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Name: Surya
Status: student
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Question:
The alternating current oscillates sinusoidally. The positive direction indicates the flow of positive electrons in forward direction and negative current indicates that it flows in opposite direction. But when the electrons move backward and forward (oscillates sinusoidally) how is the current flow in one direction? The electrons should remain at one place as they continuously move forward and backward but this does not happen. Why?


Replies:
Well, Surya, let me see if I can straighten out this bag of terms for you. As an electronic engineer dealing with currents every day, I do not usually feel that current has a "direction" in the same sense as mass flow. Positive current is not particularly "more stuff" than negative current.

True, current does have two polarities. One polarity is designated "positive" by our arbitrary convention that the charge on the electron is a "negative" charge. But if a load is getting positive current flowing in, is it loosing electrons or gaining "holes" (electron-vacancies in the solid) or gaining positive ions? And is equal and opposite current going out the other wire (which almost always exists)? I can never decide which way is "more" in a gestalt sense. When I must, I follow the electrons, flowing towards positive voltage, and consider that positive current. But that's just a personal thinking method.

AC current, because it dithers back and forth, does not even have a polarity. At least it is not just +1 or -1. It is a phase, a real number which repeats every two-pi radians or 360 degrees or 1 cycle or 4 quadrants. Phase is defined only with respect to some other sine wave of exactly equal frequency which the thinker is arbitrarily taking to be his 0-degree reference wave.

In AC power on a wire, the voltage sine-wave on the wire is usually considered to be the reference phase, considered to be 0 degrees. Then the resulting AC current can have any phase relative to that.

What _does_ have a direction of "more" is: Power, energy transferred per unit time. Power is Current x Voltage. The multiplication of the two amplitudes seems to create the direction of flow.

For DC, positive current into a load at positive voltage means the load is getting positive power input. (+1 x +1 = +1) If the voltage alone goes negative, the power flow is backwards out of the load. (+1 x -1 = -1) If the current alone goes negative, the power also goes backwards. (-1 x +1 = -1) If _both_ go negative, then the power flow is forwards again. (-1 x -1 = +1)

As an alternative to 0-360 degree phase, you can think of any AC current as having within itself two independent polarity values: polarity of the sine part and polarity of the cosine part.

The cosine is a sine advanced 90 degrees, 1/4 cycle out-of-phase. Sine and Cosine are mathematically "orthogonal" to each other. In linear systems they do not interact with each other. For AC current with respect to AC voltage, the voltage is always Amplitude times the sine part (no cosine part), and the power flow depends only on the polarity of the sine part of the current. (Integral over a cycle of sine x cosine = zero.)

The electrons do pretty much remain in one small area during AC current. But the net power flow and magnetic fields from the brisk temporary electron motions within those small areas can be felt, can have heating and motor-force effects much like DC electric current.

Jim Swenson


Surya,

The current flow is NOT in only one direction for an AC circuit. The current direction alternates, flipping back and forth. The electrons do bounce back and forth many times per second. Electrons everywhere in the circuit, in wires, in power sources, in all of the devices, begin moving back and forth as soon as the power source is turned on.

Dr. Kenneth Mellendorf
Physics Instructor
Illinois Central College



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