1. a) An electron-volt (eV) is the amount of energy needed to move a single elec
ID: 249796 • Letter: 1
Question
1. a) An electron-volt (eV) is the amount of energy needed to move a single electron from an electric potential of 0 to an electric potential of -1 V. The charge of an electron is approximately -1.602 x 10-19 C. How many joules is an electron volt?
b) The stopping potential in our experiment can range in magnitude from 0 to about 3 V. How much initial kinetic energy, in joules, must an electron have to pass through a 3 V stopping potential? (In our case, “stopping potential” means that the sign of the potential is deliberately chosen to retard the motion of the electron.)
2. Pennies are made of zinc coated with copper. Copper has a work function of 4.7 eV. The ozone layer blocks nearly all solar radiation with wavelength shorter than 320nm, but a very small amount of 200 nm light still manages to reach the Earth's surface. Can direct sunlight produce photoelectrons from a penny?
3. a) The currents in our experiment are between 0 and 20 nanoamperes. How many electrons per second are passing an ammeter if it reads 10 nA ?
b) In the United States, “presentation” lasers are limited by law to a power output of 5 mW. If a red 650nm laser pointer and a green 532nm laser pointer are set to maximum power, how many photons per second are emitted by each laser?
Explanation / Answer
1) a) An electron volt =1.602 x 10-19 J
b) It must have at least 3 eV = 3*1.602 x 10-19J = 4.806 x 10-19 J
2) We can find this out by determining if the energy of the incident photon equals the work function of the target metal. This energy is termed the threshold energy.
The energy of the incident photon: E
E = hc/ = (6.6x10^-34)(3x10^8)/200x10^-9 = 9.9x10^-19 J
Work function:
= 4.7 eV (1.6x10^-19 J/eV) = 7.52x10^-19 J
Since, the energy of the incident photon exceeds the work function of the metal, photoelectrons will be produced and have kinetic energy imparted to them by the sunlight.
E > and the remaining energy after the electron is freed will be used to give motion to the electron.
3) a) Look this up in a table of useful numbers in SI units, and you get:
e = 1.602 × 10^-19 C = 1.602 × 10^-10 nC
If there were N per second, they would carry a charge of N * e per second, which would be the current. We thus conclude that the rate of electrons passing through is:
r = I / e.
This lets us state that:
r = 10 nA / 1.602e-10 nC = 6.242 × 10^10 / second.
b) E = hc/ = (6.6x10^-34)(3x10^8)/650x10^-9 = 3x10^-19 J
P = 5x10-³ J/s
# of photons per sec emitted by red laser pointer: n = P/E
n = 5x10-³/3x10^-19 = 1.67 x 10^16 photons/second
similarly for green laser, E = hc/ = (6.6x10^-34)(3x10^8)/532x10^-9 = 3.72x10^-19 J
P = 5x10-³ J/s
# of photons per sec emitted by red laser pointer: n = P/E
n = 5x10-³/3.72x10^-19 = 1.34 x 10^16 photons/second
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.