Question

Fields on the Page Estimate the maximum magnitudes of the electric and magnetic fields of the light that is incident on a page because of the visible light coming from your desk lamp. Treat the lightbulb as a point source of electromagnetic radiation that is 6% efficient at transforming energy coming in by electrical transmission to energy leaving by visible light. SOLUTION Conceptualize The filament in your lightbulb emits electromagnetic radiation. The brighter the light, the larger the magnitudes of the electric and magnetic fields. Categorize Because the lightbulb is to be treated as a point source, it emits equally in all directions, so the outgoing electromagnetic radiation can be modeled as a spherical wave. Analyze Recall that the wave intensity I a distance r from a point source is I = where Pave is the average power output of the source and 4tr2 is the area of a sphere of radius r centered on the source. (Use the following as necessary: Mo, TT, r, Pavol Emax, and c.) Set this expression for I equal to the intensity of an electromagnetic wave given by I = Sava = ve given by I = Savg Emax max = Emax : 2 uo m 200C Solve for the electric field magnitude: KocPavg max w by visible light. (The remaining energy transfers out of the lightbulb by conduction and invisible radiation.) Let's make some assumptions about numbers to enter in this equation. The visible light output (in W) of a 60 w lightbulb operating at 6% efficiency is approximately A reasonable distance from the lightbulb to the page might be 0.39 m. Substitute these values to find Emax (in V/m): v/m Emax = Use Emax = = c to find the magnetic field magnitude (in T): Finalize This value of the magnetic field magnitude is two orders of magnitude smaller than the Earth's magnetic field. EXERCISE What is the approximate magnitude of the electric field in V/m) on a page if you are reading the book outdoors under the shade of a tree and happen to leave the book on the ground where the Sun shines on it? The intensity of sunlight reaching the surface of the Earth is about 1000 W/m. Hint V/m

Answer 1

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