H. C. Oersted discovered the influence of an electric current on a nearby compas

Question

H. C. Oersted discovered the influence of an electric current on a nearby compass needle while teaching an upper-division electrostatics class. Describing his experiment, he wrote: If the distance of the connecting wire does not exceed three-quarters of an inch from the needle, the declination of the needle makes an angle of about 45°. If the distance is increased the angle diminishes proportionally. The declination likewise varies with the power of the battery." Magnetic declination is the angle between the direction the compass needle points and true north. About how large a current, in amperes, must have been flowing in Oersted's connecting wire? Assume the horizontal component of the earth's field in Copenhagen in 1820 was the same as it is today, 0.2 gauss.

Explanation / Answer

For Current

B = u0I / 2r

u0 = 4 * 10-3 gauss m/ A

r = 1.905 * 10-2 m

B = 0.2 Gauss

So I = 1.905 A

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