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Phy122_0928

# Phy122_0928 - Physics 122 Sept 28 2010 Concluding chapter...

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Physics 122 Sept. 28, 2010 Concluding chapter 18. This lecture is the end of the material that will appear on your 1 st midterm exam. September 30. First lecture on Ch. 19 (magnetism) October 4 Review session: Javits 103 6:50-8:50 PM October 4. Review session: Javits 103, 6:50 8:50 PM October 5. Second lecture on Ch. 19 October 5. 8:30 PM Midterm exam on Ch. 15-18. O t b 6 5:00 PM HW Ch 19 is d October 6. 5:00 PM. HW on Ch. 19 is due. Room assignments for the midterm will be posted soon. Question 1: how do you want review session?

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A car battery has a voltage of 12 V when it is not under load, but when it is providing 100 Amps of current to the starter motor (load), its terminal voltage drops to 10 V. What is the internal resistance of the battery?
A car battery has a voltage of 12 V when it is not under load, but when it is providing 100 Amps of current to the starter motor, its terminal voltage drops to 10 V. What is the internal resistance of the battery? E = 12 V When 100 Amperes are flowing through R the load , the voltage across R load is 10 V, so the voltage across R int is 2 V . R int = 2 V / 100 A = 0.02 .

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For more complicated problems, we can use Kirchhoff’s rules*: 1. Sum (algebraic) of currents entering a junction is zero. 2. Sum of voltage differences around any loop is zero. *Not new physics: conservation of charge and conservative force.
Tactics: Draw and label an arrow to represent each k t unknown current. Write node equations: sum of currents flowing into each node – sum of currents flowing out = 0. Draw closed loops keeping track of starting point and Draw closed loops, keeping track of starting point and direction of each loop. W it l ti s: s f lt diff s Write loop equations: sum of voltage differences around each loop = 0. Pay attention to signs. 1. + V batt if traverse a battery from – to + terminal. 2. V batt if traverse a battery from + to – terminal. 3. + IR if traverse a resistor opposite current I . 4 - IR if traverse a resistor with current I 4. .

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Problem: What is the current through each resistor? We did this with series and parallel resistors – now try with Kirchhoff’s laws. First step: Label currents I 1 Label currents. Second step: W it d I I 1 I 2 Write node equation(s): 1 I 1 I 1 I 2 I 3 = 0.
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