Resistance in Parallel Circuits
Resistance Total is the inverse of the sum of the Reciprocals.
Phwew! That is a mouth full. But with the help of a calculator, finding
resistance total in a parallel circuit is just a matter of key strokes.
Note: The reciprocal key is the 1/x key. What happens when you press that
key is that the calculator takes the displayed number and divides it into 1.
Here's how a problem goes. Say you know that the resistance of each of four
components in parallel is 4 ohms. So you have 4 ohms, 4 ohms, 4 ohms, and 4 ohms
in the circuit connected in parallel.
To find the resistance total on the calculator it is done as follows:
enter the value of the first resistor then press the 1/x key
then press + key
enter the value of the second resistor then press the 1/x key
then press +
enter the value of the third resistance then press the 1/x key
then press +
enter the value of the fourth resistance then press the 1/x key
then press the = key
then press the 1/x key one last time.
The reading in the calculator display will be the resistance total in
parallel for these components.
Try it. Notice your answer is 1 ohm.
Tip: When all the resistors have the same value, in this case 4 ohms, then
the resistance total will always be the value of any one of the resistors
divided by how many there are.
In this case that would be 4 divided by 4 equals 1. Neat, eh?
License exams may have a question about resistance total like this: What is
resistance total in a parallel circuit where there are 6 resistors that are 36
ohms each? Easy to do, right? Just 36 divided by 6 is 6. Do it in your head,
even.
Oh, and as a rough check, resistance total in a parallel circuit will always
be less than the smallest resistor in the circuit. Just a math thing.
HERE is a worksheet where you can practice this concept.
Apply this aspect to a circuit with four components HERE.
NEXT
What about a missing resistor when you know the total?
Here is the concept. You enter the total resistance in the calculator. Then
press the 1/x key. Then subtract the reciprocal of each of the resistors you
know. When you have entered all the known resistors followed by the 1/x key,
then press 1/x one last time. The result will be the value of the missing
resistor. Let's do the problem above without knowing the value of the fourth
resistor, but knowing the total resistance at the beginning. Here we go.
On the calculator:
total resistance 1 ohm then press 1/x key
then press  (minus) key
enter 4 ohms then press 1/x key
then press 
enter 4 ohms then press 1/x key
then press 
enter 4 ohms then press 1/x key
then press = key
then press 1/x key one last time to find value of missing resistor which
should be 4 ohms.
Got it? Great.
Find practice problems HERE.
Find audio support HERE.
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