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In Physics class, we were building parallel circuits, and as more lights were attached in parallel, they got brighter (as more power was being provided to the lights, and the resistance decreases). So, when I charge a phone, why doesn't the battery charge faster when it is in use (eg. more devices powered on within the phone), than when it is not in use?
Clarification: All lights at once became brighter

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""and as more lights were attached in parallel, they got brighter "" each lamp became brighter or all the lamps in toto? –  Georg May 28 '11 at 17:22
    
All the lights at once became brighter –  IDWMaster May 28 '11 at 17:23
    
Either this is an error eg. due to variing ambient illumination, or the power circuit has some strange output characterisics. Your idea re charging some accumulator with or without current drain by a phone is wrong, anyhow. Such charging adapters deliver a more or less conststant current, this is either fed to the accumulater alone or split, the latter causing more time to charge. –  Georg May 28 '11 at 17:36
    
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2 Answers

up vote 4 down vote accepted

When you add components in parallel, the voltage across each component is the same. Thus, in a simple AC circuit, when you add an extra light bulb in parallel, the brightness of the light bulbs should stay the same, not increase.

This might seem wrong because there is less total resistance in the parallel circuit, and hence more total current, but the current is divided between the different branches. The result is that the brightness of a single light bulb is constant. The total brightness of the light bulbs combined increases. If anything, there might be some nominal decrease in brightness per light bulb due to the internal resistance of the battery - when you add more light bulbs in parallel, their effective resistance decreases, so the battery's internal resistance becomes more important and the bulbs are slightly dimmer. This should be a very small effect for a realistic circuit.

The analogy to the phone is a non-sequitur. Presumably it charges slower when in use because the phone is powered by the battery at all times, so using it drains the charge. I don't know for sure this is how it works, but absent more detailed information about the circuitry of the phone, knowing how a simple DC circuit with light bulbs in parallel works simply does not speak to the workings of the phone.

Edit: Two people have said that the charger supplies a fixed current, not a fixed voltage, and that this is the real explanation for why the phone charges slower when you are using it.

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Assuming your phone is typical, the charger can only generate a limited current. When the phone is off, all the current from the charger goes to the battery. When the phone is on, the current from the charger goes partly to charging the battery and partly to running the phone. Hence the battery takes longer to charge.

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Did You read the thread? I mentioned that exactly when the thread was some minutes old. –  Georg May 29 '11 at 10:16
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@georg Since you gave the information in a comment, it's appropriate for John to reiterate that it is, in fact, a legitimate answer to the question, not just a comment on the question. –  Mark Eichenlaub May 29 '11 at 14:35
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