1. ELECTROSTATICS - CAPACITANCE
Fields and Waves
Lesson 3.5
ELECTROSTATICS - CAPACITANCE
Darryl Michael/GE CRD

2. CAPACITANCE of Coaxial Cable
inner conductor
outer conductor
In previous class, for coaxial cable:
Note:
very general result
charge on 1 conductor
Define:
DV between conductors
Note that:

3. Calculation of CAPACITANCE
Problems on calculation of C
Find Q
1. Method - Assume rs (use symmetry)
Find V(rs)
2. Alternate method - Assume V and find Q

4. CAPACITANCE - parallel plate capacitor
Use Gauss’ Law,
z=d
z=0
C of Parallel Plate capacitor

5. CAPACITANCE - parallel plate capacitor
Parallel Plate Capacitance
increase A
increase e
decrease d
This is how electrolytics increase C
To get large C
Do problem 1a or 2a & 2b

6. CAPACITANCE - ENERGY METHOD
energy stored in capacitors is stored in the E-field
Define stored energy:
Substitute values of C and V for parallel plate capacitor:
Energy Density
Volume

7. CAPACITANCE - ENERGY METHOD
In general we can write the total stored energy as: or

8. CAPACITANCE - ENERGY METHOD
Use the Energy Formulation to compute C for the Parallel Plate Capacitor
We know that,
Compute TOTAL ENERGY:

9. CAPACITANCE
Do rest of Problems 1 or 2
Any 2 conductors have capacitance
Example:
lines on circuit board
Theremin
wires and cables
- problems 2 and 3 model lines on a circuit board
Do problem 3