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**1. Homework Statement**

An electron is confined to a nucleus of radius 4 femtometres. Estimate its minimum energy.

**2. Homework Equations**

[itex]ΔxΔp_x=h/4\pi[/itex]

[itex]E^2=p^2c^2 + m^2c^4[/itex]

As the electron's rest energy will be much less than it's kinetic energy,

[itex]E=pc[/itex]

**3. The Attempt at a Solution**

So I understand that I'm supposed to use the uncertainty principle to find the average momentum, and then plug it into the energy equation. What I don't understand is that the solutions use Δp

_{x}/2 instead of just Δp

_{x}. Why is this?