The study of electric charges Electrostatics The study of electric charges

Introduction Did you ever run a comb through your hair? What do you notice. What causes the paper holes to jump onto the comb? There are electrical forces that are in place due to the presence of charge on the comb.

The Atom An atom consists of various charged and uncharged particles. The central region is called the nucleus. Protons (+) and neutrons make up the nucleus. Electrons (-) move around the nucleus in an orbital path. Nucleus Protons (+) Neutrons Electrons (-)

The Significance of Charge As mentioned before, protons are positive and electrons are negative. An atom with balanced charges is considered neutral. The overall charge can be changed by adding or removing electrons. This makes the atom an ion. Add e- Take e- Overall Charge: (Neutral Atom) (Positive Ion) (Negative Ion)

Sample Problem (Atomic Charge) An helium atom has a net electric charge of -8.0x10-19C. Is it neutral or an ion? Are there extra electrons or a shortage of them? How many extra electrons are there? Charge Per e-: -1.60*10-19C

Sample Problem How many excess electrons are on a ball that has a charge of q = -4x10-17C?

Sample Problem (Atomic Charge) An atom has a net electric charge of 4.8x10-19C. Are there extra electrons or a shortage of them? How many electrons short is this atom? Draw this atom given it is Boron.

Electrostatic Demo’s Tape Electroscope Pith Balls

Electric Forces Charges exert a force on other charges Like Charges Repel Opposites Attract

Actual Charge of Protons/Electrons Recall, charge is measured in Coulombs (C). Even though protons and electrons are very small, they still have charge. Let us use q as a variable for charge. Electron Proton

How do atoms get charged? Work can remove electrons from the atom. Results in a positively charged atom The free Electron can be transferred to another atom. Results in negatively charged atoms

Separating Charge Charges are balanced in neutral objects. Work must be done to separate charge (free electrons). Once charge is separated, it can be used in experiments.

Separation of Charge Bring a charge rod near a neutral conductor Un-like charges are attracted Like charges are repelled

Charge by conduction A charge rod touches a neutral conductor Like charges are repelled and uniformly distribute

Charge by Induction The charges on the spheres redistribute to maximum separation The charge object is removed A charge object is placed near neutral conductors Result: Two spheres charged by induction Contact between the conducting sphere is broken Separation of charge takes place A A A A B B B B

Charging by Polarization Certain substances, such as the one below, have polar molecules. These molecules have opposite charges at each end. Charging by polarization takes place when a charged object is brought near, realigning the molecules in the substance. Magnification

Conductors and Insulators Electrical Conductors are similar to Heat conductors. Electrical Conductors allow charge to move easily. Electrical Insulators do not allow charge to move easily

Conductors and Insulators Electrical Conductors all electrons to move easily. Metals Graphite Electrical Insulators do not allow electrons to move easily Glass Plastic Rubber

Homework Ch 16.1-16.5 Page 464 1-8 11-12

Lightning Charging by induction occurs during thunderstorms The negatively charged cloud induces a positive charge on the ground Lightning: An electrical between the clouds and oppositely charged ground.

Grounding The earth stores a seemingly infinite amount of charge, both kinds. An object is grounded when it is connected to the earth or another large object. Electrical devices often have a “ground,” which prevents unwanted charge buildup. Grounding is also the principle behind lightning rods. Outlet Ground

Lightning Average Temperature 30,0000C (roughly 5x as hot as the sun) Typical charge (q) for lightning 10C to 25C How many electrons is this?

Lightning is actually a discharge of static electricity. Charge differences are developed from the friction of dust particles within the cloud. When the concentration of charges becomes too great, an electrical discharge results

The Electroscope An electroscope is a device that detects electrical charge in objects brought near. Its metallic inner contents, which are usually neutral, have to be separated from surroundings by some type of insulator. There are two metal leaves that hang inside. When a charged object is brought near, the leaves separate. Charge can also be stored in the electroscope by touching it with the rod. Leaves

Coulomb’s Law The electrostatic force one charged object exerts on an other The force is related to the amount of charge i.e more charge – more force The force is proportional to 1/d2 i.e. the further apart the charges, the smaller the force

Coulomb’s Law Symbol Unit F Force N q1 Charge C q2 Charge C d Distance m K constant N m2 / C2

Ex. Coulomb’s Law WS 20.1 #4 Object A has a charge of 6x10-6C. Object B has a charge of 3x10-6C and is 0.03m away. Calculate the force on A.

Ex. Coulomb’s Law WS 20.1 #5 The distance that separates electrons in a typical atom is 1.5x10-10m. What is the electrostatic force between them?

Electro-static Applications Electrostatic filter

Electro-static Applications Electrostatic painting

Electrostatic filters Scan pictures from p.416

Practice Problems WS 20.1 #’s 2, 3, 6

Ex. Coulomb’s Law WS 20.2 #1 Three charges are aligned as shown. Calculate the force on q2 due to q1. Electric Charge Positions Scale: 1 Square = 0.05m q1 q3 q2

Ex. Coulomb’s Law WS 20.2 #1 Three charges are aligned as shown. Calculate the net force on q2. Electric Charge Positions Scale: 1 Square = 0.05m q1 q3 q2

Ex. Coulomb’s Law WS 20.2 #1 Three charges are aligned as shown. Calculate the force on q2 due to q3. Electric Charge Positions Scale: 1 Square = 0.05m q1 q3 q2

Practice Problems WS 20.2 #’s 3

Coulomb’s Law in 2-D To find Fnet with 3 or more charges Calculate each Force vector. It helps to have a grid system on which to work. Use vector addition to find the resultant Fnet q2 q3 q1 F13 F14 F12 FNet q4

Coulomb’s Law in 2-D (cont.) Find the net force acting on q1. First find the distance between q1 and the others. Use the Pythagorean Theorem to find these distances. 1st Triangle 2nd Triangle q2 q3 C A B C B A The variable C from each triangle’s hypotenuse is the variable d used in the Coulomb’s Law equation. q1

Coulomb’s Law in 2-D (cont.) Charge (C) q1 3.0 X 10-4 q2 -2.6 X 10-5 q3 7.2 X 10-6 Attraction q2 q3 F12 q1 F13 Repulsion

Coulomb’s Law in 2-D Sample Determine the direction of each of the forces prior to vector addition. F12 F13 q2 q3 5.4N hyp opp adj hyp adj opp q1 Quad II Adjust Quad III Adjust 0.972N

Coulomb’s Law in 2-D Sample The remaining task is to use analytical vector addition. Mag Ang X Y Q F12 5.4N 146.3° -4.49 3.00 II F13 0.972N 206.6° -0.87 -0.44 III FNet 5.94N 154.5° -5.36 2.56 Quad II Adjust

And in it there’s no lightning Conclusion Electrostatics, the study of the forces between charges at rest. Hospital And in it there’s no lightning I had a dream…

Practice Problems WS 20.2 #’s 2

Coulomb’s Law in 2-D Sample Three charges are aligned as shown. Find the net force on q1. Electric Charge Positions Scale: 1 Square = 0.5 cm q1 q3 q2 Mag Ang X Y Q F12 5.4N 146.3° -4.49 3.00 II F13 0.972N 20.6° -0.87 -0.44 III FNet 5.94N 154.5° -5.36 2.56

Static Discharge Pic’s

End Ch 20

Example: Charge Distribution What is the total charge of three conducting spheres with charges of 6q,-1q, and 0q A 6q B -1q C 0q

Example: Charge Distribution cont. What is the final charge distribution if sphere A and B touch? C 0q A 2.5q A 6q B -1q B 2.5q

Example: Charge Distribution cont. What is the final charge distribution if sphere B and C touch? A 2.5q B 2.5q B 1.25q C 0q C 1.25q

Example: Charge Distribution cont. What is the total charge of three conducting spheres C 1.25q A 2.5q B 1.25q

Example: Charge Distribution cont. Sphere B is twice as large as sphere A, what will be the charge distribution after they touch? A 5q B 5q

Charge by Conduction Bring a charge rod near a neutral conductor Un-like charges are attracted Like charges are repelled

A 5q B -1q C 0q

q3 q1 Scale: 1 square =0.1cm q2

q1 q3 q2