Physics 201- Introduction to Electricity & Magnetism
October 18- Kirchoff's Rules

Kirchoff's Rules state what we've seen already-

1. The sum of the changes in potential encountered in a complete traversal of any circuit must be zero.  (Each point has a unique potential value.)
2. The sum of the currents entering any junction must be equal to the sum of the currents leaving that junction. (Charge is conserved.)

To apply these rules, there are some guidelines that are worth following:

• Draw & label the current through each component.
• Pick directions to "travel" though a loop.
• If you go across the battery from - to + then the potential difference is positive.
• If you go from + to - (opposite the way the current would flow if this were the only battery), then the difference is negative.
• If you go across a resistor with the current, then the voltage difference is negative.
• If you go across the resistor opposite the direction you've drawn the current, the voltage is positive.
• Donít worry if you've drawn the current in the right direction, as long as youíre consistent it will work out.  If the current is negative, then it travels opposite to the way you've drawn it.
• Start writing down the various loop equations.  There may be more equations than you need.
• Do lots of algebra.

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