2049_ch30C - Energy Stored in Magnetic Field Just like...

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PHY2049: Chapter 30 47 Energy Stored in Magnetic Field Î Just like electric fields, magnetic fields store energy Î Let’s see how this works 2 1 0 2 2 0 2 ε μ = = E B uE B u Electric field Magnetic field
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48 Energy of an Inductor Î How much energy is stored in an inductor when a current is flowing through it? Î Start with loop rule Î Multiply by I to get power equation Î Identify P L , the rate at which energy is being stored in the inductor Î Identify energy stored in inductor Î Similar to capacitor: di iR L dt ε =+ L L dU di P Li dt == 2 1 2 L UL i d i L i 2 I iR L i 2 2 C q U C =
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PHY2049: Chapter 30 49 Energy in Magnetic Field (2) Î Apply to solenoid (constant B field) Î Use formula for B field: Î Calculate energy density: Î This is generally true even if B is not constant ( ) 22 2 11 0 L UL i n l A i μ == l r N turns 0 B ni = 2 0 2 L B Ul A = 2 0 2 B B u = L B U uV A l V = = 2 1 0 2 E uE ε = B field E field
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PHY2049: Chapter 30 50 Energy Calculation Examples Î Calculate u B for earth field, B = 5 x 10 -5 T Î Calculate u B for neutron star, B = 10 8 T Î Calculate u B for magnetar, B = 10 11 T () 2 5 2 33 7 0 51 0 10 J/m 2 24 1 0 B B u μ π
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2049_ch30C - Energy Stored in Magnetic Field Just like...

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