ELECTROSTATICS

Electric Charge Electric Charge: a property of matter that creates a force between objects Charge can be positive (+) or negative (-) Like charges REPEL Opposite charges ATTRACT What objects (particles) do we know that have a charge?

Electric Charge An object’s charge depends on imbalance of protons (+) and electrons (-) More protons than electrons  positive More electrons than protons  negative Units of charge: coulombs (C) Protons and electrons have exactly the same amount of charge: 1.6 x 10-19 C Differ in sign (+ or -)

Charging an Object Objects become charged if they have an imbalance of protons and electrons. Can an object gain or lose protons? (Think: Can protons MOVE?) Can an object gain or lose electrons? (Think: Can electrons MOVE?) Law of Conservation of Charge: the net charge in an isolated system remains constant

Charging an Object Conductors: materials that transfer and redistribute charge easily Examples: ??? Insulators: materials that do not transfer charge easily; retain charge within a localized region Semiconductors: materials that behave as either insulators or conductors, depending on temperature

Triboelectric Series ONLY ELECTRONS CAN MOVE!!! Valence electrons, specifically How do we know if a material gains or loses electrons? Direction of electron movement depends on materials involved. A triboelectric series allows us to determine the resulting charges of two objects being rubbed together. If two materials are rubbed together, the one higher on the list should give up electrons and become positively charged.

Triboelectric Series Asbestos Fur (rabbit) Glass Mica Wool Quartz Fur (cat) Lead Silk Human skin , Aluminum Cotton Wood Amber Copper, Brass Rubber Sulfur Celluloid India Rubber

Charging by Friction Electrons are literally “rubbed” off one object onto another. Demonstration: Balloon & Hair Hair – positive; Balloon – negative Demonstration: Fur & Rubber Rod Fur – positive; Rubber Rod – negative

Charging by Conduction Conduction involves transfer of charge from one object to another by direct contact. Walk across the carpet in socks and touch the doorknob… ZAP! Charge is transferred and you experience a shock. Demonstration: Touch the negatively-charged rubber rod to the pith ball. Some electrons move from rod to pith ball. Pith ball repels rod (both slightly negative). Static Discharge: movement of charge from one object to another by conduction. Charge can JUMP! (Lightning)

Charging by Induction A temporary charge can be induced in a neutral object by bringing a charged object close to it. Electrons - attracted to a positively charged object. Electrons - repelled from a negatively charged object. Demonstration: Bring negatively charged rubber rod near pith ball. Pith ball repels. Bring negatively charged balloon near wall. Electrons in wall repel. Ball sticks to wall.

How Can We Detect Charge? Electroscope: a device used to detect charge. When a charge is present, the straw rotates. Degree of rotation or separation indicates strength of charge More rotation, more charge Less rotation, less charge Detects both positive and negative charge, but cannot tell the difference.

Electric Force Electric Force: force of attraction or repulsion between objects due to charge Depends on CHARGE and DISTANCE Increase charge  force increases Increase distance  force decreases Forces can act over a distance (no physical contact) through a FIELD Electric Field: region around a charged object in which other charged objects experience an electric force

Coulomb’s Law: Where: F = electric force in Newtons k = constant (just a #) = 9.0x109 Nm2/C2 q1 = charge of object #1 in Coulombs (C) q2 = charge of object #2 in Coulombs (C) r = distance between two charges in meters

Effects of Coulomb’s Law If the charge of one object doubles… Force doubles (x2) If the charge of both objects double… Force quadruples (x4) If the distance between the charges doubles… Force is quartered (divided by 4)

Electric Fields, Electric Energy and Electric Potential

Electric Fields An electric field is the region around a charge in which the electrostatic force is felt by other charges. Diagram electric fields using “field lines”. By convention, electric field lines always extend from a positively-charged object to a negatively-charged object, from a positively-charged object to infinity, or from infinity to a negatively-charged object. Electric field lines are most dense around objects with the greatest amount of charge. Electric field lines never cross each other. At locations where electric field lines meet the surface of an object, the lines are perpendicular to the surface.

Electric Field Lines

Electric Field Lines

Electricity, Part 2: Electric Current and Circuits Most examples and applications of electricity are not electrostatic. In order for electric charges to do useful work, they need to move. In order to move an electric charge, we need to apply a force.

Electrical Potential Energy What is needed in order to move a charge against the influence of an electric field? An external force Work If work is done on the charged particle, what happens to its energy? It increases!!! (Law of Conservation of Energy) What can do work on the charged particle to decrease its energy? The field!!!

Electrical Potential Energy Since this work changes a charge’s position in the electric field, the type of energy it gains is potential, specifically electrical potential energy (PEE) Work done against the field always increases PEE Work done by the field always decreases PEE

Electrical Potential Energy Draw field diagram for an electric field surrounding a positive charge (proton) Draw field diagram for an electric field surrounding a negative charge (electron) For a small, positive test charge… where would it have a large PE? A small PE? Would the PE be numerically the same for a slightly larger (or smaller) test charge?

Electrical Potential Electrical Potential Energy depends on the size of the test charge. If we want to get a picture for the energy independent of the test charge, we need to look at electrical potential Electrical Potential (V): potential energy per unit charge V = PEE / q0 Units: joules per coulomb (J/C) or volts (V) Electric Potential is commonly known as voltage

Potential Difference Electric potential itself is not useful, only the change in electric potential provides us with useful information. Potential Difference (ΔV): change in electrical potential energy per unit charge ΔV = Vf – Vi The potential difference between any two points A & B equals the work done against the field in moving a positive test charge from A to B with no acceleration: ΔV = WAB / q0 and ΔV = ΔPEE / q0 The work done is independent of the path taken

Potential Difference For a proton or an electron (qe) a change in potential of 1 V, produces a change in PEE of 1.6 x 10-19 J While very small in size, many atomic phenomena involve energies of this order of magnitude. A reasonable unit is needed in order to report this energy. Electron Volt (eV): amount of energy corresponding to an electron falling through a potential difference of one volt 1 eV = 1.6 x 10-19 J