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$$
\begin{array}{l}{\text { A very long, straight horizontal wire carries a current such }} \\ {\text { that } 3.50 \times 10^{18} \text { electrons per second pass any given point goint going }} \\ {\text { from west to east. What are the magnitude and direction of the mag- - }} \\ {\text { netic field this wire produces at a point } 4.00 \mathrm{cm} \text { directly above it? }}\end{array}
$$

$$
B=\frac{M_{0} I}{2 \pi r}
$$

$$
I=3.5 * 10^{18} \text { el/s } * 1.6 * 10^{-19} \mathrm{c/el}=0.56 \mathrm{A}
$$

$$
\rightarrow B=\frac{4 \pi * 10^{-7}*10.56}{2 \pi(0.04)}=2.8*10^{-6} \mathrm{T}
$$

$$
\begin{array}{l}{\text { Bacteria Navigation. Certain bacteria (such as }} \\ {\text { Aquaspirillum magnetotacticum) tend to swim toward the earth's }} \\ {\text { geographic north pole because they contain tiny particles, called }} \\ {\text { magnetosomes, that are sensitive to a magnetic field. If a transmis- - }} \\ {\text { sion line carrying } 100 \mathrm{A} \text { is laid underwater, at what range of dis- }} \\ {\text { tances would the magnetic field from this line be great enough to }}\end{array}
$$

$$
\begin{array}{l}{\text { interfere with the migration of these bacteria? (Assume that a field }} \\ {\text { less than } 5 \text { percent of the earth's field would have little effect on the }} \\ {\text { bacteria. Take the earth's field to be } 5.0 \times 10^{-5} \mathrm{T} \text { and ignore the }} \\ {\text { effects of the seawater.) }}\end{array}
$$

$$
B=0.05\left(5 * 10^{-5}\right)=2.5 * 10^{-6} \mathrm{T}
$$

$$
\rightarrow B=\frac{M_{0} I}{2 \pi r}
$$$$
\rightarrow r=\frac{M_{0} I}{2 \pi / B}
$$

$$
r=\frac{2 * 10^{-7} * 100}{2.5 * 10^{-6}}=8 \mathrm{m}
$$

$$
=25 \mathrm{ft}
$$

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