Intro to astrophysics:
(a) somewhere in our galaxy resides a cloud of neutral hydrogen gas with a radius r. the gas density is n. (a) show that the number of 21-cm photons the cloud emits every second is given by:
n_{ph}=\frac{\pi r^{3}n}{t}
(b) if the cloud is a distance d from the sun, show that the energy flux of this radiation at the sun is given by:
f=\frac{hc r^{3}n}{4d^{2}\lambda t}

Four point charges each having charge q are located at the corners of a square having sides of length a. find symbolic expressions for the following. (a) the total electric potential at the center of the square due to the four charges (use any variable or symbol stated above along with the following as necessary: ke.) vtotal= (b) the work required to bring a fifth charge p from infinity to the center of the square (use any variable or symbol stated above along with the following as necessary: w=