Topic-4 - Topic 4. Magnetic Forces and Fields Do currents...

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Topic 4. Magnetic Forces and Fields Do currents exert forces on each other in the same way that charges do? Our study of electric forces and electric fields began with a simple experimental observation: charged particles exert forces on each other. From that, we were able to deduce that a charged particle creates an "alteration" in space, and that alteration [which we called the electric field ] exerts a force on any other charged particle that is placed in its presence. We found that the force exerted by one charge on another gets smaller as their separation increases, but that the force gets larger as the charge amount increases. Another simple experimental observation leads to another whole set of physical phenomena. It is relatively easy to observe that two current-carrying conductors will exert forces on each other, even though the conductors do not have any net charge. Since they are not charged positively or negatively, this new force cannot be the same as the electrical force we investigated earlier. This new force is due to the influence of charges on each other. moving If one has two conducting wires of length placed parallel to each other, very long P carrying currents and separated by a distance , it is found that the magnitude of the mutual MM < 12 force exerted by one wire on the other is given by , where is a proportionality Jœ+MMPÎ< + constant. If the currents flow in the same direction, the force is attractive. If they flow in opposite directions, the force is repulsive. It is also observed that if one wire is rotated about its center point until it is to the other, there is on it at all! All in all, a strangely perpendicular no net force different force than the others we have studied in Physics 111 and 112. In a way that is similar to our previous analysis of electric forces and fields, one can define a physical entity called the " " [symbol: ; unit: T (tesla)] that is produced magnetic field F t by any current-carrying conductor. This magnetic field is a vector quantity: it has both magnitude and direction. For a long straight wire, the magnitude of this field depends on the distance from the wire, and is given by , where the same constant appears as <F œ + M Î < + before. Just as with the analysis of electric fields, we are often interested in the effect of the magnetic field on some "test" current, and are not interested in (or have no information about) the source currents that produced the magnetic field. If a test current consisting of a straight wire of length carrying current is placed in a uniform external magnetic field , then the PM F magnitude of the force exerted on that wire is given by sin , where is the angle JJ œ F M P ))
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between the direction of the magnetic field, and the direction in which the current is flowing.
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This note was uploaded on 02/11/2012 for the course PHYSICS 112 taught by Professor Fretwell during the Summer '08 term at Iowa State.

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Topic-4 - Topic 4. Magnetic Forces and Fields Do currents...

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