Ph272- General Physics II (Electricity and Magnetsism)

Lecture 8: Ch. 25 - Capacitor Circuits

Key Topics

Overview

• If you have two 10 microfarad capacitors but you need 5 microfarad, what do you do?
• By putting capacitors in parallel or in series you can get deserible capacitance

Capacitors in parallel

• You get a larger capcitance by putting two or more capcitor in parallel.
• C(equivalent) = C1 + C2 + C3 +...
• To derive this formula, you need to know:

The voltage across parallel capacitors are equal: V1 = V2 = V3 +...

The net charge stored is the sum: Q = Q1 + Q2 + Q3 + ...

• A heuristic way of remembering the result is that two parallel capacitors have larger area hence larger capacitance.

-> Do Problem 21-17, P.711

Capacitors in series

• You get a smaller capcitance by putting two or more capcitor in series.
• Eq. 21-25.
• You need to know:

The charge stored in each capacitors in series are the same: Q1 + Q2 + Q3 + ...

The voltage across the entire series of capacitors is the sum: V = V1 + V2 + V3 +..

• A heuristic way of remembering the result is that two series capacitors have larger separation hence smaller capacitance.

-> Do Problem 21-16,21-28, P.711

Parallel and series

• Work out the equivalent capacitance of the parallel units first.

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