It is only apparent if the overall charge density is zero; otherwise, the Coulomb repulsion overwhelms the magnetic attraction. In an electric arc, where charges are moving parallel to one another, an attractive force squeezes currents into a smaller tube. In large circuit breakers, such as those used in neighborhood power distribution systems, the pinch effect can concentrate an arc between plates of a switch trying to break a large current, burn holes, and even ignite the equipment.
Another example of the pinch effect is found in the solar plasma, where jets of ionized material, such as solar flares, are shaped by magnetic forces. The definition of the ampere is based on the force between current-carrying wires. Note that for long, parallel wires separated by 1 meter with each carrying 1 ampere, the force per meter is. Since is exactly by definition, and because the force per meter is exactly This is the basis of the definition of the ampere.
Infinite-length wires are impractical, so in practice, a current balance is constructed with coils of wire separated by a few centimeters.
Force is measured to determine current. This also provides us with a method for measuring the coulomb. We measure the charge that flows for a current of one ampere in one second. That is, For both the ampere and the coulomb, the method of measuring force between conductors is the most accurate in practice.
Calculating Forces on Wires Two wires, both carrying current out of the page, have a current of magnitude 5. The first wire is located at 0.
What is the magnetic force per unit length of the first wire on the second and the second wire on the first? Strategy Each wire produces a magnetic field felt by the other wire. The distance along the hypotenuse of the triangle between the wires is the radial distance used in the calculation to determine the force per unit length.
Since both wires have currents flowing in the same direction, the direction of the force is toward each other. Solution The distance between the wires results from finding the hypotenuse of a triangle:. The force per unit length can then be calculated using the known currents in the wires:. Watch how the parallel wires behave in each of these set-ups.
When either circuit type is selected, the Knife Switch can be lowered to complete the circuit by clicking either the switch itself or the blue Run button. Notice a stream of yellow electrons traveling through the circuit; flowing as they always do from negative to positive — opposite the direction of conventional current.
To halt the flow of current, click on the red Stop button or the knife switch. Clicking the blue Pause button will let you examine the process in mid-stream. As you can see, the wires in the series circuit repel one another, while the wires in the parallel circuit attract one another. If the currents flow in opposite directions, the electrons will "see" a higher density of electrons in the other wire due to relativistic length contraction.
And the wires will repel. Why do two wires with current flowing in the same direction attract each other, and two wires with current flowing in opposite direction repel? Paul N. Sep 5, Explanation: You can determine the direction of the magnetic field on a wire by imaging your right thumb pointing in the direction of the current. Related questions When work is done on a positive test charge by an external force to move it from one location to What is an electric circuit?
What is Ohm's law? How does a battery produce electric energy? How does the electrical circuit in my flashlight work?
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