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A 12V battery has an internal resistance of $2.0\Omega$. A load of variable resistance is connected across the battery and adjusted to have resistance equal to that of the internal resistance of the battery. Find the power dissipated.

The equation for internal resistance is $$\mathcal{E}=I(R+r)$$ where $R$ is the resistance in the circuit, $r$ is the internal resistance of the battery and $I$ is the current. We know that $\mathcal{E}=12V$. The sum of internal and external resistance would be $4 \Omega$, seeing as the variable resistor is set to $2 \Omega$, which adds to the already exisiting $2 \Omega$ from the battery.

The equation for power dissipated is

$$P=I^2R=\frac{V^2}{R}$$

So I substitued the value for $V$ and the value for $R$ to get

$$P=\frac{12^2}{4}=36$$

This is, however, wrong. Any ideas as to why?

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  • $\begingroup$ If $36\,\rm W$ is wrong then the setter of the questioned possibly wanted the power dissipated just in the external resistor. $\endgroup$
    – Farcher
    Dec 8, 2018 at 15:52
  • $\begingroup$ @Farcher That would make sense, but then wouldn't the power be $\frac{12^2}{2}$? The OP says the answer is $18\ \rm W$ in another comment $\endgroup$ Dec 8, 2018 at 16:46
  • $\begingroup$ @AaronStevens No, it would be a half of the total power dissipated ie the total power is shared between the internal resistor and the external resistor. $\endgroup$
    – Farcher
    Dec 8, 2018 at 16:49
  • $\begingroup$ @Farcher But if you have a resistor $R$ with a potential drop of $V$ across it, then isn't the power dissipated in the resistor $\frac{V^2}{R}$, regardless of what else is going on in the circuit? Is the potential drop across the external resistor not $12\ \rm V$? $\endgroup$ Dec 8, 2018 at 16:56
  • $\begingroup$ @AaronStevens There is 6V across each of the resistors to make the 12 V generated by the battery. $\endgroup$
    – Farcher
    Dec 8, 2018 at 17:00

4 Answers 4

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You aren't applying your power equation correctly. You can't just plug in any voltage, current, or resistance.

The equation $P=IV$ gives you the power delivered or dissipated by a single part of the circuit with a potential drop $V$ across it and current $I$ flowing through it.

If you want the total power dissipated in the circuit, you must apply this to each element separately. If just the outer resistance, then only consider this resistor. I will leave this to you.

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  • $\begingroup$ So would I need to add the power dissipated in the battery to the power disspiated in the circuit? If so, what's the current across the circuit? $\endgroup$
    – Pablo
    Dec 8, 2018 at 15:08
  • $\begingroup$ @Pablo Use ohms law on the variable resistor $\endgroup$ Dec 8, 2018 at 15:27
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Why do you substitute $R$ with 4? $R$ "is the resistance in the circuit", so it equals $2\Omega$, not $4\Omega$. There is another question though: do they need total dissipation or just dissipation in $R$?

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Your answer looks right to me as long as you want the total power dissipated (including in the battery).

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  • $\begingroup$ What is the given answer? 18 W? $\endgroup$
    – Ben51
    Dec 8, 2018 at 14:56
  • $\begingroup$ Yes, but $\frac{12^2}{4}=36$. How did you get to $18$? $\endgroup$
    – Pablo
    Dec 8, 2018 at 15:09
  • $\begingroup$ 18 W dissipated in the resistor, 18 in the battery $\endgroup$
    – Ben51
    Dec 8, 2018 at 17:01
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The question is a multiple choice and one of the answers state 'power dissipated in the battery is 18W'. This (C) is the correct answer as the total resistence in series is 4 ohms, the current through the load is 3A not 6A and the PD across the load is 6V.

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  • $\begingroup$ In addition to the advice at How to Ask, please search Physics Meta for guidance about how to ask homework-like questions; this one would be closed without an answer if it were posted in its current form. $\endgroup$
    – rob
    Oct 20, 2020 at 19:43

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