20) A solenoid of length 2.8 cm and diameter 0.75 cm is wound with 160 turns per

Question

20) A solenoid of length 2.8 cm and diameter 0.75 cm is wound with 160 turns per cm. When the current through the solenoid is 0.2 A, what is the magnetic flux through one of the windings of the solenoid ?


25) After the switch is closed in a series LR circuit, what is the current when a time equal to four time constants has elapsed ?

a. (1/5) Imax   b. (15/16) Imax   c. (1/16) Imax   d. 0.018 Imax   e. 0.98 Imax 20) A solenoid of length 2.8 cm and diameter 0.75 cm is wound with 160 turns per cm. When the current through the solenoid is 0.2 A, what is the magnetic flux through one of the windings of the solenoid ?


25) After the switch is closed in a series LR circuit, what is the current when a time equal to four time constants has elapsed ?

a. (1/5) Imax   b. (15/16) Imax   c. (1/16) Imax   d. 0.018 Imax   e. 0.98 Imax

Explanation / Answer

a ) Length of the solenoid is l = 2.8 cm                                              = 0.028 m    diameter of the solenoid is d = 0.75 cm        Area of the solenoid A = p ( d^2 / 2 )                                            = 0.44*10^-4 m^2    Number of turns N = 160 current in the solenoid is I = 0.2 A              magnetic flux through the solenoid is f _B = µ _0 N I A / l                                         = 4p * 10^-7 * 160* 0.2 A * 0.44*10^-4 / 0.028 m                                          = 6.3*10^- 8 Weber b ) I = I _max e^ - t / t            = I _ max e ^-4t / t             ( time equal to 4 times constant elapsed )            = I _max e ^-4            = 0.018 I _max     option d .

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