ay45c3-page11 - between  and  d is u d = 1 c Z  I d...

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Unformatted text preview: between  and  + d , is u d = 1 c Z  I d d and the total energy density is u = u d = 1 c Z ZZ I d d : Pressure is force per area, or momentum transfer per area per time. Consider photons bouncing back and forth between parallel perfect mirrors of area A and separation L. A L Each photon contributes u = E=(AL) to the energy density (since the volume is AL). Each photon gives a momentum transfer 2p each time it bounces o the mirror (+p goes to p, so the di erence is 2p). These bounces occur (on a given mirror) every 2L=c time, so the photon's contribution to the pressure is pc E 2p P = A(2L=c) = LA = LA = u : Adding up all the photons, we would nd P = u (but wait!) We have forgotten that photons are moving at di erent angles. A photon moving at angle  to the normal still contributes E=(AL) to u, but 35 ...
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