Pieces of potassium (3.68x10^-19 J) and sodium (4.41x10^-19 J) metal are exposed to radiation of wavelength of 435nm.

Question

what is the velocity of the electrons from potassium?

 

what is the velocity of the electrons from sodium?

Answer 1

This is a question on Photo electric effect
Given Work Function for potassium = 3.68 x 10 -19 J and for sodium = 4.41 x 10 -19
Energy of each photon of 435 nm
E = h x frequency = h x c /wave length , (h = Planks const , c = speed of light )
E = 6.62 x 10 -34 x 3 x 108/ 435 x 10 -9 J = 4.56 x 10 -19 J
Which is more than the work function of both materials and so Photo electric effect will occur in each case and photo electrons will be emitted

The difference in case of potassium = (4.56 – 3.68) x 10 -19 J
= 0.88 x 10 -19 J
Which is equal to KE of electron ejected =1/2( mv2)
= velocity of ejected electron = 0.88x 10 -19 J
Gives v = Sqr ( 0.88 x 10 -19 x 2 / 9.1 x 10-31) m/s Taking mass of electron as 9.1 x 10 -31kg
v = ( 1760/91) ½ x 105 m/s
Same way we can find for other material

For Sodium the difference is 0.15 x 10 ^ -19 J . Now equate it ti KE of ejected electron ie 1/2 mv ^2 Calculate V using m of electron as 9.1x 10^ -31kg . You get the answer.