One of my teaching approach uses the ISEE (Identify, Setup, Evaluate, Execute) methodology to solve problems. In the example below, the solution is illustrated using the ISEE methodology. In the tuition class, this method is taught as a thinking process, which will become intuitive to students when they approach the questions. Students are able to quickly tackle the questions at a snap of the fingers.
An isolated copper sphere of radius 5.00 cm, initially uncharged, is illuminated by ultraviolet light of wavelength 200 nm. What charge will the photoelectric effect induce on the sphere?
(The work function of copper is 4.70 eV.)
- Photoelctric effect suggests that the electrons are being emitted due to the incident uv
- As more and more electrons leave the copper sphere due to photoelectric effect, the copper sphere will progressively become more positively charged.
- The positively charged sphere will now set up a positive potential to impede the outflow of the electrons (i.e. attracting the electrons back).
- This potential serves a similar function to the stopping potential in the usual photoelectric experiment.
- Once the electrons stop leaving the copper sphere, the copper sphere is now induced with a certain positive charge.
- Let KEmax be the maximum KE of the photoelectrons emitted.
- Let V be the electric potential of the copper sphere when it becomes positively charged.
- Photoelectric equation will be first used to find the KEmax.
- When the copper sphere becomes so positive as the electrons leave, it sets up effectively a stopping potential to prevent further emission of electrons.
- Make use of the electric potential formula of a point charge to find the charge acquired.
The value of the “stopping potential” is comparable to typical values in photoelectric experiments.