Tuesday, February 24, 2009
Notes on Circuit Theory
For a "simple" circuit consisting of a battery and a resistor (and wires), like we discussed in class on Monday, there are some subtle and tricky things to bear in mind. Familiarity with these will help you conceptualize and carry out circuit problems like those on the HW for this week (see earlier post).
The first has to do with current, I, which has units of coulombs per second. A key premise of circuit theory is that current is the same at each point in the circuit! It does not diminish!
(Also, as an aside, when the voltage is first applied, the current starts moving everywhere in the circuit at once. The charge carriers (electrons) in the metal of the wire and in the resistor all start moving together at the same time and in the same "direction" as soon as the voltage is applied. (When we cover relativity we will see that there is a slight correction to this related to the finite speed of light.))
As electrons go through the resistor, they go from a region of ____ potential energy to a region of ___ potential energy (fill in). In the resistor, potential energy is converted to heat energy (or sometimes to something else like light (from the point of view of circuit theory, a light bulb is a resistor). Kinetic energy does not play a role in this. Circuits are all about conversion of potential energy to heat or other forms of energy!
The equation for this is P = V I, where V is associated with the size of the potential energy change (drop) and I is associated with the number of charge carriers (electrons) that get converted. Units are Volt-coulombs/second , which is the same as Joules/second, also known as Watts.
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what is the solution to the fill in problem?
ReplyDeletelow to high?
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