ON A METER SCALE , TWO LONG PARALLEL CONDUCTORS AT RIGHT ANGLE TO IT PLACED AT 2cm & 6cm. THEY CARRY CURRENTS OF 1A & 3A RESPECTIVELON A METER...

The figure is attached
below.

By definition the magnetic field induction `B`  of a
current `I`  that flows to a straight wire, at distance `R` from the wire
is

`B = mu*I/(2*pi*R)`

where
`mu` is the magnetic permeability of the medium.

Because
the currents are parallel and in the same direction the condition of having zero
magnetic field happens is a point between the two wires where the field of one wire
enters the page and the other exits the page.

`B_1 =B_2
rArr mu*I_1/(2*pi*R_1) =mu*I_2/(2*pi*R_2)`

`R_1/R_2
=I_1/I_2 rArr R_2 =3R_1`

with the condition `R_1 +R_2 =
6-2 =4 cm`

`R_1 +3*R_1 =4`

`4R_1 =4 rArr R_1 =1cm`

Thus
the field will be zero at `R_1 =1 cm` distance to the right of first wire. This means
that on the cm scale this point will be at the 3 cm
mark.

The correct answer is C) 3
cm

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