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PHY231
Kirchoff’s Law 1
Experiment 1: Simple Series and Parallel Circuits
Recall and Predict
Assume that there is ½ Amp of current through bulb A in Circuit 1 below. Based on your work from last week, predict the amount of current at all the listed points in the following circuits:
1. point a in Circuit 1? 2. point b in Circuit 1? 3. through the battery in Circuit 1? 4. Bulb A in Circuit 2? 5. Bulb B in Circuit 2? 6. point a in Circuit 2? 7. point b in Circuit 2? 8. point c in Circuit 2? 9. through the battery in Circuit 2? 10. Bulb A in Circuit 3?
A
B
Circuit 2
A
Circuit 1
b
a a
b
c
A B
Circuit 3
a b
d
c
11. Bulb B in Circuit 3? 12. point a in Circuit 3? 13. point b in Circuit 3? 14. point c in Circuit 3? 15. point d in Circuit 3? 16. through the battery in Circuit 3?
Measurements with Ammeters and Voltmeters
Set up the circuits shown on the previous page-‐-‐ one at a time. We will be using commercial resistors now rather than bulbs because their resistance is more uniform and precise. Make measurements of current by inserting the ammeter at appropriate points. Record the actual measurements of current for the points listed below.
17. point a in Circuit 1? 18. Bulb A in Circuit 1? 19. Did you need to move the ammeter to make this measurement? Explain.
20. point b in Circuit 1? 21. through the battery in Circuit 1? 22. point a in Circuit 2? 23. point b in Circuit 2? 24. point c in Circuit 2? 25. Where could you put the ammeter to measure the current through Bulb A? How about
the current through the battery?
26. point a in Circuit 3? 27. point b in Circuit 3? 28. point c in Circuit 3? 29. point d in Circuit 3?
30. Do your measurements match up with your ideas about current flow in circuits? Explain.
31. Calculate the voltage across each resistor assuming that the resistance of each is 110 Ω.
32. Measure and record the voltage across each resistor. Do your calculations and measurements agree?
33. What is the total resistance in each circuit? Show your calculations for the parallel circuit.
34. Use the meter to measure the voltage across the power supply. Record that value.
35. Based on the recorded voltage and the total resistance in the circuit, calculate the predicted total current in each circuit (this is also the current through the battery). Show your work.
36. Your calculated and measured currents should agree to a good extent. What is the %
error between them? Show your work. Experiment 2
37. Calculate the equivalent resistance of the circuit shown above assuming all the resistors are 110 Ω. Show your work.
38. Set up the circuit using all 110 Ω resistors. Measure the voltage across the power supply in case it has changed. Record that value.
39. Based on the measured voltage and equivalent resistance, calculate the current through each resistor.
40. Calculate the voltage across each resistor.
41. Measure the currents and voltages and record them.
42. Replace resistor C with a 500 Ω resistor. Repeat steps 37-‐41.
43. In what ways did current and voltage measurements change when you replace the resistor? How is this consistent with something you learned in lab last week?
Some More Equivalent Resistance Calculations
44. What is the equivalent resistance in this circuit if all resistors are 100 Ω?
45. Calculate the equivalent resistance for the circuit shown below if Resistors A and C are 500 Ω and Resistors B, D and E are 100 Ω.
46. What is the total resistance in circuit shown below? Let R1=8 Ω, R2=2 Ω, R3=6 Ω, R4=4 Ω and R5=5 Ω.
R2
R4
R3
R1
R5
47. The figure below shows a multi-loop circuit containing one battery and four resistors with the following values:
R1 = 20 Ω, R2 = 20 Ω, ∆VB = 12 V, R3 = 30 Ω, and R4 = 8.0 Ω. (a) What is the current through the battery? (b) What is the current i1 through R1? (c) What is the current i4 through R4? (d) What is the voltage across R1? (e) What is the voltage across R4? (f) Write the KCR and two KVR equations for this circuit using only symbols. (g) What is the current i3 through R3? (Hint: You will need to apply KVR aka “the loop rule”
here)
Fig. 26-42