Cobalt-60 is often used as a radiation source in medicine. It has a half-life of

Question

Cobalt-60 is often used as a radiation source in medicine. It has a half-life of 5.25 years. How long after a new sample is delivered will its activity have decreased a. to about one-eighth its original value b. to about one-third its original value c. What is N/N_O after 20 years? 8) The photoelectric work function of potassium is 2.3eV. If light having a wavelength of 250 nm falls on potassium, find a) the stopping potential in volts; b) the kinetic energy, in electron volts, of the most energetic electrons ejected; c) the speeds of the electrons.

Explanation / Answer

(a) KE = hf – work func = hc/wavelength – work func = (1.24 eV-m)/(0.250 m) – 2.3 eV = 2.66 eV
stopping potential eVo = KEmax = 2.44 eV
(b) KE = 2.66 eV
(c) 2.66 eV (1.6 x 10-19 J/eV) = 4.25 x 10-19 J
KE = ½ mv2
4.25 x 10-19 = ½ (9.11 x 10-31)v^2
v = 9.65 x 10^5 m/s

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