A battery of negligible internal resistance powers a decorative string containing a single identical light bulb rated for 3.0 V operation. A normally open push-button switch can connect a second, identical bulb directly across the first bulb (in parallel) when the button is pressed.
Initially the switch is open and the circuit has been operating long enough for conditions to be steady.
(a) When the push-button is pressed, how does the equivalent resistance of the portion of the circuit that includes the bulb(s) change? State whether it increases, decreases, or remains the same and describe the reasoning that leads to your answer.
(b) Based on your answer to part (a), state and justify the change, if any, in the magnitude of the current that leaves the battery at the instant just after the switch is pressed.
(c) Compare the brightness of the original bulb before and after the switch is pressed. Provide a qualitative explanation that makes explicit reference to the current through that bulb and to the electrical power it dissipates.
(d) Suppose instead the battery has a small but non-negligible internal resistance. Discuss qualitatively how the presence of this internal resistance would alter the magnitude of the changes you predicted in parts (b) and (c).