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Electrostatics

Electrostatics. Coulomb’s Law. +. +. -. -. +. -. +. +. -+. -. -. +-. Attraction/Repulsion. Repulsion Repulsion Attraction Nothing Nothing or Attraction (electrons move) Nothing or Attraction (electrons move). What is a Charge?.

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Electrostatics

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  1. Electrostatics Coulomb’s Law

  2. + + - - + - + + -+ - - +- Attraction/Repulsion Repulsion Repulsion Attraction Nothing Nothing or Attraction (electrons move) Nothing or Attraction (electrons move)

  3. What is a Charge? • All materials have neutrons, protons, & electrons • Hard to move neutrons, and protons are stuckto neutrons so they don’t move either • But electrons are far away - they move easily • # of protons = # of electrons • If we remove a few electrons # of protons ≠ # of electrons • Charge is measured in Coulombs • 1 proton or electron has acharge of 1.6x10-19C(Quantum of Charge - e) neutral atom charged atom (ion)

  4. Cu2O Example How many electrons are gained if anobject has a charge of -1.00C? Q = N e • Q: Net charge on object (C) • N: Number of charged particles (# of electrons gained or lost) • e: Charge on 1 electron/proton (quantum of charge) • N = Q/e • N = (-1.00C) / (-1.60x10-19C/electron) • N = 6.25x1018 electrons N is always positive

  5. - - - + + + - - 6- 3+ 3+ 3+ + + + - - - - - - - - - 3- 6- - + - + - - + How do we charge an object? Get Same Charge Charging by contact Protons Don’t Move NEUTRAL ELECTRONSTRANSFER NEGATIVE

  6. 1- - - - + + + - 3+ 3+ 3+ 2- + + + - - - 3- 1- - + + + How do we charge an object? Get Opposite Charge Charging by induction - - NEUTRAL ELECTRONS LEAVE THROUGH GROUNDING POSITIVE

  7. Coulomb’s Law

  8. Coulomb’s Law Push or Pull • A force between 2 charged particles • Has magnitude & direction (vector) • Depends on: • Distance between objects • Amount of charge on objects • Type (+ or -) of charge on two objects • k = Coulomb’s Constant (9.00x109 Nm²/C²) • q = Charges on the charged particles (C) • r = distance between centres of charged particles (m) Where do yourecognize this from?

  9. Coulomb’s Law Example 3 charged particles lie on a table, what is net force on “B”? • FAonB = KqAqB / rAB² • FAonB = (9E9)(0.005)(-0.003) / (0.4m)² • FAonB = -843,750N • FConB = KqBqC / rBC² • FConB = (9E9)(-0.003)(-0.008) / (0.5m)² • FConB = 864,000N • FBnet = FAonB + FConB • FBnet = (843,750N [Left]) + (864,000N [Left]) • FBnet = 1.7x106N [Left] Attraction Repulsion Look at diagram for direction 5mC -3mC -8mC A B C 0.4m 0.5m

  10. 0.3m 0.4m Coulomb’s Law Example 2 3 particles lie at 90°, what is net force on the 6C particle? • F-3on6 = Kq-3q6 / r-3,6² • F-3on6 = (9E9)(-3)(6) / (0.4m)² • F-3on6 = -1.0E12N • F1.5on6 = Kq1.5q6 / r1.5,6² • F1.5on6 = (9E9)(1.5)(6) / (0.3m)² • F1.5on6 = 9.0E11N • F6net = √[F-3on6² + F1.5on6²] • F6net = √[(-1.0E12N)² + (9.0E11N)²] = 1.35E12N • θ = tan-1[F-3on6 / F1.5on6] • θ = tan-1[(-1.0E12N) / (9.0E11N)] = 48° • F6net = 1.35E12N @ 48°WofS Attraction Repulsion 1.5 -3 6

  11. Conclusions • Opposite charges attract (same repulse) • Quantum of Charge (e) = 1.6x10-19C • Charge (Q) = N*e • Things can be charged by contact and induction • Coulomb’s Law: • FE = Kq1q2/r² • K = 9.00x109 Nm²/C²

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