Physics????

Question

a particle of charge 1 coulomb is at rest. a body of mass 200 mg and charge 1 coulomb is approaching this particle with velocity 1.5*10^5 m/s. then this body will---
a. stick to charge
b.carry the charge with
c.cross the charge
d.stop at a distance of 4m from the charge

Answer 1

Firstly, I am assuming the body is approaching from infinity, otherwise there is not enough information.

I am thinking the answer is D. A does not seem right because they have the same sign of charge. and B and C don't seem quite right. but...

Well, let's see here... it starts out with a velocity of 1.5E5 m/s, so it has kinetic energy 0.5 (.0002kg)(1.5E5 m/s)^2 = 2.25MJ. If it's going to stop 4 meters away, the electrical potential energy at 4 meters apart would need to be 2.25MJ, exactly. If the electrical potential at 4m is more than this amount, the charges will never get that close, and if it is less than this amount, then the body will approach closer than 4m. The electrical potential energy is given by...

kQq/r {k = 9E9 NM^2 /C^2}

= (9E9 * 1 * 1 / 4) J = 2.25 GJ.

So it seems as though the 2.25MJ of kinetic energy will be completely converted to electrical potential energy before the charges are within 4 meters of each other. Hmm... let me think about this some more.

I am thinking perhaps you meant 200 gm rather than mg. If this is the case, your initial kinetic energy would be 1000x as large, which would make it 2.25GJ and would be equal to the electric potential energy at 4m apart. I am compelled to believe this in large part because I don't believe any of A-C are reasonable answers. C doesn't seem right because this would mean at some point the distance between the two particles would be zero, at which point the electrical potential would be infinite. Thus, at any initial speed you would never expect the body to cross the charge (assuming one dimensional motion).

Answer 2

You have given neither the mass of the first particle nor whether the charges are like or opposite, so the problem is unsolvable. God is great, but your question is not.

Answer 3

Answer is c. cross the charge