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# Experiment 18 - Formula Internal Resistance

 For the calculation of the internal resistance $$R_i$$ you used two measured values... the voltage of the battery without load $$U_0$$ and the current flowing in the load circuit $$I$$. Moreover you know the resistance of the load resistor $$R_L$$... Illustration of the voltage and current in the load circuit.

According to the Kirchhoff's Voltage Law (KVL) in the load circuit... $$U_0 = U_i + U_L$$ and according to the Ohm's Law you can replace $$U_i$$ and $$U_L$$... $$U_0 = R_iI_i + R_LI_L.$$ Since this is a simple circuit with no nodes, furthermore the Kirchhoff's Current Law (KCL) applies... $$I = I_i = I_L$$ and so you get... $$U_0 = R_iI + R_LI.$$ Rewriting the formula you get an expression for the internal resistance $$R_i$$... $$R_i = {U_0 \over I} - R_L.$$ All values ​​on the right side are known to you and therefore you can use this formula to calculate the internal resistance of the battery.