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A cell supplies a current of 0.9 A through a 2 Ohm resistor and 0.3 A through a 7 Ohm resistor What is the internal resistance of the cell?
Sreejamohan
We picture the cell as its Thevenin equivalent ... an ideal voltage source ' V ' in series with an internal resistance ' R '. You're asking for the value of ' R '.
With the 2-ohm resistor: I = V / (R + 2) = 0.9 ===> V = 0.9 (R + 2)
With the 7-ohm resistor: I = V / (R + 7) = 0.3 ===> V = 0.3 (R + 7)
Since ' V ' is constant, 0.9 (R + 2) = 0.3 (R + 7)
0.9R + 1.8 = 0.3R + 2.1
0.6R = 0.3
R = 0.3 / 0.6 = 1/2 ohm
We can confirm this result either by substitution or by measuring the cell open-circuitly. It should be 2.25 volts.
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∙ 14y ago
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