***PLEASE NOTE: Answer has been given as B...I need to understand the work done

Question

***PLEASE NOTE: Answer has been given as B...I need to understand the work done to get that answer. Thank you.***

If the concentration of electrons flowing at speed v in the +x direction is equal to the concentration of protons flowing at speed v in the x-direction, what can we say about the current density, J, in this region? A. IJ is half what it would be if only the electrons were flowing. B. IJI is twice what it would be if only the electrons were flowing. C. J D. The magnitude of is the sa e as it would be if all the flowing charges were protons

Explanation / Answer

we know that the current density is J = current per unit area

   J = I/A

and the conventioan current direction is in the opposite direction to that of the electrons motion

her given that the electrons are moving in the +x direction so the current due to the electrons is in the -ve x direction and

the protons are moving in the -ve x direction , then the current will be in the +x direction

the curernt is I_e+Ip = 2I
       J = 2I/A

if only electrons are moving then the current density is J = I/A

so the answer is B

J is twice what it would be if only the electrons were flowing

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