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Unformatted text preview: 1 Lecture 13 EM Waves Intensity Radiation Pressure Polarization Doppler Shift October 11 Physics 104, Fal 2011 1 Electric Potential Energy in a Capacitor One way to view electric potential energy is that the potential energy is stored in the electric field itself Whenever an electric field is present in a region of space, potential energy is located in that region The potential energy between the plates of a parallel plate capacitor can be determined in terms of the field between the plates: Section 18.8 PE elec = 1 2 Q V = 1 2 QEd = 1 2 o E 2 Ad ( ) as E = Q o A where Ad is the volume of the region Electric Energy Density The energy density of the electric field can be defined as the energy / volume: These results give the energy density for any arrangement of charges Potential energy is present whenever an electric field is present u elec = 1 2 o E 2 Section 18.8 Magnetic Potential Energy in an Inductor Energy is stored in the magnetic field of an inductor The energy stored in an inductor is PE ind = L I 2 Very similar in form to the energy stored in the electric field of a capacitor The expression for energy can also be expressed as In terms of the magnetic field, Section 21.6 PE ind = 1 2 LI 2 = 1 2 N 2 A l I 2 as L = N 2 A l for a solenoid PE ind = 1 2 NI l 2 Al ( ) = B 2 2 Al ( ) as B = NI l for a solenoid Magnetic Energy Density Energy contained in the magnetic field actually exists anywhere there is a magnetic field, not just in a solenoid Can exist in empty space The potential energy can also be expressed in terms of the energy density in the magnetic field This expression is similar to the energy density contained in an electric field...
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