A thin metallic ribbon is incorporated into a simple circuit consisting of an ideal 9.0 V battery and a fixed external resistor Rext. The circuit is initially at room temperature (25 °C). After the switch is closed, the ribbon’s temperature rises because of the current passing through it. Assume the temperature of the external resistor and the battery remains effectively constant.
(a) Describe qualitatively how rising temperature in the metallic ribbon affects the motion of free electrons within the ribbon.
(b) Explain why, with a constant 9.0 V source, the current measured in the circuit gradually decreases as the ribbon warms.
(c) Suppose the battery were replaced by an ideal current source that keeps the current through the circuit constant as the ribbon warms. Describe what would happen to the potential difference across the ribbon in that situation and explain your reasoning.
(d) Consider a second ribbon made from a semiconducting material whose resistivity decreases with temperature. Contrast the current–time behavior of a circuit containing this ribbon with the behavior described in part (b).