Physics 201- Introduction to Electricity & Magnetism
October 16- Simple Electric Circuits

Simple, single-loop circuits

• Current is constant throughout.  (You cannot creat or destroy charges, you can only push them along the loop.)
• If you traverse around the loop, you must return to the same potential when you return to the same point on your circuit.  (This simply comes from the fact that a point cannot have multiple values of potential.)

Power

• Power is energy per unit time- it can be used to describe the energy used or the energy supplied (power output of a power supply or power consumption of a component)
• Do resistors use or supply electrical energy to the circuit?
• Where did this energy go?  You still believe in conservation of energy, right?

The power lost in a resistor is found by looking at a small amount of charge dropping through a potential difference-

Or, rearranging the dt

This is the "power consumed" by the resistor.  (Of course, we really know that the energy hasnít been destroyed.  The electrical energy has actually been converted to another form- thermal energy.  The language of "power consumed" is just what we typically use.)

With Ohm's law we can also say that the power "used" by an ohmic component is given by:

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 Jeff Phillips phillips@lmu.edu Loyola Marymount University Fall 2002