How does the current flow in one direction if the electrons are hoping back and forth? Shouldn’t the net current be zero?
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It is. But power ISN’T directional. Your Light bulb doesn’t feed power back into the system on the AC down cycle. It still uses power.
Have you ever seen a mechanical crank, as in the type you have in crank shafts in engines or the ones seen on the outside of the wheels of steam engines. There is a piston pushing a rod back and forth. You can ask the same question with this, how is the power transmitted through the rod though it changes direction after half a cycle? This is because not only is the direction of movement changed but also the direction of force. The piston is only pushing on the rod when it is moving away and the crank attached at the other end is resisting this motion. But when the rod is moving towards the piston the piston is not pushing or in the case of steam engines it is pulling the rod.
AC current is exactly like the rod in a crankshaft. The movement of the rod is similar to the current in a power line while the force applied through the rod is similar to the voltage of the line. When the current changes direction so too does the voltage. The generator is basically pushing and pulling on the electrons to make sure the current alternates while the appliance is resisting this current. So power is transfered through an alternating current line.
Anytime the electrons are moving they can do “work” the direction really doesn’t matter the movement does. https://youtu.be/8-glVkryAvA
Yes, but more importantly no.
The electrons, as you say, are wiggling back and forth as the voltage sweeps between positive and negative. But we’re not really interested in the motion of *the individual electrons,* but rather the propagation *of the electromagnetic field they interact with.*
The first quantity is called the “drift velocity,” and at one amp through a wire 2mm in diameter, this drift velocity is on the order of 8 centimeters per hour.
However, the second quantity, “signal velocity,” is what we’re really interested in when it comes to electric circuits, and that value is much closer to the speed of light in a vacuum.
https://www.wtamu.edu/~cbaird/sq/2014/02/19/what-is-the-speed-of-electricity/
Imagine a really long belt running along the pylons instead of electrical wires. This belt could run continuously forwards, and people could extract useful power by having the belt rotate a wheel which is then linked to their machinery.
Or the belt could reciprocate backwards and forwards. There is no net motion of the belt, but we can still extract useful power. The belt would be attached to a connecting rod, which would move backwards and forwards and so rotate a crank, just like in any reciprocating engine. This crank would be attached to a shaft which would rotate forwards continuously and be attached to the machinery, just as in the example above.
We have motors that can make use of electrical fields that oscillate backwards and forwards and turn that into continuous motion to do useful stuff. In fact Alternating Current (AC) motors have historically been easier/better to make (modern motors are likely to be AC, but variable frequency).
Think about jumping on a jump rope. It doesn’t take much work to swing the jump rope around yourself, but it does take a little bit of work. But the jump rope isn’t going anywhere, it is just going up and down. Nevertheless it still takes a bit of work to make it go around. Now imagine if it is the jump rope that’s the one doing the work and it is dragging your hands in small circles like a crank.
The power company is the one jumping the rope and you simply attach small cranks to it and pull out power.
Light are like the air resistance in our jump rope analogy. When electricity is going through a wire there is nearly zero air resistance, like doing jump rope in space. But when you turn on a light you are adding resistance. Just like jumping on a jump rope makes you get sweaty and hot, so does the light, only it gets so hot that it emits light. You do too, but our eyes simply can’t see it.
AC current is transmitted through power lines by changing the voltage and frequency of the current. A transformer is used to step up the voltage of the current as it travels along the lines, and the current is transmitted as an alternating current (AC) which changes direction regularly after a half-cycle proportionally in the opposite direction. This is done by using a combination of three components: a generator, a transmission line, and a transformer. The generator produces an alternating current with a certain frequency and voltage. This is then transferred to the transmission line where it is stepped up to a higher voltage. Finally, the transformer reduces the voltage back down to the appropriate level for distribution to the consumer. The alternating current also maintains a consistent frequency as it travels along the power lines.
The net current *is* zero. But power isn’t transferred by current going from the battery to an item like a lamp, it is transferred by the electrons being forced to move through the lamp. The lamp doesn’t care which direction the electrons are flowing in.
Think about a bike chain. When you pedal the wheel turns because the chain connects your feet to the wheel. If you keep changing direction the wheels would spin one way and then the other. You don’t have to give energy to one part of the chain and then wait for it to get to the wheel. As soon as you pedal the wheel moves. Only in this case if you are pedalling one way and then the other, the bike wouldn’t go anywhere. But there would be constant energy transfer between you and the wheel. This is what is happening in an ac supply. The electrons go back and forth and as they do this in your appliances they transfer the energy to the appliance.
If you want to go slightly more advanced, what matters is the power transfer not just the current, which goes like the current squared. Current squared is always positive so there is always energy transferred if current is flowing. Over one cycle the current created by a generator, which is like a sin wave, has an average of zero, while the current-squared has an average of 1/2 of the peak current in the cycle. This leads to the idea of the mains voltage, which is actually 1/(root2) times the peak voltage. It is the equivalent dc supply that would convert the same energy each second.
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