|MadSci Network: Physics|
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 them. 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! ibiblio 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
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