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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: 113zami on June 16, 2008, 04:00:38 PM

Title: what is the nuclear binding energy of Helium nucleus?
Post by: 113zami on June 16, 2008, 04:00:38 PM
the nuclear binding energy, EB, can be found from the mass defect using Einstein's equation for mass-energy equivalence: EB=delta mc^2, where  c  is the speed or light (3x10^8m/s). if mass is measured in kilograms and energy in joules, then 1kg <--> 9x10^16J. But in the nuclear domain, masses are often expressed in atomic mass units(1amu=1.66x10^(-27)kg), and energy is expressed in electronvolts (1eV=1.6x10^(-19)J). In terms of these units, the equation for the nuclear binding energy, EB=deltamc^2, can be written as EB(in eV)= delta m (in amu)x 931 MeV

the above  paragraph was VERY confusing and I tried for 2 hours to make sense out of it but failed, can you Please explain that paragraph and how does it relate to the below problem, and why do they ignore the speed of light from the EB equation?:

1) the mass defect of helium nucleus is 5x10^(-29)Kg what is the nuclear binding energy?

my answer:
since EB=delta mc^2
then EB= 5x10^(-29)Kg ( 3x10^8m/s)= 1.5x10^(-20)

the correct answer:
the equation EB=delta mc^2 implies that 1kg <--> 9x10^16J, so a mass defect of 5x10^(-29)Kg is equivalent to an energy of (5x10^(-29)Kg) ( 9x10^16J)=4.5x10^(-12)J

thank you very much
Title: Re: what is the nuclear binding energy of Helium nucleus?
Post by: Borek on June 16, 2008, 05:45:07 PM
Quote from: 113zami on June 16, 2008, 04:00:38 PM
since EB=delta mc^2
then EB= 5x10^(-29)Kg ( 3x10^8m/s)= 1.5x10^(-20)

^2?
Title: Re: what is the nuclear binding energy of Helium nucleus?
Post by: 113zami on June 16, 2008, 07:20:12 PM
sorry I don't understand, can you please elaborate a bit more?
Title: Re: what is the nuclear binding energy of Helium nucleus?
Post by: 113zami on June 16, 2008, 08:23:20 PM
this is c raised to the 2nd power, don't know if that's what you meant
Title: Re: what is the nuclear binding energy of Helium nucleus?
Post by: Borek on June 17, 2008, 03:08:35 AM
Check your math. Especially around ^2 ;)
Title: Re: what is the nuclear binding energy of Helium nucleus?
Post by: 113zami on July 18, 2008, 11:06:43 AM
thanks, i forgot to squre it, but I am also confused about the units which they give in their answer:

(5x10^(-29)Kg) ( 9x10^16J)=4.5x10^(-12)J why is the9x10^16J given in (J)?
 
it should be in m^2/s^2 if we want the answer in J, since the units for 1J=(1kg)(m^2)/s^2

Title: Re: what is the nuclear binding energy of Helium nucleus?
Post by: 113zami on July 19, 2008, 10:11:01 AM
can someone please help me...I don't get it  ??? and I really NEED this for me test
thanks
Title: Re: what is the nuclear binding energy of Helium nucleus?
Post by: Astrokel on July 19, 2008, 12:37:14 PM
hey!

delta E = delta m c^2

For 1kg, delte E = (1) x c^2 = c^2 = 9x10^16 J

Therefore for 5 x10^-29 Kg, delta E = (5x10^-29) c^2 = 5x10^-29Kg x 9x10^16J

It's like ratio proportion,

good luck
Title: Re: what is the nuclear binding energy of Helium nucleus?
Post by: enahs on July 19, 2008, 02:13:26 PM
Basic units, follow your units!

1 J = 1kg · m2
            s2


c = m
      s


c2 =  m2
        c2


mass (in kg) * c2 = kg · m2
                             c2
Title: Re: what is the nuclear binding energy of Helium nucleus?
Post by: 113zami on July 19, 2008, 04:53:04 PM
THANK YOU both   :)