Static Electricity

Static Electricity It’s what we see…

Charges • Rubbing plastic or glass makes them attractive to bits of paper or dust • Benjamin Franklindecided to call the charge on rubbed glass positive

Top & Bottom

Charges • Objects charged the same way will repel each other • Oppositely charged objects attract each other

Charges • All charged objects either attract or repel each other • This implies 2 types of charges

Check yourself • Two small spheres hung on threads attract one another as shown. It is known that a positively-charged rod will attract ball A. • Ball A has a positive charge. • Ball B has a negative charge. • III. Balls A & B have opposite charges. • Which of the statements above are correct?

Electricity • One of the first objects to hold this charge was amber • The Greek word for amber is elektron • Electricity • Electron

Charge • To understand Charge, let’s look at the atom • Protons (nucleus) are positively charged • (repels charged glass) • The electrons are negatively charged • (attracts charged glass)

Charge • Even though protons are much bigger (2,000x’s bigger than electron), the – and + charges are the same size. • If an atom has equal number of electrons and protons the charge is zero. ____? • 3 electrons (blue) • 3 protons (red) • Net charge is zero (neutral charge)

Charge Rules • In order to be charged, an object must have extra protons or electrons • The following facts help understand charges: • In solids, positive charges do not move protons are fixed in place • But… Negative charges can move

Conductors • Conductors: electrons move throughout the material • Because like charges repel the electrons and protons will be as evenly spaced • Conductors can be charged easily

Charge Rules • Insulators: electrons mostly move around their atom • It is very difficult to charge an insulator because insulators do not allow charges to transfer easily.

Examples of Conductors & Insulators

Charge Rules In fluids, both protons and electrons can move all around • Charges need to be distributed evenly throughout the liquid What is wrong with this picture? • Protons repel protons! • Electrons repel electrons!

Charge Rules • Objects want to be neutral. • If given a path to the earth, electrons will flow out until the object is neutral. • This is called grounding. v

Brain Break! • Simpsons & Electrostatics

Charge by Rubbing • Every atom attracts electrons (e-) a certain amount • When you rub two objects together the object that attracts electrons more will steal e- + −

Charge by Rubbing • This stealer becomes negative (more e-) and the other object becomes positive (lost e-) What will happen to the charge when the bars separate?

Transferring Charge • This stealer becomes negative (more e-) and the other object becomes positive (lost e-) • Only works if there is contact

Conduction A charged conductor touches a neutral conductor Total Charge = protons - electrons Q1 = -4 Q2 = 0

Conduction A charged conductor touches a neutral conductor

Conduction A charged conductor touches a neutral conductor The electrons spread out so that the charges on both are equal Q1 = -2 Q2 = -2

What was wrong with this movie?

Induced Polarity • A neutral balloon has no effect on wall’s e- • How can charged objects attract neutral insulators? • They induce a charge on the neutral object.

Induced Polarity • Let’s charge the balloon! • Now the e- in the wall are being repelled • This leaves the exposed wall acting positive • Negative and positive charges attract each other

Induction = Induced Polarity + grounding • You charge an object without touching it with another charged object (conduction)? • Induce polarity with a charged object • Ground the object • Remove the ground Induced polarization Induction! Grounding

Visualizing Electric Charge pg 183 Neutral Draw 3 units of charge (protons & electrons) • In each bar

Induced Polarization pg 183 6 units of charge (protons or electrons) Q2 = 0 Q1 = 6 9 protons & 9 electronstotal

Conduction pg 184 Initially Neutral Q2 = 0 Q2 = +3 Q1 = 6 Q1 = +3 What can happen when charged objects touch? Electrons can transfer! Charge has to equally distribute

Charging by Induction pg 185 Protons attract Electrons How many electrons should enter from the ground? 9 protons & 9 electronstotal

The image below is an example of: • Conduction • Induction • Induced Polarization • Charging by rubbing • Grounding

The image below is an example of: • Conduction • Induction • Induced Polarization • Charging by rubbing • Grounding Induced polarization Induction! Grounding

Electrostatic Force • A Force is a push or a pull • Electric Force is 1039 times largerthan gravity • Coulomb studied charge with charged metal balls • Let’s look at how the amount of charge on each object and the distance effect the electrostatic force

Coulomb’s Law • Electrical force is proportional ( ) to both • Particles with more charge (±q) experience a greater force. • Charges separated by a smaller distance experience a larger force.

Coulomb’s Law • These lead to one equation called Coulomb’s Law: Electric Force in Newtons (N) charge in Coulombs (C) distance between centers in meters (m)

Coulomb’s Law • A Newton is about equal to ¼ pound • A Coulomb is the amount of charge in 6.25 x 1018 electrons or protons

Example 1Two charged spheres, one with 1x1012 extra electrons and one with 2x1012 extra protons are separated by 50 cm. What force do they exert on each other? Given: Want:

Example 1solve for FE - force is attractive + force would be repulsive!

Example 2By what factor does the Electric Force change by if the distance between 2 charged spheres is doubled? Given: +q +q d +q +q 2d Unknown:

Time to Check Yourself! pg. 188 #1-6 Skip #9 (unless you want extra credit)

Answers • A • C • A • C • E • A • B • A • D • B

5. When two small charged spheres are separated by 2.0 m, the electric force of attraction between them is 6.0 N. If the charge on each sphere is doubled and the separation is reduced to 1.0m, the force of attraction will now be: Given: Unknown:

Start Homework! #1-4 pg. 190 Hint: #4)μC = 10-6 C

Discussion Question 1 Two identical charges Q, separated by a distance R, feel an electric force F. By what factor does the force change if both charges are doubled (2Q) and the distance is doubled (2d)? (A) 2F (D) F/2 (B) 4F (E) F/4 (C) F

The Electric Field • All charged objects produce electric fields • We know that a positive charge will repel another positive charge and attract a negative one • If we draw lines to represent this we would be drawing an electric field

The Electric Field • Let’s look at the rules for drawing field lines: • Field lines always point in the direction that a positive test charge would move - +

Static Electricity