Consider a current carrying wire (current I) in the shape of a circle. Note that as the current progresses along the wire, the direction of `vecJ` (current density) changes in an exact manner, while the current I remains unaffected. The agent that is essentially responsible for is

Options

A. the source of emf
B. electric field produced by charges accumulated on the surface of wire.
C. the charges just behind a given segment of wire. Which push them just the right way by repulsion
D. the charges ahead

Answer

B

Explanation

Current density is a vector quantity so it changes the direction when current is moving in circular path. It I is the current and A is the area, then current density is given by.
`J = (I)/(A)`
The current flowing through the wire is given by

`I =(V)/(R )`
`rArr J = (V)/(RA)`
Also, `R= rho (l)/(A)`
`rArr RA= rho l`
`rArr J = (V)/(rho l)`
`:. sigma (1)/(rho) and (V)/(l)= E rArr i= sigma E`
`rArr vec(J) = sigma vec(E)`
From the above relation, we can see that the direction of current density is given by the direction of electric field. Therefore, option (b) is correct