Answer to Question #292852 in Physics for Jayson

Question

Two concentric solenoids carry currents in opposite directions. The inner solenoid has 50 turns per cm. The outer

solenoid has 20 turns per cm. The inner coil carries a current of 0.5 A. (a) Calculate the current needed in the

outer coil so that its fields cancels the field produced by the inner coil. (b) How large a field is produced by the

inner coil?

Answer 1

Assuming the solenoids are equal and the only difference is the current and number of turns, for equal magnetic fields, we can write:

$$N_1I_1=N_2I_2,\\ I_2=I_1N_1/N_2=1.25.$$

The field is

$$B=\dfrac{N_1I_1}{L_1}=25/L\text{ T},$$

where L is the length of the solenoid.