Electricity and MagnetismWhat we learned last time (II)What we learned last time (I)TodayCoulomb’s LawSuperposition principleSuperposition principleSuperposition principleFeb 11 2002Electricity and Magnetism•Recap:– Fundamental Forces–E.S. Induction– Coulombs law (qualitative)• Coulombs Law (quantitative)•Induction (Demos)Feb 11 2002What we learned last time (II)• Strength of Electrostatic Force (qualitatively):– If distance gets larger, force gets weaker– If charge gets bigger, force gets stronger• Coulomb’s Law (1780):• Electrostatic Induction– Force on (globally) neutral objects by redistributing balance of positive and negative chargesFeb 11 2002What we learned last time (I)• Four fundamental forces– Weak, Strong, Electromagnetic, Gravity• All mediated by Exchange Particles• Exchange particle for E.M. Force is Photon– mass m=0 -> Long Range• E.M. Force dominates from 10-10to 1 m• E.M. Force is much stronger (x1035) than GravityFeb 11 2002Today• Study Coulomb’s Law– Check ‘Inverse square law’ (F~1/r2) experimentally • Superposition principle– What happens when there are many charges?• ‘Challenge’ Demo on E.S. Induction– Summarize what we know about E.S.Feb 11 2002Coulomb’s LawxyAngle αx2F21x1mgFeb 11 2002Superposition principle• Just add the forces on Q1!Q3Q1Q2F12F13F1,totalFeb 11 2002Superposition principle• Just add the forces on Q1!• Works for arbitrary number of charges:F1= Σ kQ1Qi/(ri/ri3)Feb 11 2002Superposition principle• What to do for many, many charges?–109e-on glass rod...• Calculus makes life easier (for a change)!• Replace sum with
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