Chapter 18: Electrostatic amber – electron (Greek word)

Units of Chapter 18 Static electricity and Charge Conservation of charge Conductors and Insulators Coulomb’s Law

By the end of this section, you will be able to: Define electric charge, and describe how the two types of charge interact. Describe three common situations that generate static electricity. State the law of conservation of charge. 3

Explain The spark that jumps from your finger to a metal doorknob after you walk across a wool carpet. Is there any connection lighting and static electricity. When we pump gasoline, we are warned to discharge ourselves on a metal surface before grabbing the gas nozzle. Attendants in hospital operating rooms must wear booties with aluminum foil on the bottoms to avoid creating sparks which may ignite the oxygen being used. 4

Electric charge The charge is defined as a property of a system that can affect its interaction with other charged systems. System – matter (object) which is composed with atoms and molecules. Atom - sub particles 5

Summary of Subatomic Particles Proton Neutron Electron In nucleus In nucleus Outside nucleus Tightly Bound Tightly Bound Weakly Bound Positive Charge No Charge Negative Charge Massive Massive Not very massive Charged versus Uncharged Particles Positively Charged Negatively Charged Uncharged protons > electrons electrons > protons protons = electrons

The most basic characteristics of static electricity The effects of static electricity are explained by a physical quantity called electric charge. There are only two types of charge, positive and negative. Like charges repel, whereas unlike charges attract. The force between charges decreases with distance. ELECTROSTATICS: The study of the behavior of stationary charges.

8 What will happen? Between positive charged object and neutral object Between negative charged object and neutral object 8

9 Methods of charging By Friction - two objects are rubbed together By Induction - without touching By Conduction - with touching Grounding (the removal of a charge) - the process of removing the excess charge on an object by another object of substantial size. 9

Rubbing certain electrically neutral objects together (a glass rod and a silk cloth) tends to cause the electric charges to separate. In the case of the glass and silk, the glass rod loses negative charge and becomes positively charged while the silk cloth gains negative charge and therefore becomes negatively charged. After separation, the negative charges and positive charges are found to attract one another. 10

If the glass rod is suspended from a string and a second positively charged glass rod is brought near, a force of electrical repulsion results. Negatively charged objects also exert a repulsive force on one another. These results can be summarized as follows: unlike charges attract and like charges repel.

Experiments performed early in this century have led to the conclusion that protons and neutrons are confined to the nucleus of the atom while the electrons exist outside of the nucleus. When solids are rubbed together, it is the electrons that are transferred from one object to the other. The positive charges, which are located in the nucleus, do not move. Rubber scrapes electrons from fur atoms.

Polarization – redistribution of charge a. Electrons in the sphere will move to the balloon. b. Protons in the sphere will move to the balloon. c. Electrons in the sphere will move to the left side. d. Electrons in the sphere will move to the right side. e. Protons in the sphere will move to the left side. f. Protons in the sphere will move to the right side. 13 A negatively charged balloon is brought near a neutral conducting sphere as shown below. The presence of the charged balloon will cause something to happen in the sphere. Which of the following would you expect to occur? Select all that apply.

Polarization – redistribution of charge c. Electrons in the sphere will move to the left side. f. Protons in the sphere will move to the right side. 14 A negatively charged balloon is brought near a neutral conducting sphere as shown below. The presence of the charged balloon will cause something to happen in the sphere. Which of the following would you expect to occur? Select all that apply.

The conduction charging process A: A teacher holds a negatively charged metal bar by its insulating handle and touches it to a metal sphere (attached to an insulating stand). B: The teacher pulls the metal bar away and the metal sphere acquires a charge. C: The excess negative charge spreads uniformly about the surface of the metal sphere. 15

The conduction charging process Diagram A is the charging step. How does the sphere become charged? a. Electrons move from the insulating stand into the sphere. b. Electrons move from the charged metal bar into the sphere. c. Protons move from the sphere into the negatively charged bar. 16

The conduction charging process When the metal bar is pulled away in Diagram B, the metal bar is _____. a. positively charged b. electrically neutral c. still negatively charged, but has fewer excess electrons than it previously did. 17

The conduction charging process Diagram C shows the excess negative charge distributed differently than it is in Diagram B. Explain why the excess negative charge would distribute itself as it does in Diagram C. 18

Two different processes are shown A: A negatively charged metal sphere is touched. B: The hand is pulled away and the sphere is then electrically neutral. C: A positively charged metal sphere is touched. D: The hand is pulled away and the sphere is then electrically neutral. 19

Check understanding The process of neutralizing the charged spheres as depicted above is known as _____. a.charging b.polarization c.induction d.grounding 20

Check understanding When the negatively charged sphere is touched, ______ move from the _______ to the _______. a. electrons, sphere, hand b. electrons, hand, sphere c. protons, sphere, hand d. protons, hand, sphere 21

Consider the following process: An uncharged metal soda can is attached to a Styrofoam cup (which acts as an insulating stand). A negatively charged balloon is brought near the can. While the balloon is held near, the can is touched. When the balloon is pulled away, the can is charged. 22

Consider the following process: This process is known as _____. a.charging by conduction b.charging by induction c.polarization d.grounding 23

Check understanding When the balloon is held near to the soda can (and before being touched by the hand), the distribution of charge on the can is best depicted by diagram _____. 24

Check understanding When the soda can is touched by the hand, ______ move from the ______ to the _______. a. protons, hand, can b. protons, can, hand c. electrons, hand, can d. electrons, can, hand 25

Check understanding This process causes the can to acquire a _____ charge. a. negative b. positive c. neutral 26 When the induction charging process is complete, the balloon is _____. a.positively charged b.electrically neutral c.still negatively charged, only having fewer excess electrons as before the process began d.still negatively charged, having the same amount of negative charge as it previously had

27 Van de Graaff generators An electroscope is an early scientific instrument that is used to detect the presence and magnitude of electric charge on a body.

The total electric charge of the universe is a constant: Electric charge is conserved. electric charge cannot be created or destroyed, however, it can be transferred from one object to another. CONSERVATION OF ELECTRIC CHARGE In the process of rubbing two solid objects together, electrical charges are not created. Instead, both objects contain both positive and negative charges. The quantity of excess charge on each object is exactly the same.

Conservation of Charge 29 Total charge is constant in any process.

The SI unit of charge is the coulomb (C). The charge carried by the electron is represented by the symbol -e, and the charge carried by the proton is +e. A third particle, which carries no electrical charge, is the neutron. q = 1.6 x C Charge is physical quantity 1 C = 6.25 x electrons or protons How many electron or proton is equal 1C?

Insulators and Conductors Conductor: A material whose conduction electrons are free to move throughout. Most metals are good conductors. Insulator: A material whose electrons seldom move from atom to atom. There is no such thing as a perfect insulator, however examples of good insulators are: glass, rubber, plastic and dry wood.

Insulators and Conductors If a conductor carries excess charge, the excess is distributed over the surface of the conductor.

Check your understanding After a comb is rubbed on a wool, it is able to pick up small pieces of paper. Why does the comb lose that ability after a few minutes? During a lighting storm, what is it about staying inside a car that makes it safe? 11

Check your understanding How could you find out which piece of plastic, A and B, is positively charged? Bring a positively charged glass rod near the two plastics. The one that is repelled by the rod is positive. You can charge rubber rod negatively by rubbing it with wool. What happens when you rub a copper rod with wool? 11

Check your understanding Which one of these spheres is made of an insulating material and which is made of a conducting material? 11 A B

Electric Force The electrical force is a push or pull exerted upon an object as a result of an interaction between electrical charges. the electrical force is a non-contact force. The electrical force is vector quantity 12

COULOMB’S LAW Coulomb’s Law states that two point charges exert a force (F) on one another that is directly proportional to the product of the magnitudes of the charges (q) and inversely proportional to the square of the distance (r) between their centers. The equation is: F = electrostatic force (N) q = charge (C) k = 9x10 9 N. m 2 /C 2 r = separation between charges (m)

The value of k can also be expressed in terms of the permittivity of free space (ε o ): 9x10 9 N. m 2 /C 2 k is a proportionality constant (the Coulomb's law constant) The value of this constant is dependent upon the medium that the charged objects are immersed in. If the charged objects are present in water, the value of k can be reduced by as much as a factor of 80 ( ε ).

Coulomb’s Law The forces on the two charges are action-reaction forces.

Coulomb’s Law If there are multiple point charges, the forces add by superposition.

Coulomb’s Law example problem Sphere A, with a charge of +6.0 μC, is located near another charged sphere B. It has a charge of – 3.0 μC and is located 4.0 cm to the right of A. What is the force of sphere A on sphere B?

Coulomb’s Law example problem A third sphere C, with a μC charge, is added to the configuration. If it is located 5 cm directly below sphere B. What is the net force on sphere B? 18

1. Two charges q 1 = - 8 μC and q 2 = +12 μC are placed 120 mm apart in the air. What is the resultant force on a third charge q 3 = - 4 μC placed midway between the other charges? q 1 = - 8 μC q 2 = +12 μC q 3 = - 4 μC r = m -q1-q1 +q2+q2 -q3-q3 F1F1 F2F2 FRFR = 80 N = 120 N F R = = 200 N, to the right

2. Three charges q 1 = +4 nC, q 2 = -6 nC and q 3 = -8 nC are arranged as shown. Find the resultant force on q 3 due to the other two charges. q 1 = +4 nC q 2 = -6 nC q 3 = -8 nC F2F2 F1F1 37˚ θ = 2.88x10 -5 N = 6.75x10 -5 N FRFR

F2F2 F1F1 37˚ θ From the FBD: Σ F x = F 2 - F 1 cos 37˚ = (6.75x10 -5 ) - (2.88x10 -5 )(cos 37˚) = 4.45x10 -5 N Σ F y = F 1 sin 37˚ = (2.88x10 -5 )(sin 37˚) = 1.73x10 -5 N = 4.8x10 -5 N θ = 21˚ F R (4.8x10 -5 N, 21˚) FRFR

Problems from text book 46

Summary Electrons have a negative charge, and protons a positive charge, of magnitude Unit of charge: Coulomb, C Charge is conserved, and quantized in units of e Insulators do not allow electrons to move between atoms; conductors allow conduction electrons to flow freely

Summary The force between electric charges is along the line connecting them Like charges repel, opposites attract Coulomb’s law gives the magnitude of the force: Forces exerted by several charges add as vectors