PY208-Lecture-14

# PY208-Lecture-14 - Physics 208N-004 Lecture #14 14 February...

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Some circuits cannot be broken down into series and parallel connections. For these circuits we use Kirchhoff’s rules. Physics 208N-004 Lecture #14 – 14 February 2011 Chapter 26: 3-4 – Kirchhoff’s Laws Junction rule: The sum of currents entering a junction equals the sum of the currents leaving it. 26-3 Kirchhoff’s Rules I 3 = I 1 +I 2

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Loop rule: The sum of the changes in potential around a closed loop is zero. 26-3 Kirchhoff’s Rules Problem Solving: Kirchhoff’s Rules 1. Label each current, including its direction. 2. Identify unknowns. 3. Apply junction and loop rules; you will need as many independent equations as there are unknowns. 4. Solve the equations, being careful with signs. If the solution for a current is negative, that current is in the opposite direction from the one you have chosen. 26-3 Kirchhoff’s Rules
26-3 Kirchhoff’s Rules Example 26-9: Using Kirchhoff’s rules. Calculate the currents I 1 , I 2 , and I 3 in the three branches of the circuit in the figure. EMFs in series in the same direction: total voltage is the sum of the separate voltages. 26-4 Series and Parallel EMFs; Battery Charging

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EMFs in series, opposite direction: total voltage is the difference, but the lower-voltage battery is charged. 26-4 Series and Parallel EMFs; Battery Charging EMFs in parallel only make sense if the voltages are the same; this arrangement can produce more current than a single emf. 26-4 Series and Parallel EMFs; Battery Charging
26-4 Series and Parallel EMFs; Battery Charging Example 26-10: Jump starting a car. A good car battery is being used to jump start a car with a weak battery. The good battery has an emf of 12.5 V and internal resistance 0.020 . Suppose the weak battery has an emf of 10.1 V and internal resistance 0.10

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## This note was uploaded on 09/18/2011 for the course PY 208M 87878 taught by Professor George during the Spring '09 term at N.C. State.

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PY208-Lecture-14 - Physics 208N-004 Lecture #14 14 February...

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