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}

Lightning is an example of what phenomenon? a release of a large amount of energyan absorption of a large amount of energya natural electric circuita natural electric current

Large electric fields in cell membranes cause ions to move through the cell wall. the field strength in a typical membrane is 1.0 x 10 7 n/c. what is the magnitude of the force on a calcium ion with charge +e? what is its acceleration? is it possible for a particle with the same charge as as calcium to have a different acceleration if placed at this location? what would change the acceleration: mass of the particle? diameter of the particle?

Two conductors, a and b, are each in the shape of a tetrahedron. but of different sizes. they are charged in the following manner: 1. tetrahedron a is charged from an electrostatic generator to charge q. 2. tetrahedron a is briefly touched to tetrahedron b. 3. steps 1 and 2 are repeated until the charge on tetrahedron b reaches a maximum value. if the charge on tetrahedron b was q/4 after the first time it touched tetrahedron a. what is the final charge qbmax on tetrahedron b?