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NOTE: I'm not requesting a solution to this problem; rather, I'm seeking clarification on an objection that arose throughout the process of solving it.

In order to refresh my understanding of AC and waves, I was attempting to answer some conceptual problems. Then, I encounter this problem :

It takes enter image description here hours for a direct current I0 to charge a storage battery. How long will it take to charge such a battery from the mains using a half-wave rectifier, if the effective current value is also equal to I0? (effective current = Rms value of current)

When I looked for a solution, I got this one:

[ https://www.youtube.com/watch?v=uTR4fQOXWC4 ]

enter image description here

enter image description here

The speaker in the video says:

'so when we say 'To charge a Battery' , "BATTERY HOLDS CHARGE" , so we will equate the Total charge stored in the battery to solve the problem'

The phrase "Battery holds charge" was utilised in the solution key to answer this question.

But What we should be equating in both cases is the energy needed/consumed to charge the battery rather than the charge stored, according to what I was taught. A battery stores energy in the form of EMF rather than a physical charge.

How does the "charge stored" idea work, and can we replace it with the "energy consumed" idea? What is the concept behind it ?

A response that could explain all this to me and clear my thoughts would be greatly appreciated !!!

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  • $\begingroup$ You might want to have a look over at the chemistry stack exchange and answers like mine here: chemistry.stackexchange.com/a/128269/79678. The key concepts are ‘free energy’ and ‘redox’ (oxidation-reduction) chemical reactions. $\endgroup$
    – Ed V
    Oct 25, 2022 at 15:13

2 Answers 2

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Energy = charge * voltage difference.

For constant voltage, you want some number of charges to pass through the given voltage difference.

It's true that a battery does not hold "charges" and most batteries end up with 0 net charge.

Because Energy = charge * voltage, you can calculate the total number of charges that pass through the battery to fill it up, and hence the amount of time you need to apply a current to cause enough charges to pass through the battery to fill it up.

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When you "charge" a battery, that doesn't mean that the battery gains an electrostatic charge: overall, the battery remains neutral. What you do is to take electrons from the positive terminal and push them into the negative terminal. This has the effect of taking them away from a substance with a high electron affinity and giving them to a substance with low electron affinity. But there's no buildup of electrostatic charge internal to the battery: ions move through the electrolyte and (almost) completely neutralize it.

The difference between the electron affinities is what produces the EMF, V. The charge "stored" in the battery, Q, represents the amount of charge you can transfer around a circuit before you exhaust the ability of one of the electrodes to accept or release electrons. The energy stored is QV (to a first approximation).

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