A proton orbits with speed just outside a charged sphere of radius . We have to find the charge on the sphere.

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Let Q be the charge on the sphere. As the proton is orbiting the sphere in a circular orbit of radius . The centripetal force for circular motion will be provided by the Coulomb attraction of the proton by the charge on the sphere. We therefore have the equation

where mass of the proton, , and charge of the proton, .

It is given that

,

and

.

Therefore,

As the proton is attracted by the charge on the sphere, the sign of the charge is negative.

Therefore,

.