# Lect13 - 1 Lecture 13 EM Waves – Intensity Radiation...

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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 Δ V = 1 2 QEd = 1 2 ε o E 2 Ad ( ) as E = Q ε 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 ε 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 = ½ 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 μ N 2 A l I 2 as L = μ N 2 A l for a solenoid PE ind = 1 2 μ μ NI l Λ Ν Μ Ξ Π Ο 2 Al ( ) = B 2 2 μ Al ( ) as B = μ 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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Lect13 - 1 Lecture 13 EM Waves – Intensity Radiation...

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