(a) In the figure what value must R have if the current in the circuit is to be 0.79 mA? Take E1 = 2.1 V, E2 = 4.7 V, and r1 = 12 = 3.9 0. (b) What is the rate at which thermal energy appears in R? (a) Number 999.2 Units V (b) Number i 1.69e-9 Units W