value. The second band reveals the second digit of the value. The third band is used to multiply the value digits. The fourth band tells the tolerance of the accuracy of the total value. If no fourth band is present, it is assumed that the tolerance is plus or minus 20%.
Here are the digits represented by the colored bands found on a resistor:
Black 0
Brown 1
Red 2
Orange 3
Yellow 4
Green 5
Blue 6
Violet 7
Gray 8
White 9
Ohm’s law states this mathematical formula:
Voltage is equal to resistance multiplied by the current flow, or E=IR.
As with any algebraic formula, it is possible to rearrange the terms in order to solve the equation for a specific unit of measurement. Two algebraic equivalents of the formula would be:
I=E/R R=E/I
A very handy magic triangle is available that makes it easy to remember the different permutations of this formula.
Cover the value to be determined with your finger, and the relationship of the other two are already in the proper form.
(Example: you need to know the amount of current flowing in a circuit with 100Ω of resistance and 100 volts of pressure.
Cover I, the symbol for current, and the
remaining two symbols, E and R, appear
in their correct relationship E/R.)
Ohms law and other formulae like it will yield an accurate result if and only if all of the units of measurement (such as Volts, Amps, and Ohms) use the same multiplier prefix within the same algebra problem.
Otherwise, your answer will be off by some order of magnitude, or power of ten.
Most often, it is easiest just to convert any readings you have into units, where no prefix is required. But this could leave you with a large number of 0s to keep track of.
On occasion, it may be more expedient to maintain a prefix such as Mega, if all of the measurements are given using that prefix. If the latter method is used, the answer to the problem will automatically come out in the same prefix used for the component parts.
For example:
#1
E (in volts) = I (in amps) x R (in ohms) E = 2A x 100Ω
E = 200v
___________________________
#2
E (in Megavolts) = I (in MegaAmps) x R (in MegaΩ)
E = 2MA x 100MΩ E = 200Mv
___________________________
In the first problem, units were used throughout, so the answer is simply given in volts. In the second problem the Mega prefix (M) is used for both amps and ohms, so the answer will also be given using the Mega prefix. Of course, these are very unusual values that are unlikely to occur in any sort of practical work.
It also possible to “mix and match”
prefixes to make the final answer come out as units.
For example:
E (in units) = I (in milliA) x R (in kiloΩ) In this problem the prefixes on the right hand side of the equation cancel each other out since “milli” means 1/1000 and Mega means 1000. 1/1000 x 1000 = 1.
These problems can also be worked with exponents using the form milli = 10
-3and Mega = 10
3. Again, 10
-3x 10
3= 1, so the end result would be an answer given in units.
Commonly used prefixes:
Mega Kilo Units milli micro
X,000,000 X,000 X .00X .000,00X