Electrostatics Coulomb’s Law

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

What is a Charge? neutral atom charged atom (ion) All materials have neutrons, protons, & electrons Hard to move neutrons, and protons are stuck to 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 a charge of 1.6x10-19C (Quantum of Charge - e) neutral atom charged atom (ion)

Cu2O Example How many electrons are gained if an object 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

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

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

Coulomb’s Law

Where do you recognize this 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 you recognize this from?

Look at diagram for direction 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

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 0.3m -3 6 0.4m

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