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 = mc ^{2}**

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.

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.