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Electro statics

Do NOW: What do you think electrostatics means?. Electro statics. Electro - Electricity/charged particles Static -stationary/ not moving Electrostatics - the study of stationary charges. Three Particles. Proton (p) Charge= +1.6x10 -19 C=+1 e Mass=m p =1.66x10 -27 kg Electron (e)

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Electro statics

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  1. Do NOW: What do you think electrostatics means? Electrostatics Electro- Electricity/charged particles Static-stationary/ not moving Electrostatics- the study of stationary charges

  2. Three Particles • Proton (p) • Charge= +1.6x10-19C=+1e • Mass=mp=1.66x10-27kg • Electron (e) • Charge= -1.6x10-19C=-1e • mass=me=9.11x10-31kg • Neutron (n) • Charge= neutral =0e • Mass=mn=1.67x10-27kg BOTH protons and electrons are elementary charges!

  3. The Coulomb • The basic unit of charge is the Coulomb • You MUST be in Coulombs when you plug into a formula • 1C=6.25x1018e (elementary charges) • 1e=1.6x10-19C • Elementary Charges CAN NOT be cut in half. • This means objects have charges that are multiples of 1.6x10-19C.

  4. Practice • How many elementary charges are in 1.5C of charge? • How many coulombs of charge are 1.88x1019 protons?

  5. Conductors vs. Insulators Conductors • Allow electrons to flow freely through the material • Usually materials with loosely bound valence electrons • Metals • Ionic solutions Insulators • Do not allow electrons to flow freely through the material • Usually materials with tightly bound valence electrons • Rubber • Wood

  6. Separation of charge • Neutral objects have an equal number of protons and electrons. • If a charged object is brought near a neutral object, the charged object can cause the charges in the neutral object to separate. • ONLY ELECTRONS CAN MOVE!!!! Example: Pith Ball • NEUTRAL OBJECTS • ATTRACT BOTH • POSITIVE AND • NEGATIVE THINGS!! As the negative rod comes close, it repels the negative charges. Because opposites attract, the positive side of the pith ball will attract to the negative rod. As the positive rod comes close, it attracts the negative charges. Because opposites attract, the negative side of the pith ball will attract to the positive rod. - + - + - + - + - + - - - + + + + - - - + + - - + + - - + +

  7. Coulomb’s Lawthe force between two charges • Fe– Electric Force (N) • k – Electrostatic constant (8.99x109Nm2/C2) • q1- the charge of the first object (C) • q2- the charge of the second object (C) • r- the distance between the centers of the two charged objects(m)

  8. Examples • What is the electrostatic force between two protons separated by a distance of 2m? • What is the electrostatic force between a 3C sphere and a -5C sphere separated by 50m? • What happens to the electrostatic force between two objects if the • distance is tripled? • distance is halved? • charge of one object is quadrupled? • charge of both objects are halved?

  9. Electrostatic Force Graphically Fe Fe q r

  10. Example Problems • What is the magnitude of the electrostatic force between a charge of +3.0x10-5 C and a charge of + 3.0x10-6 C separated by 0.3m? • Do they repel or attract? Example Problems • A point charge of -1.0x10-9C and a charge of + 3.0x10-9C separated by 5.0x10-2 m what is the magnitude of the electrostatic force between them? • Do they repel or attract?

  11. How to charge an object? Glass and Silk • When a glass rod is rubbed with silk, the silk strips electrons from the glass. • Silk gains electrons, so it becomes negative • Glass looses electrons so it becomes positive Rubber and Fur • When a rubber rod is rubbed with fur, the fur is striped of electrons by the rubber. • fur looses electrons, so it becomes positive • rubber gains electrons so it becomes negative

  12. Electroscopes (devices that show charge. They can not measure charge) • Leaf electroscope • Braun Electroscope

  13. Charging by Conduction(with a negative Rod)Charging by conduction means there is contact • Charge the rod negatively by rubbing it with fur • What does that mean about its electrons? • Touch the electroscope with the negative rod • The excess charges from the rod will seek equilibrium and move into the electroscope • Remove the rod • The electroscope ends up with the same (negative) charge as the rod • How do I know the electroscope is charged? • What if I bring a negative rod back near it? • What if I bring a positive rod near it?

  14. Charging by Induction(with a negative rod)No contact, the charge is induced • Charge the rod negatively • Bring the rod close to, but do not touch, the electroscope (what do the electrons do) • Ground the electroscope by touching the top or the leaves (what do the electrons do?) • The excess charges seek equilibrium through the ground • Remove the ground first, then remove the charged rod (why?) • The electroscope is charged opposite the charge of the rod + + + +

  15. GOAL:How many electrons are on your balloon? • assumptions • Each balloon has the same charge • The string has no mass • Hints: • Free body diagram of each balloon • Use the protractor to find an angle

  16. Calculations • Show all work neatly on a separate sheet of paper. Include • Diagrams • Formulas • Units • Verbal explanations

  17. Setup-uncharged setup- charged Electrostatic Force on Balloons Materials: -ring stand -2 balloons • Tape • String • Protractor • Ruler • scale Make sure strings are vertical

  18. Plan of Action Goal: number of electrons (elementary charges) • Need charge in Coulombs first (1e=1.6x10-19 C) • How do we get the charge in Coulombs? • Can find them using coulomb’s law • They were assumed to be the same so… • Need Fe and r • Fe can be found using a free body diagram and the idea of equilibrium • r can be measured • Fe = Fx which is the horizontal component • We need the vertical component and the angle to find it • The angle can b • e measured • The vertical component is equal to the force of gravity • The force of gravity can be calculated (Fg =mg) • g=9.81m/s2 • m can be measured on an electric balance (in Kg)

  19. Balloon Free Body Diagram Ft Fe Fg

  20. Coulombs spheres +4C +4C +5C +3C Examples: What is the charge on each conducting sphere after they are brought together and then separated. ASSUME THEY ARE EQUAL IN SIZE 1) 2) 3) -6C -2C -4C -4C -7C +3C -2C -2C +3C -2C -7C -2C -2C -2C

  21. Conducting Spheres of unequal Size • Both conducting spheres have a charge of –Q • They are then connected with a conducting wire. What happens? • Charges will flow until the spheres have • Equal electric potential • Equal charge density • This means electrons will flow from the smaller sphere to the larger one. -Q -Q

  22. Electric Fields

  23. P+ e- Electric Fields(vectors) • Use a Tiny Imaginary Proton (TIP) to see which way it would move near the object in question. The direction the TIP would move is the direction of the electric field.

  24. P+ P+ P+ e- Examples-Draw the Electric Fields 1. 2. 3. 4. + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + +

  25. Electric Field StrengthE=Fe/q • What is the field strength if a 3.0C charge feels a force of 12N? • A -3.1µC charge is placed is an electric field of 2.3 N/C directed to the right. • What is the magnitude of the force acting on the charge? • What is the direction of that force?

  26. Charge on and Electric Field for a Conducting Sphere

  27. Electric Potential Difference

  28. V - Electric Potential difference (Voltage): volt = J/C= eV/e W – Electrical Energy: Joule or eV q – charge: Coulomb or e • If 5 Joules of work is done on 0.5 coulombs of charge, what is the electric potential difference? • A 4e charge is moved through a potential difference of 40 volts. • How much energy does it gain? • Is that energy in eV or Joules?

  29. Electric Potential Difference due to a Point Charge

  30. Ue - Electric Potential Energy (J) k – Electrostatic Constant (9x109Nm2/C2) q – charge: Coulomb r - distance between the charges (m) • How much electric potential energy is stored between two protons that are separated by a distance of 2.5nm?

  31. Millikan Oil Drop Experiment • Figured out the charge/mass ratio of an electron. This proved that charge is quantized. • Charge comes in multiples of 1.6x10-19 C + + + + + + + + + + + + Fe=qE Fe=Fg qE=mg qV/d=mg q/m=dg/V m d V=Ed E=V/d Fg=mg - - - - - - - - - - - - - - - -

  32. Current and Resistance An introduction to Ohm’s Law

  33. Recall: Potential Difference

  34. Potential Difference • Provided by the battery • AKA voltage measured in volts • It is a “push” for the electrons in the wire that causes them to flow. • The battery DOES NOT supply the electrons, they are already in the wire.

  35. Current: The rate at which electrons flow through a conductor

  36. Conservation of charge – Junction Rule Total current into a junction must equal the total current out of that junction. 3 Amps Magnitude and direction of current flow? 6 Amps 4 Amps The unknown current is 5 amps out of the junction (to the right)

  37. Simple Circuit • Every Circuit must have • A source of potential difference (battery) • A conducting material (wires) • An electrical load (resistor/lightbulb)

  38. Circuit Symbols • Draw a circuit with a battery, a lamp and a switch using the symbols above

  39. Resistance • A resistor is any electrical device that you plug in. • It impedes the flow of electrons • The resistance of an object is DIRECTLY related to temperature. • The hotter the wires, the more resistance they have. • The resistance of a wire depends on • Its length • The material its made of (resistivity) • The cross-sectional area of the wire

  40. Resistance is measured in Ohms (Ω) Calculating Resistance

  41. Ohm’s Law I Slope = Resistance Slope = 1/Resistance V R V Resistance and current are INVERSES of each other. As one increases the other decreases! I I

  42. Electrical Power • When resistance is constant • “ohmic” • “Follows ohm’s law” • “constant temperature” P P I When Potential Difference is constant I

  43. Electrical Power • When resistance is constant • “ohmic” • “Follows ohm’s law” • “constant temperature” P When current is constant P V V

  44. Electrical Power When Voltage is constant When current is constant P P R R

  45. Electrical Energy (AKA work) Measured in Joules

  46. Circuits Measurements Ammeter Voltmeter Measures potential difference (in Volts) Wired IN PARALLEL with the resistor Has a very HIGH resistance • Measures current (in Amps) • Wired IN SERIES with the resistor • Has a very LOW resistance

  47. Types of Circuits Series One complete loop Parallel Multiple paths

  48. Circuits

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