Collected questions so far this evening.

So far, two questions this evening. Here they are, along with my responses. If I get any more interesting questions by email this evening, I'll try to post the answers here for all to see.

1. Quick question on #7 from fall 2007 final: In part b do you use the E=mc^2 equation?

On #7 you do want to use E=mc^2 - I reused this question on a homework from Spring 2008 (HW12, here). The result is as silly as you would expect - half of the mass of the bullet would have to be turned directly into energy for this to work, which can't be done with any known technology.

2. An FM radio transmitter has a power output of 130kW and operates at a frequency of 98.3MHz. How many photons per second does the transmitter emit?

This is one like the HW question, where you need to convert power and energy. In fact, it is from spring 2008 HW11, here.

First, 130kW means 130e3=1.3e5 Joules per second, since a watt is a joule per second.

The frequency 98.3Mhz means 98.3e6 Hz. If the transmitter has this frequency, than means each photon emitted has an energy of

E = hf = (6.6e-34 J*sec)*(98.3e6 sec^-1) = 6.5e-26 Joules (per photon)

So, if the transmitter puts out 1.3e5 joules per second, and each photon is worth 6.25e-26 joules, that means the transmitter must put out

# photons = (1.3e5 J/sec) / (6.5e-26 J/photon) = 2e30 photons per second.

Basically, find the total energy being emitted per second (just the power given) and divide by how much energy a single photon has to figure out how many photons per second must be coming out. In this case, the intermediate step of converting to electron volts isn't really necessary - the power and energy per photon both need to have the same units, so you can convert to eV or just use Joules.