Physics problem. Calculating photon energy.?

T

the1ine

New member
I'm doing some home-learning physics, and have been given an example of how to calculate the energy of a photon with the following:

hf = & + Ek

Ek = kinetic energy in joules
f = frequency in Hz
h = planck's constant 6.3 x 10(-34)
& = work function in eV

QUESTION:
copper has a work function of 4.7eV and releases a photon of frequency 7.2 x 10(15) Hz - what is the kinetic energy of the photon?

ANSWER:
Ek = hf - &
= 6.63 x 10(-34) x 7.2 x 10(15) - 4.7
= 6.63 x 7.2 x 10(-34) + 15 - 4.7
= 31.16 x 10(-19) J - 4.7
= 31.161 x 10(-19) / 1.6 x 10(-19) eV - 4.7 eV
= 19.48 eV = 4.7 eV
= 14.78 eV

MY QUESTION: IS THIS CORRECT?

I think I maybe missed something, when I go to do this i do...

Ek = hf - &
= 6.63 x 10(-34) x 7.2 x 10(15) - 4.7
= 4.77 x 10(-18) J - 4.7
= 4.77 x 10(18) / 1.6 x 10(-19) eV - 4.7
= 29.835 - 4.7
= 25.135 eV

Can someone please tell me where I'm going wrong? Or is the example answer flawed?

The line Ek = 6.63 x 7.2 x 10(-34) + 15 - 4.7 in the example answer really confuses me, im not sure how it gets from the previous step to that one.

BTW... numbers in brackets are powers, eg 1 x 10(2) = 10 to the power of 2 = 100
 
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