| Themes > Science > Physics > Electromagnetism > Magnetostatics > Currents from magnetism > Force between current-carrying conductors |
Another consequence of Oersted's discovery is the expectation that two current-carrying conductors might exert forces on each other, since the magnetic effect produced by one could interact with the current of the other, and vice versa. It should be noted, however, that this argument is not a proof and requires experimental verification. As was pointed out by Arago, a French academician of the time, a magnet exerts a force on each of two unmagnetized pieces of iron, but the pieces of iron, when separated from the magnet, exert no force on each other. The situation with current-carrying conductors could conceivably be analogous: the presence of the magnet may be essential, the current-carrying conductors may be magnetically neutral in its absence. Within a week after receipt of the news of Oersted's discovery in France, André Marie Ampère (1775 - 1836) reported to the French Academy that two stiff conductors attract one another when current pass through both in the same direction and repel when the currents are in the opposite opposite direction. Note that Ampère was using the already well-established positive-current convention. The following critical question might immediately be asked of Ampère about this phenomenon: You are connecting wires to an electrical device-the electric battery. How do you know that the force between the wires is essentially magnetic? How do you know it is not simply an electrostatic force arising from electrical charges acquired by the conducting wires? Ampère anticipated this question, and his paper carries this answer: These attractions and repulsions between electric currents differ fundamentally from the effects produced by electricity in repose. First, they cease, as chemical decompositions do, as soon as we break the circuit. Second, in ordinary electric attractions and repulsions, opposite charges attract, and like charges repel; in the attractions and repulsions of electric currents, we have precisely the contrary; it is when the two conducting wires are placed parallel in such a way that their ends of the same sign are next to each other that there is attraction, and there is repulsion when the ends of the same sign are as far apart as possible. Third, in the case of attraction, when it is sufficiently strong to bring the movable conductor into contact with the fixed conductor, they remain attached to one another like two magnets, and do not separate after a while, as happens when two conducting bodies, oppositely electrified, come to touch. We might add to Ampère's list of evidence still another item, namely that three parallel conductors carrying current in the same direction all attract one another simultaneously. No three electrically charged bodies will all attract one another; at least one pair will repel. |
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