1. The problem statement, all variables and given/known data To a source of current, one by one are added 2 resistances, first R1=1ohm, then R2=4ohm. In both cases, in the same period of time (t=t1=t2), the resistors give the same amount of heat (Q=Q1=Q2). Determine the internal resistance ("r" small r) of the electric source. 2. Relevant equations I=V/R 3. The attempt at a solution I found somewhere a formula which states: r=(V2-V1)/(I1-I2), so including the basic R=V/I into it would give: r=R2-R1, and the result will be 3ohm? looks too simple for an engineering problem.