1. Electrostatics

2. Assignment Read Chapter 20 Static Electricity
Do these in Chapter 20 Static Electricity
20/10,13,47

3. I. Electric Charge
Neutral atom contains protons and electrons in equal amounts
When atoms lose or gain electrons they acquire a charge
Gaining electrons - acquire a negative charge
Losing electrons - acquire a positive charge
m= n = 10-9 p =

4. Electric charge, cont.
B. By rubbing two objects together, electrons can be transferred from one object to another
1. like charges repel one another
2. unlike charges attract one another
3. charged objects attract uncharged or neutral objects.

5. C. Field or force - region around a charged object within which the force of the charge can be detected.
Objects will move toward or away from the charged object (depending on their charge) when they enter this field of force
Strength of the force depends on
a. the larger the charge, the greater the force
b. the closer the object, the greater the force (force follows the inverse square law - F a 1/r2)

6. Lines of Force

7. Lines of Force
By convention they are drawn from positive to negative.

8. Coulomb’s Law F = k qaqb d2 F, force in newtons, N
q, Coulomb - SI unit of charge.
1 C = charge of 6.24 x 1018 electrons or protons. Charge of 1 electron or proton is the reciprocal of above # or 1.60 x C
k = 9.0 x 109 N m2/C2
d, distance in m

9. Problem
Force of attraction

10. A B C N

11. d = d

12. Two equally charged pith balls, each of mass 0
Two equally charged pith balls, each of mass 0.10g, are suspended from the same point by threads 20. cm long. The balls come to rest 10. cm apart due to repulsion. Determine the charge q on each ball.
Draw out the situation.
Fg = mg = 1x10-4kg(9.8m/s2) = 9.8x10-4n
Find angle, q:
= sin-1(o/h) = sin-1(.05/.2) = 14o
Fr = Fg tan q = 9.8x10-4 n(tan14o) =
Fr = 2.4 x n
Fr = k qa qb / d2
q = F r d2 / k
q = x10-4 n(0.05m)2 / 9.0 x 109 Nm2/c2
q = 8.2 x 10-5 c
0.20m q Fr
0.05m Fg

13. D. Electroscope
Any device used to detect an electric charge

14. E. Charging Objects
Conduction - charged object transfer its charge to a neutral object by touching
Result - same charge
Induction - placing a charge on an object by placing it near a charged object
a. Result - opposite charge can be permanent if object is connected to a much larger object (ground)

15. Conduction

16. Induction
online.cctt.org/.../lessonelectrostatics.asp

17. F. Conductor
Any substance that permits electricity or heat to move through it readily ; good charge carriers
Charges collect on outer surfaces and on sharp or pointed surfaces of objects.

18. Insulators / Semiconductors
Insulators do not conduct electricity well or at all because they do not have free electrons
Semiconductors are between conductors and insulators in their ability to transfer charge

19. Cardinal Rules Series: Parallel E=Vt=V1+V2+V3 E=Vt=V1=V2=V3
It=I1=I2=I3 It=I1+I2+I3
Rt=R1+R2+R Rt=
1/R1+1/R2+1/R3
Ct= Ct=C1+C2+C3
1/C1+1/C2+1/C3

20. II.Static Electricity
Accumulation of positive or negative charges causing
Lightning
Shocks from door handles
Clothes in dryer
Neutralized by
grounding objects
using fabric softener

21. D. Lightning
Static electricity charges build up around 15,000 to 25,000 feet above sea level
When enough charge builds up, the charge moves downward until it encounters something on the ground that is a good connection.
The return stroke is a flow of charge (current) which produces a luminosity much brighter than the part that came down. This entire event usually takes less than half a second.

22. Thunder is caused by lightning.
Extremely high temperatures 50,000 oF) causes rapid expansion of air which sends shock and sound waves in all directions. What you hear is the result of the expansion.

23. III. Uses of Static Electricity
Van de Graaff generator
Robert Van de Graaff's machine:
Friction at the bottom deposits charges on a moving belt
The belt carries charges up to the interior of a metal sphere
The charges move from the belt and collect on the sphere

24. Capacitors - a combination of conducting plates separated by an insulator, used to store electrical charge. Unit: farad or coulomb/volt

25. Variable Capacitor
Capacitor with 2 plates that can move different distances from one another allowing a radio to pick up different frequencies (tuning)

26. Capacitance C = q/V C, capacitance in farads q, charge in coulombs
V, voltage (potential diff.) in volts

27. Capacitance Problem C = q/v farad = coulomb/volt
If a capacitor has a capacitance of 4 pf with a voltage of 0.2 v, what is the charge that is built up on the capacitor?
q = Cv
q = 4 x f (0.2v)
q = 8 x c
(p, pico, 10-12)

28. Capacitors in Series
Math: reciprocally additive
= …
Ct C1 C2

29. Capacitors in Parallel
Ct = C 1 + C 2 + …

30. Comparing Capacitors
Placing different dielectrics (insulators) between capacitor plates, produces greater than capacitance than with just air.
Another source:

31. Capacitance info with building info
“…this site … has a bunch of info about historical leyden jars, how to build them and how to make an electrostatic generator…”
From: Mike Maloney

32. Cardinal Rules Series: Parallel E=Vt=V1+V2+V3 E=Vt=V1=V2=V3
It=I1=I2=I3 It=I1+I2+I3
Rt=R1+R2+R Rt=
1/R1+1/R2+1/R3
Ct= Ct=C1+C2+C3
1/C1+1/C2+1/C3