What potential difference is needed to accelerate a he+ ion (charge +e, mass 4u) from rest to a speed of 1.0×106 m/s ? express your answer using two significant figures?

For the law of conservation of energy, the loss in potential energy of the He+ ion should be equal to the gain in kinetic energy:

which can be rewritten as

where
is the charge of the ion,
is the mass of the ion,
is the speed of the ion.
By using these values, we find the potential difference needed:

and the negative sign means the final point is at lower voltage than the initial point, and this is correct, because the ion has positive charge and a positive charge travels naturally from higher voltages to lower voltages.