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Chapter 30 - 30.1

1. (a) The magnitude of the magnetic ﬁeld due to the current in the wire, at a point a distance r from

the wire, is given by

B = µ 0 i

2 πr .

With r =20ft=6 . 10m, we ﬁnd

B = (4 π

10 − 7 T

m / A)(100 A)

2 π (6 . 10m)

=3 . 3

10 − 6 T=3 . 3 µ T .

(b) This is about one-sixth the magnitude of the Earth’s ﬁeld. It will aﬀect the compass reading.