(32) Electric Field Lines and the Electric Dipole

(32) Electric Field Lines and the Electric Dipole - Lecture...

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Lecture 32 Electric field lines. The electric dipole.
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Example: The electric dipole + - d +Q -Q Definition: Electric dipole moment p Q d = r r Direction: From – to + p r
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What happens to a dipole in an external electric field?
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An electric dipole is placed in a uniform electric field E ext . + - F = QE E sin int o t he page net d Q E τ θ = θ net p E = r r r The CM of the dipole does not move but the dipole rotates. d 0 net F = θ E p
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- When the dipole rotates an angle d θ , the work done by the electric field is: dW d τ θ = For a finite displacement: θ θ θ = - = + - = -∆ ext sin (cos cos ) f i i f W pE d pE U E int o t he page pE τ = p θ sin d = - d θ is out of the page cos U p E p E = - = - r r
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Dipole in a uniform electric field: ext net p E U p E τ = = - r r r r r sin cos net pE U p E θ = = - r E - + p E - + p E - + p MAX 180 0 U pE U = = = = o min 0 0 U pE U = = = - = o 90 0 pE U = = = o Stable equilibrium Unstable equilibrium Maximum torque
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Dipoles are important in Physics, Chemistry, Biology, Engineering… The water molecule (makes water a good solvent!) H H O p
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This note was uploaded on 03/27/2008 for the course PHYS 221 taught by Professor Herrera-siklody during the Fall '08 term at Iowa State.

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(32) Electric Field Lines and the Electric Dipole - Lecture...

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