C 3

1. Example: determine the capacitance of a single capacitor that will have the same effect as the combination shown. Use C1 = C2 = C3 = C. C2 C1 C3 I don’t see a series combination of capacitors, but I do see a parallel combination. C23 = C2 + C3 = C + C = 2C

2. Now I see a series combination. C23 = 2C C1= C

3. Example: for the capacitor circuit shown, C1 = 3F, C2 = 6F,C3 = 2F, andC4 =4F. (a) Find the equivalent capacitance. (b) if V=12 V, find the potential difference across C4. I’ll work this at the blackboard. C1 C2 C4 Homework Hint: each capacitor has associated with it a Q, C, and V. If you don’t know what to do next, near each capacitor, write down Q= , C= , and V= . Next to the = sign record the known value or a “?” if you don’t know the value. As soon as you know any two of Q, C, and V, you can determine the third. This technique often provides visual clues about what to do next. C3 V

4. (a) Find Ceq. (b) if V=12 V, find V4. C1=3F C2=6F C1 and C3 are not in parallel. Make sure you understand why! C4=4F C3=2F C2 and C4 are not in series. Make sure you understand why! V=12 V C1 and C2 are in series. Make sure you use the correct equation! Don’t forget to invert: C12 = 2 F.

5. (a) Find Ceq. (b) if V=12 V, find V4. C12=2F C12 and C4 are not in series. Make sure you understand why! C4=4F C3=2F V=12 V C12 and C3 are in parallel. Make sure you use the correct equation!

6. (a) Find Ceq. (b) if V=12 V, find V4. C123=4F C4=4F C123 and C4 are in series. Make sure you understand why! Combined, they make give Ceq. V=12 V Make sure you use the correct equation! Don’t forget to invert: Ceq = 2 F.

7. (a) Find Ceq. (b) if V=12 V, find V4. Ceq=2F Ceq = 2 F. V=12 V If you see a capacitor circuit on the test, read the problem first. Don’t go rushing off to calculate Ceq. Sometimes you are asked to do other things. Truth in advertising: there’s a high probability you will need to calculate Ceq at some point in the problem.

8. (a) Find Ceq. (b) if V=12 V, find V4. Q1=? C1=3F V1=? Q2=? C2=6F V2=? Q4=? C4=4F V4=? Homework Hint: each capacitor has associated with it a Q, C, and V. If you don’t know what to do next, near each capacitor, write down Q= , C= , and V= . Next to the = sign record the known value or a “?” if you don’t know the value. As soon as you know any two of Q, C, and V, you can determine the third. This technique often provides visual clues about what to do next. Q3=? C3=2F V3=? V=12 V We know C4 and want to find V4. If we know Q4 we can calculate V4. Maybe that is a good way to proceed.

9. (a) Find Ceq. (b) if V=12 V, find V4. Q123=? C123=4F V123=? Q4=? C4=4F V4=? C4 is in series with C123 and together they form Ceq. Therefore Q4 = Q123 = Qeq. V=12 V

10. You really need to know this: Capacitors in series… all have the same charge add the voltages to get the total voltage Capacitors in parallel… all have the same voltage add the charges to get the total charge (and it would be nice if you could explain why)

11. Homework Hint! C1 C2 What does our text mean by Vab? C4 a b Our text’s convention is Vab = Va – Vb. This is explained on page 759. This is in contrast to Physics 1135 notation, where Vab = Vb – Va. C3 V In the figure on this slide, if Vab = 100 V then point a is at a potential 100 volts higher than point b, and Vab = -100 V; there is a 100 volt drop on going from a to b.

12. A “toy” to play with… http://phet.colorado.edu/en/simulation/capacitor-lab (You might even learn something.) For now, select “multiple capacitors.” Pick a circuit.