1 \begin{problem*}{Q19.7} % resistor networks
2 If two identical resistors are connected in series to a battery, does
3 the battery have to supply more power or less power than when only one
4 of the resistors is connected? Explain.
8 We know that the power provided by the battery is given by
10 so the power supplied increases if the current $I$ increases (because $V$ remains constant for batteries).
12 From Kirchoff's loop rule, we know the voltage drop across the
13 resistors is the same as the voltage gain across the battery.
15 We also know that the voltage across the resistors relates to the current via Ohm's law
17 Finally, we know that the effective resistance of two identical resistors in parallel is given by
18 $$R_2 = R_1 + R_1 = 2R_1$$
20 Putting these together in the case of a single resistor, we find a current of
21 $$I_1 = \frac{V_R}{R_1} = \frac{V_b}{R_1}$$
22 and in the case of the two resistors in series
23 $$I_2 = \frac{V_R}{R_2} = \frac{V_b}{2R_1} = \frac{I_1}{2}$$
24 So with two resistors in series, we have less current and need less power.