Many species in nature apparently use the Earth\'s magnetic field as an aid for

Question

Many species in nature apparently use the Earth's magnetic field as an aid for navigation. Among them are freshwater magnetotatic bacteria. If there is enough iron in their enviornment, these bacteria are able to synthesize chains of small magneticparticles within their bodies. These are basically internal bar magnets. Just like compass needles, they cause the bacteria to align with the Eart h's magnetic field. The creatures then use the whiplike flagella to move along the field lines. a) The bacterias' one goal in life is to avoid the oxygen rich upper layers of the water. In the Northern hemisphere they have evolved to swim in the same direction as the earth's B-field lines, while in the southern hemisphere, they swim in the opposite direction. Why? b) Near the equator, they are equally likely to travel in either direction. How does the earth's B-field help them in this case ? c) In each of the three cases(northern,southern,equatorial latitudes), say which is at the front of each bacteria? Its N or S pole?

Explanation / Answer

MAGNETIC FIELDS A magnetic field is a region of space where a north magnetic monopole experiences a force. The direction of the field is by definition the direction of the force on the north end of a magnet. Since most texts contain diagrams of magnetic fields they will not be reproduced here. CHARGES IN A MAGNETIC FIELD Use your right hand to determine the direction of force on a moving positively charged particle in a magnetic field. With the fingers pointing from south to north (the same direction as the field), and the thumb pointing in the direction of the velocity of the particle, the palm points in the direction of the force on the particle. In the example below, the force is away from the observer. For a negative particle, use your left hand. The magnitude of the force on the particle, F, is F = qvBsin

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