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PHY132_L06

# PHY132_L06 - Classical Physics II Classical Physics II...

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lassical Physics II Classical Physics II PHY132 Lecture 6 The Electric Potential - II Lecture 6 1 02/05/2010

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Work by Work by a Varying Force a Varying Force Work is a scalar quantity , and we can thus “add little pieces of work” together, • i.e. sum a series of dot-products of the force F ( s ) with i small parts d s i of the total trajectory… F j F i θ i θ j •T h u s , the general definition of Work : F k θ k e for onstant: 1 0 1 lim ( ) n ii n i Wd d →∞ = ≡⋅ = s s Fs F s s Lecture 6 2 i.e. for F ( s ) constant: () 11 00 10 cos d F s θ =⋅ = = = Δ = Δ ∫∫ ss F,s F s sF s s F s 02/05/2010
(Electric) (Electric) Potential Potential Energy and Potential Energy and Potential When the Electric force does net work W F · d l (either positive or negative) on a test charge , the result is a change in its kinetic energy: F net = m a Î W net F net · d l = Δ K = Δ mv 2 ) The electric force is a conservative force, so we may associate an ELECTRIC POTENTIAL ENERGY with the force (as for ANY conservative force!): W = Δ U : f( f f) Δ U = U f U i = W E,i Æ f ≡− i f F E · d l s we did for the electric field we can now make the As we did for the electric field, we can now make the energy INDEPENDENT of the value of the test charge by defining a POTENTIAL V ( r ) as the Potential-Energy-per- nit st harge: Lecture 6 3 unit-test-charge: 1 () ( ) ff if ii fi E test U VV V d d qq Δ Δ= = = rr F r E r 02/05/2010

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If the electric field is zero everywhere, the potential … A. is zero everywhere B. is constant and the same everywhere C. varies as you go along 02/05/2010 Lecture 6 4
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PHY132_L06 - Classical Physics II Classical Physics II...

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