Physics experts wanted

<p>Do we have any physics experts? Specifically, I'm looking for explanations of some of the more difficult topics that may appear on the AP Physics B exam.</p>

<p>(If we do have an expert here, I have a few specific questions to ask.)</p>

<p>I'm certainly not an expert, but post your question and I'll see if I can help.</p>

<p>Two large, flat, parallel, conducting plates are 0.04 m apart. The lower plate is at a potential of 2 V with respect to ground. The upper plate is at a potential of 10 V with respect to ground. Point P is located 0.01 m above the lower plate. (The electric potential at point P is 4 V.)</p>

<p>What is the magnitude of the electric field at point P?</p>

<p>(Answer is 200 V/m.) Can anyone explain this? How do you figure this out?</p>

<p>Electric field = potential difference / distance = (10-2)/(0.04) = 200 V/m. The electric field is constant at every point between the two plates, but the electric potential decreases linearly from 10V to 2V.</p>

<p>Thanks! How about this -
A 50,000 W radio station transmits waves of wavelength 4 m. What is the best estimate of the number of photons it emits per second?</p>

<p>(Answer is 10^30)</p>

<p>Energy of each photon = hc/lambda = 5E-26 J, where h = Planck's constant and c = speed of light. 50,000 W = 50,000 J/second, so number of photons per second = 50,000 J / 5E-26 J = 1E30.</p>

<p>Thanks again! How about this -
A hollow metal sphere 1.0 m in diameter carries a charge of 4.0 (micro)C. The electric field at a distance of 2.0 m from the center of the sphere is most nearly ...</p>

<p>(Answer is 9.0 x 10^3 N/C)</p>

<p>the charge on the surface of a conducting spherical shell acts as though it is concentrated at the centre of the shell so you can treat it as a point charge.</p>

<p>So Using Coulombs law </p>

<p>E= kQ/r^2</p>

<p>where r=2m</p>

<p>E= 9x10^9x4x10^-6/2^2= 9.0x10^3 N/C</p>