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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?

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closed as too broad by Eugene Sh., winny, Oleg Mazurov, Mitu Raj, StainlessSteelRat Aug 28 at 0:09

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    \$\begingroup\$ Current flows in a loop. No loop -> no current. \$\endgroup\$ – brhans Aug 21 at 19:51
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    \$\begingroup\$ For the same reason batteries have two prongs, one prong has voltage with respect to the other prong. \$\endgroup\$ – Justme Aug 21 at 19:51
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    \$\begingroup\$ 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. \$\endgroup\$ – Evan Aug 21 at 20:05
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    \$\begingroup\$ @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. \$\endgroup\$ – DKNguyen Aug 21 at 20:13
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    \$\begingroup\$ 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.) \$\endgroup\$ – wbeaty Aug 21 at 20:37
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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.

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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.

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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.

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