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Phy 3031 Modern physics, Spring 2007
Homework set #6. NOT Due at the start of class on Wednesday, February 7, 2007.
For numerical answers give two significant digits. The first should be correct, the second can
be approximate.
1. (a) Compute the rest energy of 1 g of dirt.
(b) If you could convert this energy entirely into electrical energy and sell it for
10 cents per kilowatt-hour, how much would you get?
(c) If you could power a 100 W light bulb with the energy, how many years would it
last?
9 × 1013 J
= 9 × 1011 s ≈ 3 × 104 yr
100 W
2. Consider the following, all moving in free space: a 2.0 eV photon, an 0.40 MeV electron,
and a 10 MeV proton. The energy given is the kinetic energy of the object.
(a)
(b)
(c)
(d)
Which is moving fastest?
The slowest?
Which has the greatest momentum?
The least momentum?
3. If an electron has total energy .511 GeV, and if v is the speed of the electron, then
what is c − v?
4. When a proton and an anti-proton, at rest, annihilate each other and create two gamma
rays, what is the total energy of each individual gamma?
5. If a neutron at rest were to decay into an electron and a proton, what would be the
approximate amount of the kinetic energy of the electron?
6. (a) What is the surface temperature of a star whose spectrum has a peak at λ =
500 nm, which is the wavelength of green light.
(b) Does the star appear to be green?