Physics 4B Bli: lab15--4/28(CAPACITORS AND CAPACITIVE CIRCUITS)

Answer to the in- class quiz:

Energy stored inside capacitor:

Activity: Measuring How Capacitance Depends on Area or on Separation

a. Devise a way to measure how the capacitance depends on either the foil area or on the separation between foil sheets. If you hold the area constant and vary separation, record the dimensions of the foil so you can calculate the area. Alternatively, if you hold the distance constant, record its value. Take at least five data points in either case.

Describe your method and then create a data table with proper units and display a graph of the results.

material:

2 pieces aluminum foil, 12 cm  12 cm

• 1 textbook

• 1 multimeter (with a capacitance mode)

• 2 insulated wires (stripped at the ends, approximately 12" long)

• 1 ruler

• 1 Vernier caliper

7pg=0,47nF

1pg o,96nF

4 pg 0.66nF

14 pg 1.4nF

...

Graph Capacitance vs distance

Activity: Derivation of Capacitance vs. A and d

a. Given that Q = emf*EA = k emf(0)*EA where the net positive charge on the bottom plate is denoted by Q. Notes: (1) We are assuming that since the positive charges on the bottom plate are attracted to the negative charges on the

top plate, the excess charges are on the inside surface of each plate. Remember that in a uniform electric field the voltage drop over a distance d is given by V = Ed. Plug the definition for charge and voltage into the

definition of charge to write an expression for capacitance as a function of k*emf(0)*dA