# Lec 9 Kirchoff`s rules circuits

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```Kirchhoff’s Rules for Complex circuits
Sum of voltage changes around a loop is zero
Current into any junction must equal the current
out of the junction.
Complex circuits can’t simplify to one battery and one
resistor Reff.
Have to use Kirchhoff’s rules to write equations and
solve for unknowns.
• Guess any direction for any currents…if you’re
wrong, that current will be _________ in the
solution.
• Write equations for as many loops or junctions
as you need to find unknowns
• As you go along a loop, if you go through a
resistor “downstream” (with current) ∆V is
_____; if you go “upstream” ∆V is _____.
• Use sign on emfs (batteries) to know sign of
∆V as you go through it in either direction.
Electrical shock danger
Sign near ESC huge pendulum:
Caution:
10,000 Ohms!
What is it that kills: Current, voltage, or total charge?
Smallest you can feel?
Enough to kill you?
Skin resistance
Frequency
P1. Write the voltage change equation for the loop we
didn’t use in the solution.
A. I did it and checked with another classmate
B. I did it
P2. In circuit I, which bulb puts out more light?
A. high R
B. low R
C. same from each
high R
high R
low R
low R
P3. In circuit II, which bulb puts out more light?
A. high R
B. low R
C. same from each
```