1. Class 12: Outline Hour 1: Working with Circuits
Monday 2/28/2005 / Tuesday 3/1/2005
Class 12: Outline
Hour 1:
Working with Circuits
Expt. 4. Part I: Measuring V, I, R
Hour 2:
RC Circuits
Expt. 4. Part II: RC Circuits
Class 12

2. Last Time: Resistors & Ohm’s Law

3. Resistors & Ohm’s Law

4. Measuring Voltage & Current

5. Measuring Potential Difference
A voltmeter must be hooked in parallel across the element you want to measure the potential difference across
Voltmeters have a very large resistance, so that they don’t affect the circuit too much

6. Measuring Current
An ammeter must be hooked in series with the element you want to measure the current through
Ammeters have a very low resistance, so that they don’t affect the circuit too much

7. Measuring Resistance
An ohmmeter must be hooked in parallel across the element you want to measure the resistance of
Here we are measuring R1
Ohmmeters apply a voltage and measure the current that flows. They typically won’t work if the resistor is powered (connected to a battery)

8. Experiment 4: Part 1: Measuring V, I & R

9. RC Circuits

10. (Dis)Charging a Capacitor
When the direction of current flow is toward the positive plate of a capacitor, then
Charging
2. When the direction of current flow is away from the positive plate of a capacitor, then
Discharging

11. Charging A Capacitor
What happens when we close switch S?

12. Charging A Capacitor NO CURRENT FLOWS!
1. Arbitrarily assign direction of current
2. Kirchhoff (walk in direction of current):

13. Charging A Capacitor A solution to this differential equation is:
RC is the time constant, and has units of seconds

14. Charging A Capacitor

15. PRS Questions: Charging a Capacitor

16. Discharging A Capacitor
What happens when we close switch S?

17. Discharging A Capacitor
NO CURRENT
FLOWS!

18. Discharging A Capacitor

19. General Comment: RC All Quantities Either:
t can be obtained from differential equation
(prefactor on d/dt) e.g. t = RC

20. Exponential Decay Very common curve in physics/nature
How do you measure t?
Fit curve (make sure you exclude data at both ends)
(t0,v0)
(t0+t,v0/e)
2) a) Pick a point
b) Find point with y value down by e
c) Time difference is t

21. Demonstrations: RC Time Constants

22. Experiment 4: Part II: RC Circuits

23. PRS Question: Multiloop circuit with Capacitor in One Loop