Chapter 1 #23

[Jocelyn Martinez-1K]

#23 is asking to "wavelength of the y-rays" and we're giving an energy of 140.511 keV. My question is why do we convert from eV to joules? Is it required to convert to joules in order to solve this problem?

[Joyce Lee 1C]

Yes, you must convert to joules! It has to do with the cancellation of units.
I just looked it up on google
1eV = 1.602 x 10^-19 J
Convert the energy, and you get 2.2510 x 10^-14 J
If you use the formula lambda =hc/E, the joules cancel out. I will show you the solution below...
lambda = [(6.626 x 10^-34 Js) (3.0 x 10^8 m/s)]/(2.2510 x 10^-14 J) = 8.8310 pm
As you can see, if you convert from eV to Joules, the joules as well as the seconds cancel out to give you an answer in meters.

[Gwen Peng 1L]

Yes in order to solve this problem you must convert to joules, so it should be:
E= (140.511x10^3 eV)(1.0622x10^-19 J*eV^-1) = 2.2513x10^-14 J
Then you can plug this value into the following equation:
wavelength = (hc)/E = ((6.626x10^-34 J*s)(3.00x10^8 m*s^-1))/2.2513x10^-14 J = 8.8237x10^-12 m or 8.8237 pm
You must convert to joules in order for the units to cancel and leave you with only meters.
Hope this helped :)

[Sarah Brauer]

Is 1.6022x10^-19 a constant number to convert to Joules?

[Kendall Schemmer 1I]

1.60218e-19 J is not necessarily a constant, but just a conversion factor. You can use (1 eV/1.60218e-19 J) to convert between eV and J in any problem.

[Radha Patel 1D]

Exactly, 1.602×10−19 is just the amount of Joules equivalent to electron volts.

[Karan Singh Lecture 3]

1.602*10^-19 is just a conversion factor in order to go from eV to joules.