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Electrostatics for M.6
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Electrostatics for M.6



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  • 1. Electrostatics Teacher : Piyanuch Plaon Subject : Physics 4
  • 2. Discovery of charge Benjamin Franklin arbitrarily called the two kinds of charge positive and negative. In most cases, only the negative charge is mobile.
  • 3. Properties of charge Like charges repel, and unlike charges attract. Charge is conserved, meaning it cannot be created or destroyed, only transferred from one location to another. In all atoms, electrons have negative charge and protons have positive
  • 4. Insulators In insulators, electrons are bound in “orbit” to the nucleus in each atom. When charge is placed on an insulator, it stays in one region and does not distribute. Wood, plastic, glass, air, and cloth are good insulators.
  • 5. Conductors In conductors electrons can move from atom to atom, thus electricity can “flow”. When charge is placed on a conductor, it redistributes to the outer surface. Metals (copper, gold,
  • 6. Charging by Friction When insulators are rubbed together, one gives up electrons and becomes positively charged, while the other gains electrons and becomes negatively charged.
  • 7. Charging by ConductionWhen a charged conductor makes contact with a neutral conductor there is a transfer of charge. Electrons are transferred from the rod to the ball, leaving them both negatively charged. Electrons are transferred from the ball to the rod, leaving them both positively charged. CHARGING NEGATIVELY CHARGING POSITIVELY
  • 8. Step 1. A charged rod is brought near an isolated conductor. The influence of the charge object polarizes the conductor but does not yet charge Step 2. The conductor is grounded to the Earth, allowing charge to flow out between it and the Earth. Charging by Induction
  • 9. Charging by Induction (cont.) Step 3. The ground is removed while the charge rod is still nearby the conductor. Step 4. The rod is removed and the conductor is now charge (opposite of rod).
  • 10. Electric Forces and Electric Fields CHARLES COULOMB (1736-1806) MICHAEL FARADAY (1791-1867)
  • 11. Electrostatic Charges The charge of an electron (qe) is -1.6 x 10-19 C Electrostatic charge is a fundamental quantity like length, mass, and time. The symbol for charge is q. The SI unit for charge is called the coulomb (C). ATTRACTION AND REPULSION
  • 12. The Electrostatic Force The constant of proportionality, k, is equal to 9.0 x 109 Nm2/C2. COULOMB’S LAW OF ELECTROSTATIC FORCE Fe  kq1q2 r2 consta nt distan ce charg es electrost atic force The electrostatic force depends directly on the magnitude of the charges. The force depends inversely on the square of distance between charges (another “inverse square law”)!
  • 13. Electric Field Strength g  Fg m E  Fe q0 DEFINITION OF GRAVITATIO NAL FIELD DEFINITION OF ELECTRIC FIELD g field  force mass E field  force charge SI unit of electric field newton coulomb  N C Electric field is a vector quantity E field points toward negative charges E field points away from positive charges q0 is a small, positive test charge
  • 14. Electric Field Lines Density of field lines indicates electric field strength Definition of E Field for single point charge POSITIVE CHARGE NEGATIVE CHARGE E  Fe q0  kq0q / r2 q0 E  kq r2 consta nt distan ce charg e electri c field Single Point Charges
  • 15. Electric Field Lines Electric fields for multiple point charges POSITIVE AND NEGATIVE POINT CHARGES TWO POSITIVE POINT CHARGES
  • 16. E  kq r2 EXAMPL E 1 EXAMPL E 2 E  9 109 Nm2 /C2  5 103 C  2 m 2 Electric Fields Find the force on an proton placed 2 meters from the 5 millicoulomb charge in the problem above. E  Fe q Fe  qE  1.6 10-19 C 1.13107 N/C  1.8110-12 N, to the right Fe  9 109 Nm2 /C2  5 103 C 1.6 10-19 C  2 m 2  1.8 10-12 N, to the right OR Find the electric field strength at 2 meters from the 5 millicoulomb charge. E=1.13107 N/C, to the right E
  • 17. PE for Two Point Charges PE  kq1q2 r Potential energy is zero at infinite distance Potential energy is positive for like chargesPotential energy is negative for opposite charges Potential Energy is force times distance PE  Fed  kq1q2 r2  r charg es distanc e electric potential energy consta nt Exampl eHow much electrostatic potential energy in a hydrogen atom, which consists of one electron at a distance of 5.3 x 10-11 meters from the nucleus (proton). PE  kq1q2 r  (9 109 )(1.6 1019 )(–1.6 1019 ) 5.31011  4.35 1018 J
  • 18. Potential Difference (Voltage) Potential  Energy Charge V  PE q A volt (v) is the unit for voltage named in honor of Alessandro Volta, inventor of the first battery. 1volt  1 joule 1 coulomb SI Units source voltage (V) common dry cell 1.5 car battery 12 household (US) 120 comb through hair 500 utility pole 4,400 transmission line 120,000 Van de Graaff 400,000 lightning 1,000,000,000 V  J C A good analogy: potential is to temperature, as potential energy is to heat. Electric potential is average energy per charge. Potential difference is often called voltage. Energy is a relative quantity (absolute energy doesn’t exist), so the change in electric potential, called potential difference, is meaningful. Voltage is only dangerous when a lot of energy is transferred. Voltage, like energy, is a scalar.
  • 19. Potential Difference for Constant Electric Field V  EdV  PE q  qEd q voltag e E field distanc e Potential energy is often stored in a capacitor. Most capacitors have constant electric fields. Capacitors are made by putting an insulator in between two conductors. Exampl e Calculate the magnitude of the electric field set up in a 2- millimeter wide capacitor connected to a 9-volt battery.V  Ed  9  E(0.002)  E  4500 N/C
  • 20. Consider a test charge to measure potential Potential Difference for Point Charge V  kq r charg e distanc e potential difference consta nt V  PE q0  kqq0 / r q0 Exampl e V1  kq1 r  (9 109 )(6 109 ) 0.3  180 V V2  kq2 r  (9 109 )(4 109 ) 0.4  90 V V3  kq3 r  (9 109 )(10 109 ) 0.5  180 V V  V1  V2  V3  180  90 180  270 V -4 nC 10 nC 6 nC 0.3 m 0.4 m find ∆V here
  • 21. CAPACITORS  A basic capacitor has two parallel plates separated by an insulating material  A capacitor stores an electrical charge between the two plates  The unit of capacitance is Farads (F)  Capacitance values are normally smaller, such as µF, nF or pF
  • 22.  Basic capacitor construction Dielectri c material Plate 1 Plate 2 The dielectric material is an insulator therefore no current flows through the capacitor CAPACITORS
  • 23. Storing a charge between the plates  Electrons on the left plate are attracted toward the positive terminal of the voltage source  This leaves an excess of positively charged holes  The electrons are pushed toward the right + - + _ + _ CAPACITORS
  • 24. Types of capacitors  The dielectric material determines the type of capacitor  Common types of capacitors are: Mica Ceramic CAPACITORS
  • 25.  Variable capacitors are used in communication equipment, radios, televisions and VCRs  They can be adjusted by consumers by tuning controls CAPACITORS
  • 26.  Fringing – At the edge of the capacitor plates the flux lines extend outside the common surface area of the plates. CAPACITANCE
  • 27. THE CURRENT : IC Current ic associated with the capacitance C is related to the voltage across the capacitor by  Where dvc/dt is a measure of the change in vc in a vanishingly small period of time. The function dvc/dt is called the
  • 28. CAPACITORS IN SERIES AND PARALLEL - Capacitors, like resistors, can be placed in series and in parallel. - When placed in series, the charge is the same on each capacitor.
  • 29. CAPACITORS IN SERIES AND PARALLEL  Placing capacitors in parallel the voltage across each capacitor is the same. The total charge is the sum of that on each capacitor.