20.0-mH inductor potential energy problem

[boomboompoop]

Inductance
You are given a 20.0-mH inductor by your laboratory instructor, who then asks you to place it in series with a power supply. Before instructing you to turn on the power supply, he quizzes you on your understanding of the behavior of solenoids.

a. How much magnetic potential energy is stored in the inductor when it carries a current of 0.2 A?
Possible answers: a) 1.3 * 10^-4 J
b) 2.7 * 10^-4 J
c) 4.0 * 10^-4 J
d) 5.1 * 10^-4 J

b. What should the current in the coil be in order that it contains 1.0 J of energy?
Possible answers: a) 10.0 A
b) 15.0 A
c) 20.0 A
d) 25.0 A

i think i got a) energy stored = (1/2)L*I^2
= .5 * .020 * .2^2
= .00040 J -> 4*10^-4 J

can anyone help me with letter b)?

 

[Cyrus]

a.) looks good.

for b.) why not use the same equation as part A?

 

[boomboompoop]

so b is 1=(1/2)*.020* I^2 then?
is the answer .01 A?

 

[Cyrus]

Check your math.