Q 12. We have a square loop having side as 12 cm and its sides are parallel to x and the y-axis is moved with a velocity of 8 cm/s in the positive x-direction in a region which have a magnetic field in the direction of positive z-axis.  The field is not uniform whether in case of its space or in the case of time. It has a gradient of 10−3Tcm−1 along the negative x-direction(i.e its value increases by 10−3Tcm−1 as we move from positive to negative direction ), and it is reducing in the case of time with the rate of  10−3Ts−1. Determine the direction and the magnitude and direction of induced current in the loop (Given: Resistance = 4.50  mΩ).


 

Side of the Square loop, s = 12cm = 0.12m

Area of the loop, A = s × s = 0.12 × 0.12 = 0.0144 

The velocity of the lop, v=8cms−1=0.08ms−1

The gradient of the magnetic field along negative x-direction,

dBdx=10−3Tcm−1=10−1Tm−1

And, the rate of decrease of the magnetic field,

dBdt=10−3Ts−1

Resistance, R=4.50mΩ = 4.5×10-3Ω

The rate of change of the magnetic flux due to the motion of the loop in a non-uniform magnetic field is given as:

dϕdt=A×dBdx×v=144×10−4m2×10−1×0.08=11.52×10−5Tm2s−1

Rate of change of the flux due to explicit time variation in field B is given as:

dϕ′dt=A×dBdt=144×10−4×10−3=1.44×10−5Tm2s−1

 

Since the rate of change of the flux is the induced emf, the total induced emf in the loop can be calculated as:

 

e=1.44×10−5+11.52×10−5=12.96×10−5V∴ Induced current, i=eR=12.96×10−54.5×10−3i=2.88×10−2A

 

Hence, the direction of the induced current is such that there is an increase in the flux through the loop along the positive z-direction.