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Kirchoff’s Rules

Kirchoff’s Rules. http://physics.bu.edu/~duffy/py106/Kirchoff.html. Junction: a point where at least three branches meet. Branch: a path connecting two junctions. Junction Rule:. Loop Rule:. ΣV = 0 The total voltage of the circuit is equal to 0. ΣI in = ΣI out

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Kirchoff’s Rules

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  1. Kirchoff’s Rules

  2. http://physics.bu.edu/~duffy/py106/Kirchoff.html

  3. Junction: a point where at least three branches meet. Branch: a path connecting two junctions.

  4. Junction Rule: Loop Rule: ΣV = 0 The total voltage of the circuit is equal to 0. ΣIin= ΣIout The sum of the current going into a junction is equal to the sum of the current flowing out of a junction.

  5. Steps for solving for current using Kirchoff’s rules • Use the junction rule to write an equation for the currents in the circuit. • Use the loop rule and write equations for each loop. Use your finger to trace a path around the loop and be sure to end where you began. • Plug in what you know, to your loop equations. • Using the equation found by the junction rule, rearrange it so that you can substitute this equation in for one of your currents. • Simplify your equations. • Using a system of equations, one current variable should drop out and you will be left with one current variable you can solve for. • Use the value from step 6 to solve for the other two currents by plugging it in to the other equations.

  6. Example: Using Kirchoff’s rules, find the current in each branch. R1 = 6 Ω R2 = 4 Ω R3 = 4 Ω R4= 1 Ω Ε1 = 30 V E2 = 12 V E3 = 6 V

  7. Using the loop rule we get: left loop: E1–I1R1-I3R4-E3=0 right loop: E3+I3R4-I2R3-E2=0 Using the junction rule we get: I1 = I2+ I3

  8. Use the equations on the following slide, solve for I1, I2, I3.

  9. Power

  10. Power: The rate at which energy is transferred Power is equal to the current times the potential difference. P = IV Power is measured in Watts, denoted W.

  11. A car battery causes a current of 2.0 A through a lamp and produces 12 V across it. What is the power used by the lamp?

  12. A flashlight blub is rated at .9 W. If the light bulb drops 3 V, how much current goes through it?

  13. A circuit made of a battery and a resistor of 5 Ω has a current of 2 A. What is the power delivered to the resistor?

  14. Homework Pg 611 # 65, 66, 69, 70 • 960 W • 144 W • 19.1 A • 4.5 W Pg 615 # 1, 7 • .83 A 7) .00125 W

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