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Binding Energy – A-level Physics

Binding Energy – A-level Physics

Units

To thoroughly understand this work we must have a firm grasp of the units involved:

Joules (J),   electron volts (eV)   and   atomic mass units (u)

 

There is a discrepancy between the mass of a nucleus and the sum total of the individual masses of its constituents.

This difference is called the mass defect.

We know that energy is equivalent to mass in Einstein’s equation,

E = mc2

where E is energy(J), m is mass(kg) and c is the velocity of light (ms-1).

So the mass can be given in terms of energy. That is in Joules(J).

There is however another energy unit much used in nuclear physics called the ‘electron volt‘ (eV).

One electron volt is the kinetic energy an electron gains when accelerated through a potential difference of 1 volt.

 

Since,

energy = charge x potential difference

 

E = eV

 

e is the charge on the electron 1.602 x 10-19 Coulomb
V is the p.d., 1 volt in this case

So the energy of 1eV is 1.602 x 10-19 Joules.

However, there is still another mass unit to consider. This is called the atomic mass unit (u).

By definition 1 u is equal to 1/12 the mass of a carbon-12 atom.

 

 

the atomic mass unit

 

To summarize, mass defects can be given in:

kilograms (kg),   atomic mass units (u),   Joules (J),   electron volts (eV).

The key to solving problems on binding energy is to know these units and how they inter-relate to one another.

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