When a capacitor is placed in a circuit, current does not actually travel across it. Rather, equal and opposite charge begins to build up on opposite sides of the capacitor --- mimicking a current --- until the electric field in the capacitor creates a potential difference across it that balances the voltage drop across any parallel resistors or the voltage source itself (if there are no resistors in parallel with the capacitor). The ratio of charge on a capacitor to potential difference across it is called capacitance.

It is important to break down a complicated circuit into the equivalent capacitance using the rules for capacitors in series and capacitors in parallel. Also remember that capacitors in parallel have the same voltage while capacitors in series have the same charge.

#### Example

In the circuit shown below, determine (a) the total capacitance and (b) the total charge stored.

(a): In solving this problem, we'll call the

Our first step will be to find the equivalent capacitance of

Next, we'll combine the capacitance of

Finally, we can combine

(b): Now we can use this value to find the total charge stored on all the capacitors by also using the voltage provided on the diagram.

### Simulation

Circuit Constructoin Kit (AC+DC) (PhET Simlation)

### Review

- Consider the figure above with switch,
S , initially open and the power supply set to 24 V:- What is the voltage drop across the
20Ω resistor? - What current flows thru the
60Ω resistor? - What is the voltage drop across the
20 microfarad capacitor? - What is the charge on the capacitor?
- How much energy is stored in that capacitor?
- Find the capacitance of capacitors
B ,C , andD if compared to the20μF capacitor where...B has twice the plate area and half the plate separationC has twice the plate area and the same plate separationD has three times the plate area and half the plate separation

- What is the voltage drop across the
- Now the switch in the previous problem is closed.
- What is the total capacitance of branch with B and C?
- What is the total capacitance of the circuit?
- What is the voltage drop across capacitor
B ?

### Review (Answers)

- a.
6V b.0.3A c.18V d.3.6×10−4C e.3.2×10−3J f. i)80μF ii)40μF iii)120μF - a.
26.7μF b.166.7μF