The strength of a magnetic field around a wire carrying a current of 20 A is 0.004 T. What is the strength of the magnetic field if the current is changed to 40 A?

For the same wire and distance, the magnetic field strength is directly proportional to the current. The current doubles from 20 A to 40 A, so the field doubles: 0.004 T × 2 = 0.008 T. The correct choice is 0.008 T.

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