Physics 122 4/8/11 Prof. E. F. Redish 1 4/8/11 1 Physics 122 (composed by Steve Reich) 4/14/10 11 Physics 122 Δ12mv2()= F||netΔr =FnetiΔr = WnetiiWU =Δ−4/8/11 13 Physics 122 ΔUE= −FE⋅Δr (small step)ΔUE= −FE⋅Δr()∑ (lots of little steps)ΔUE= −FE⋅ drstartfinish∫General case Simplest case (one source charge, one test charge) ΔUE= −FE⋅drstartfinish∫= −kCqQr2drrirf∫= −kCqQdrr2rirf∫= −kCqQ1ri−1rf⎛⎝⎜⎞⎠⎟UE(r) =kCqQrNOT A VECTOR – JUST A NUMBERPhysics 122 4/8/11 Prof. E. F. Redish 2 4/8/11 15 Physics 122 Field of force felt by a positive test charge near a positive source. Electric Field felt by a positive test charge near a positive source. F =kqQr2E =kQr24/8/11 17 Physics 122 Field of force felt by a negative test charge near a positive source. Electric Field felt by a negative test charge near a positive source. F =kqQr2E =kQr24/14/10 18 Physics 122 qUVqFE ==Physics 122 4/8/11 Prof. E. F. Redish 3 4/8/11 19 Physics 122 Potential energy of a positive test charge near a positive source. Electric Potential of a positive test charge near a positive source. U =kqQrV =kQr4/8/11 20 Physics 122 Potential energy of a positive test charge near a positive source. Electric Potential of a positive test charge near a positive source. U =kqQrV =kQr4/14/10 21 Physics 122 Δ(12mv2) =Fnet⋅Δr = mg ⋅Δr = −mg ΔhΔUgrav= mgΔhPhysics 122 4/8/11 Prof. E. F. Redish 4 4/14/10 22 Physics 122 ΔU = −FiΔrF = −ΔUΔr(stepping in the directionof the steepest descent)4/14/10 23 Physics 122 g =FmVgrav=Ugravm= ghNot equal!4/14/10 24 Physics 122
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