Capacitors, Batteries

Capacitors

• Create a difference in Potential based upon how much charge is stored

V = q/C (V) C : Capacitance

C = k εo A /dk : dielectric constant

Energy stored in Capacitor

•E = ½ C V2 = ½ q2 /c

Batteries

• Produce change in voltage by chemical reaction.

• Change in Voltage depends on type of chemical, not how much.

• There is no electrical energy stored in a battery, only chemical potential energy.

We can use a battery to “charge up” a capacitor.

V C

-Change in voltage around entire loop must be zero!

q=C V

Capacitors in parallel

• Several capacitors in parallel behave like one larger capacitor: Capacitance increases.

• CE = C1 + C2 + C3 + ……….

2μF

3μF

10μF

Capacitors in Series

• Several capacitors in series block the flow of charge: Capacitance decreases.

• 1/CE = 1/C1 + 1/C2 + 1/C3 + ……….2μF 3μF 10μF

Example: What is the equivalent capacitance of the entire circuit?

20V

C1 =2μF

C2 = 4μF

C3 = 4μF

C4 = 2μF

What is the voltage across C1?What is the voltage across C3?What is q3 the voltage on C3?

20V

C1 =2μF

C2 = 4μF

C3 = 4μF

C4 = 2μF

What is the voltage across C2?What is the voltage across C4?

20V

C1 =2μF

C2 = 4μF

C3 = 4μF

C4 = 2μF