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Assignment

Read Chapter 20 Static Electricity

Do these in Chapter 20 Static Electricity

20/10,13,47

I. Electric Charge

A. Neutral atom contains protons and electrons in equal amounts

1. When atoms lose or gain electrons they acquire a chargea. Gaining electrons - acquire a negative charge

b. Losing electrons - acquire a positive charge

= 10 -6 n = 10-9 p = 10 -12

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.

C. Field or force - region around a charged object within which the force of

the charge can be detected.1. Objects will move toward or away from the

charged object (depending on their charge) when they enter this field of force

2. 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 1/r2)

Lines of Force

Lines of Force

• By convention they are drawn from positive to negative.

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 10-19 C

k = 9.0 x 109 N m2/C2

d, distance in m

Problem

Force of attraction

A B C

d =

d

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, :

= sin-1(o/h) = sin-1(.05/.2) = 14

Fr = Fg tan = 9.8x10n(tan14) =

Fr = 2.4 x 10-4 n

Fr = k qa qb / d2

q = F r d2 / k

q = 2.4x10-4 n(0.05m)2 / 9.0 x 109 Nm2/c2

q = 8.2 x 10-5 c

Fg

Fr

0.20m

0.05m

D. Electroscope

• Any device used to detect an electric charge

E. Charging Objects

1. Conduction - charged object transfer its charge to a neutral object by touching

a. Result - same charge

2. 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)

Conduction

Induction

•online.cctt.org/.../lessonelectrostatics.asp

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.

www.napa.ufl.edu/2002news/lightningrodph.htm

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

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+R3 Rt= 1

1/R1+1/R2+1/R3

Ct= 1 Ct=C1+C2+C3

1/C1+1/C2+1/C3

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

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.

 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.

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

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

www.vk2zay.net/article.php/55

Variable Capacitor

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

Capacitance

• C = q/V

• C, capacitance in farads

• q, charge in coulombs

• V, voltage (potential diff.) in volts

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 10 -12 f (0.2v)

• q = 8 x 10 -11 c

• (p, pico, 10-12)

Capacitors in Series

• Math: reciprocally additive

1 = 1 + 1 …

Ct C1 C2

Capacitors in Parallel

• Ct = C 1 + C 2 + …

Comparing Capacitors

• Placing different dielectrics (insulators) between capacitor plates, produces greater than capacitance than with just air.

• Another source:

• http://prettygoodphysics.wikispaces.com/PGP+E%26amp%3BM

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…” http://www.tufts.edu/as/wright_center/personal_pages/bob_m/

• From: mmaloney@somerville.mec.edu  Mike Maloney

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+R3 Rt= 1

1/R1+1/R2+1/R3

Ct= 1 Ct=C1+C2+C3

1/C1+1/C2+1/C3