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Electricity flows from area of highest electric potential to lowest, I get that. What I don't get is why any battery charger would use two prongs on an outlet (or a few other items of similar purpose). Hypothetically, it's a below fully charged state that's filling up. It should just have an intake and then once it's full, flip that off.

I've often been taught to envision electricity flow similar to water flow, flowing downhill from high potential to low potential... but when filling a bucket, we don't put holes in the bottom of buckets, so why do we need outflow for battery chargers?

lilHar
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    Current flows in a loop. No loop -> no current. – brhans Aug 21 '19 at 19:51
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    For the same reason batteries have two prongs, one prong has voltage with respect to the other prong. – Justme Aug 21 '19 at 19:51
  • @brhans I know that's a common design, but lightning doesn't flow in a loop. So we know it -can- do a direct point to point. – lilHar Aug 21 '19 at 19:52
  • Oh wait... lightning is static-generated direct current... it's due to a limitation of alternating current, isn't it? – lilHar Aug 21 '19 at 19:53
  • Wait... no... I take that back, downed electrical lines (AC) will definitely pump current through a person and into the ground.... so... still wondering why we can't just dump it into a battery in a similar fashion. – lilHar Aug 21 '19 at 19:57
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    Even lightning involves current loops. The lightning arc is the "forward" current, but the return current is discharge of the capacitor formed between the clouds and the earth. Likewise, downed power lines can send current to ground because some transformer or generator somewhere is connected to ground for the return current to flow. – Evan Aug 21 '19 at 20:05
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    @Evan Indeed. It would be like mistaking an animal as an infinite source of...uhhh...fluid because you did not notice the fact that it takes a drink every now and then. – DKNguyen Aug 21 '19 at 20:13
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    @Evan Is that right? My understanding is that lightning comes from clouds which have an electrostatic imbalance - if all the electrons did a round trip, the cloud would still be charged after a lightning strike. As another example, you can have two charged capacitor plates in a vacuum, and discharge them by touching both - the current only flows one way from the higher potential plate to the lower. You need a loop for a sustained current, but a one-time discharge doesn't require a loop. See https://electronics.stackexchange.com/questions/303609/why-can-current-only-flow-in-loops – Nuclear Hoagie Aug 21 '19 at 20:28
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    You ARE aware that batteries don't store electricity. Right? They store chemical fuel. The number of electrons inside a "charged" battery is the same as in a "discharged" one. Batteries are charged with energy, not with charge. Think of them as being like wind-up clocks which are wound up by using leather belts and pulleys. The leather belt passes **through** the clock, and the clock-spring stores energy, not leather. The path for charge is through the battery, and none builds up inside. (And, two different things flow in electric circuits: slow electrons, and fast EM energy.) – wbeaty Aug 21 '19 at 20:37
  • @wbeaty One could ask about plates capacitor... – Eugene Sh. Aug 21 '19 at 21:06
  • @wbeaty I was only partially aware of that. I thought it was chemicals that were receptive towards more free electrons or something. That's actually pretty informative. – lilHar Aug 21 '19 at 22:31
  • @liljoshu batteries are fuel cells. But with batteries, the fuel is contained onboard. So, imagine that batteries are really just Pt-electrode fuel cells with internal H2 and O2 tanks. When we "charge" the battery, we are splitting H2O to create hydrogen/oxygen gas which fills the internal tanks. During "discharge" these gases react, producing an electron-pumping effect (and creating H2O byproduct.) Note that during electrolysis, there is electric current through the liquid between the two electrodes. Zinc battery? It's electroplating, not hydrogen release, since zinc metal is the fuel. – wbeaty Aug 22 '19 at 01:35
  • @NuclearWang -- it depends on what you consider current. See my answer to the "why does current flow in loops" question you referenced for a detailed explanation. There is a "displacement current" related to the change in electric field after the lightning discharge. If you only include free current then yes lightning discharge is temporarily unidirectional, but lots of other things stop making sense (like capacitors) if you don't include displacement current. – Evan Aug 22 '19 at 03:44
  • I am seeing this question for 9999th time in SE – Mitu Raj Aug 27 '19 at 12:12
  • Water flows down the hill, but how did it get up the hill? There is your loop. And water is an analogy, not reality. – StainlessSteelRat Aug 28 '19 at 00:09

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The water/hydraulic analogy for electricity is OK but honestly not that great given how popular it is and this is one of the big areas where it fails.

When you charge a battery you aren't filling it up with charge like a bucket. The battery stays electrically net neutral. By charging it you are driving a chemical reaction between ions in the electrolyte and the electrodes. If you had a capacitor instead of a battery you would be building up equal and opposite charges on the plates, but the net charge would be zero.

Current always flows in a loop (Kirchhoff's law) so if you want to think about water pressure consider a closed hydraulic system where you have a pump pushing water out at high pressure and receiving it back at low pressure. Loads can extract work from the fluid due to the pressure difference but they don't store material quantities.

Evan
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Using the water and bucket analogy: it's more like the buckets in a water wheel, where the buckets around the water wheel empty as the wheel turns, but the wheel lifts a weight up higher as it turns, thus storing potential the energy provided by the running or falling water. If the buckets didn't empty, the water wheel would quickly stop turning.

In a battery cell, the charging electrons flow through, but boost up the energy stored in the chemical potential of the battery as they get "pushed" through by a voltage differential.

hotpaw2
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...taught to envision electricity flow similar to water flow, flowing downhill from high potential to low potential...

At the voltages typical of electronics circuits that we might encounter (i.e., tens of volts, even the low hundreds of volts) it's better to envision electricity as flowing in pipes. Electrical current is mostly carried by electrons, which are strongly attracted to the protons in atomic nuclei. You can't rip very many electrons off of a piece of wire before they get lonely and want to jump back.

All analogies break down at some point, and the "electric current is like water in a pipe" analogy breaks down at the point where water will drip out of a pipe -- electrons won't, in general, drip off of a wire.

...but lightning doesn't flow in a loop...

I just did some research on this (strictly Wikipedia, because I'm lazy), and I can't find a reference that directly contradicts this. However, it must. Why? Because electrostatic attraction is hugely more powerful than gravity. So something must equalize the charge between the cloud and the ground after a lightning strike, or there's some mechanism that carries charge from the cloud to the ground before the strike.

The lighting strike may not flow in a loop -- but it's the consequence of a capacitor getting charged up, and that capacitor will discharge, eventually.

TimWescott
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