MadSci Network: Physics

Re: Why is it that in parallel circuits the voltage divides evenly?

Date: Sat Sep 8 13:41:36 2001
Posted By: Karl Kolbus, Staff, Data processing, Mequon Consulting Corp.
Area of science: Physics
ID: 999691869.Ph

Hi jawed!

I hope that’s just a nickname, and not something that actually happened to 
you. During the last few weeks, we have had a number of deadly shark 
attacks in the shallow waters off the coasts of Florida and the Carolinas.

Firstly, let’s make sure we’re talking about the same thing.  Your 
question: “Why is it that in parallel circuits the voltage divides 
evenly?” clearly indicates that each of the globes would be connected to 
the voltage source (battery) by two wires; one to the positive, and one to 
the negative.  They are said to be ‘in parallel’.  The body of your 
question, however, indicates that the positive wire of the battery goes to 
the first wire of the first globe, the second wire from the first globe 
goes to the first wire of the second globe, the second wire of the second 
globe goes to the first wire of the third, and so on until the second wire 
of the LAST globe goes back to the battery’s negative terminal. They are 
now said to be ‘in series’.  We will limit our discussion here to parallel 
resistances, and how to use Ohm’s Law to determine current flow through 

Ohm’s Law is simply stated: I = E / R, where I is current in Amperes, E is 
Electromotive force in Volts, and R is Resistance in Ohms.  For a  
parallel circuit, the total resistance is:
Rtotal = R1 * R2 / R1 + R2
Therefore, if  R1 = 10 Ohms  and R2 = 20 Ohms, substituting into our 
equation we get:
Rtotal = (10*20)/(10+20) = 6.667 Ohms
Assuming a battery voltage of 12 volts and substituting our resistance 
into Ohm’s law, we see that the total current through the circuit is:
I = 12/6.667 = 1.799, or rounding off, 1.8 Amperes.
We can prove this by calculating the current flow through each resistance 
individually using Ohm’s Law, and adding them together. Try it!	

The following link will take you to an interesting website where you can 
learn, in far more detail than we have covered here, about various 
electrical circuit types and the calculations we use to analyze them. I 
think you’ll enjoy it!  
When doing calculations like this, it is assumed that there is an 
unlimited supply of current (Amperes) at any given voltage. That is why 
the current is not ‘used up’. In real life, however, the electricity 
flowing through the globes is taken from the battery (or other power 
source) and converted to light and heat. As a result, the amount of energy 
in the battery is reduced and must be replaced, or the globes will become 
dim and eventually cease to emit light entirely. That is why motor 
vehicles have alternators or generators. Their job is to take the KINETIC 
ENERGY from the rotating engine, and convert it into electrical energy. 
This energy is then stored in the battery as POTENTIAL ENERGY. I suggest 
you look up the capitalized terms above and familiarize yourself with 
their meanings. 

Good luck, and Happy Experimenting!

Your not-so-mad scientist,
Karl Kolbus

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2001. All rights reserved.