One can not align the apparatus to eliminate entirely the effects of the Earth\'

Question

One can not align the apparatus to eliminate entirely the effects of the Earth's magnetic field. Consider the case where electrons having accelerated through 34.0 V then enter the uniform field region between the Helmholtz coils. With the coil current set to zero, the electron beam was observed to have a slight curvature due to the earth's magnetic field. The current in the coil had to be set to 0.533A to set the electron beam exactly straight. If each coil has 65 turns and has a mean radius of 66.5 cm, calculate the component of the earth magnetic field perpendicular to the entering electron beam:

mT

Explanation / Answer

Given N=65 turns

I=0.533 A

R=66.5 cm =0.665 m

For Helmolthz coil ,the magnetic field is given by

B=[(4/5)^(3/2)]*(uoNI/R)

B=[(4/5)^(3/2)]*(4pi*10^-7*65*0.533/0.665)

B=0.0468 mT

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