WELCOME TO DAY 57!
Current and Capacitance
SELF-GRADE PS28
DAY 57 Wednesday
2-22-17
Goes with Chapter 17, Sections 2 and 3, and PS29.
I. INTRO TO ELECTRIC CIRCUITS
II. CURRENT IN AN ELECTRIC CIRCUIT
III. PARALLEL PLATE CAPACITORS
It is essential that there be an electrical potential difference
between the two sides of the source – this is what makes the
electrons flow. In fact, “source” is short for “source of
electrical potential difference.”
Section of a copper (a metal) wire:
 Copper nuclei and 28 of the 29 electrons in each atom (red
circles) are bound in place in the solid by electric forces.
(2 in n=1, 8 in n=2 and 18 in n=3 and 1 in n=4)
 One valence electron from each copper atom leaves the
atom and roams freely throughout the metal (blue circles).
These are called the “free electrons.” All metals have
“free electrons.”
the free electrons in a metal – no potential difference
http://www.youtube.com/watch?v=dyX5I_io7bg
Animation showing an applied potential difference
https://www.youtube.com/watch?v=KprFTxjQAoE
If current is 1 A,
 how much charge moves past line A in 0.160 sec?
 how many electrons is this?
1. Suppose that 3.2 x 1016 electrons pass a given point
along a wire every second. What is the current?
photo of a teaching capacitor p. 604 in text
drawing of a parallel-plate capacitor
end
What is the capacitance of a parallel plate capacitor that has an area of
2.0 cm2. Its plates are separated by 2.0 mm. (Air in between the
plates)
8.8 x 10-13 F
What is the capacitance if the space between the plates is filled with
paper?
3.7 * 8.8 x 10-13 F
photos of a circuit boards with capacitors and other circuit elements on
them