Lecture08

# Lecture08 - 1 Physics 132-Winter 2008 Prof Jim Beatty-Ohio...

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Unformatted text preview: 1 January 23, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 1 Physics 132 Introductory Physics: Electricity and Magnetism Winter Quarter 2008 Lecture 8 January 23, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 2 Review of Potential Energy i f U x Potential energy U(r) can be used to describe a conservative force. Reference point (U=0) can be chosen for convenience. Work done by F : W = F • d s ∫ = F dx ∫ (1D) Change in P.E. : Δ U = U f − U i = − W Force from P.E. : F x = − dU dx (more rigorously, F x = − ∂ U ∂ x ) Work is independent of the path. U is unchanged if we return to the same point. For conservative forces, K + U = E E is constant without nonconservative forces E 2 January 23, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 3 Potential Energy of a Charge in a Uniform Electric Field What is the change in potential energy of a 3C charge in a uniform electric field of 10 N/C if it is moved along the path shown? Δ U = − dW = − F • d s ∫ ∫ = − Eq Δ x = − 10 N C 3 C ( ) 5 m ( ) = − 150 Nm = − 150 J The potential energy gets lower. U x If a positive charge were released in this field it would accelerate to the right. Reference point (U=0) is arbitrary here. 5m January 23, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 4 The Electric Potential Just like we defined E as the force per unit charge, define the electric potential V(r) as the potential energy per unit charge. V = U q E = F q Δ U = − F • d s ∫ ⇒ Δ V = − E • d s ∫ E and F are vectors. V and U are scalars. V is measured in volts. Moving 1C through 1V potential difference does 1J of work. 1 V = 1 J C = 1 Nm C E x = − ∂ V ∂ x ; E y = − ∂ V ∂ y ; E z = − ∂ V ∂ z Alternate units for E: Volts/meter (same as Newtons/Coulomb) 3 January 23, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 5 Electric Potential of a Charge in a Uniform Field What is the change in the electric potential between two points in a uniform electric field of 10 N/C separated by 5m along the field?uniform electric field of 10 N/C separated by 5m along the field?...
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Lecture08 - 1 Physics 132-Winter 2008 Prof Jim Beatty-Ohio...

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