A long straight wire AB carries a current I. A proton P travels with a speed v, parallel to the wire, at a distance d from it in a direction opposite to the current as shown in figure. What is the force experienced by the proton and what is its direction?

Explanation

Magnetic field induction at P due to current I in long straight wire AB is
`B=(mu_0)/(4pi)(2I)/(d)`
It acts perpendicular to the plane of paper inwards, represented by `hatn`.
Lorentz force acting on proton at P can be given by
`vecF=q(vecvxxvecB)=e(vecvxx(mu_02I)/(4pid)hatn)`
or `vecF=(mu_0Iev)/(2pid)(hatvxxhatn)` `:. F=(mu_0Iev)/(2pid)`
acting in the plane of paper away from the wire.