# What is a Capacitor?

• A capacitor is an electric device used to store electrical energy

• Two conducting plates

• Insulating material between (dielectric)

• Place opposite charges on each plate

• Develop a potential difference across the plates

• Energy is stored in the electric field between the plates # Capacitance

• Capacitance (C) is the ratio of the charge separated on the plates of a capacitor to the potential difference between the plates

• Units of capacitance are coulombs/volt, or farads (F)

• A farad is a very large amount of capacitance # Calculating Capacitance

1. Assume a charge of +Q and -Q on each conductor

2. Find the electric field between the conductors (Gauss's Law)

3. 4. # Example 1: Parallel Plates • Determine the capacitance between identical parallel plates of area A separated by a distance d
1. Assume +Q and -Q

2. 3.   4. # Example 2: Cylindrical Capacitor

• 1. Assume +Q and -Q
1.   2.   3. # Example 3: Spherical Capacitor • 1. Assume +Q and -Q
1.   2.   3. # Energy Stored in a Capacitor

• Work is done charging a capacitor, allowing the capacitor to store energy.

• • • • •  # Field Energy Density

• •  # Dielectrics

• Insulating Materials  # Dielectric Constant (κ)  # Example 4: Capacitors in Series • Determine the equivalent capacitance of two capacitors in series

• •  # Example 5: Capacitors in Parallel • Determine the equivalent capacitance of two capacitors in parallel

•  # Example 6: Capacitance

• A capacitor stores 3 microcoulombs of charge with a potential difference of 1.5 volts across the plates. What is its capacitance? How much energy is stored in the capacitor?

• • # Example 7: Charge on a Capacitor # Example 8: Designing a Capacitor # Example 9: Calculating Capacitance # Example 10: Two Conducting Spheres # Example 11: Inserting a Dielectric 