Homework Help: Work required to disassemble a helium atom

1. Apr 28, 2012

tevatron1

1. The problem statement, all variables and given/known data

A helium atom has a rest mass of He = 4.002603u. When disassembled into its constituent particles (2 protons, 2 neutrons, 2 electrons), the well-separated individual particles have the following masses: p = 1.007276u, n= 1.008665u, e = 0.000549u.

How much work is required to completely disassemble a helium atom? (Note: 1 u of mass has a rest energy of 931.49 MeV.)

2. Relevant equations

E=mc^2

3. The attempt at a solution

2(1.007276u + 1.008665u + 0.000549u) = 4.03298u

4.03298u - 4.002603u = .030377u

E = (0.030377u)*(931.49 MeV/u)*(3.0*10^8)^2

E = 2.5466*10^18 MeV

*I tried using c = 2.9979*10^8, however it is still wrong.

2. Apr 28, 2012

Office_Shredder

Staff Emeritus
Did you notice that the units of (0.030377u)*(931.49 MeV/u)*(3.0*10^8)^2 are m4/s4?

The conversion 931.49 MeV/u is not a conversion between one mass and another mass, so multiplying by this and then using E=mc2 afterwards is pretty suspect

3. Apr 28, 2012

tevatron1

I had considered this. I'm honestly not sure where else to use the conversion. Can you give a hint? Am I correct to solve for the change in mass?