• $summary_{type}$: application
• $word_{count}$: 170

EM waves carry energy with density u = $epsilon_0$$E^2$ = $B^2$/$mu_0$ (electric and magnetic contributions are always equal). The Poynting vector S = $\frac{1}{mu_0}$(E x B) gives instantaneous energy flow per unit area. Average intensity: I = $\frac{1}{2}$c$epsilon_0$$E_0^2$ = $E_0$$\frac{B_0}{2*mu_0}$. For a point source of power P: I = $\frac{P}{4*pi*r^2}$. EM waves carry momentum: p = $\frac{U}{c}$ (absorbed) or 2U/c (reflected). Radiation pressure: $P_{rad}$ = $\frac{I}{c}$ (absorbing surface) or 2I/c (reflecting surface). Force = $P_{rad}$A. Numerical scale: sunlight intensity ~1400 W/$m^2$ gives radiation pressure ~5 x 10^-6 Pa on an absorbing surface — negligible on Earth but significant in space (solar sails) and stellar interiors. JEE calculations: (1) Given $E_0$ or $B_0$, find intensity using I = $\frac{1}{2}$c$epsilon_0$$E_0^2$. (2) Given power and area, find $E_0$. (3) Pressure and force from intensity.