Theme B · Particulate Nature of Matter · SL + HL · 6 hours
B.5 Current and Circuits
How do charged particles flow through materials, and what are the consequences of resistance?
Current is the flow of charge carriers — electrons in metals, ions in electrolytes, and holes in semiconductors. The rate of flow, measured in amperes, is charge per unit time. Potential difference is the energy per unit charge available to drive that flow; resistance is the opposition to it. Ohm's law — V = IR — holds for metallic conductors at constant temperature, but not for all materials or all conditions.
Internal resistance is the part of Ohm's law that textbooks gloss over but real circuits cannot ignore. Every cell has an internal resistance r; when current flows, some of the EMF is "used up" driving current through the cell itself, so the terminal voltage is always less than the EMF. Understanding this is essential for analysing real circuits, where the battery is not an ideal voltage source.
A battery of EMF 12 V and internal resistance 0.8 Ω is connected to an external resistor of 3.2 Ω. Calculate the current, the terminal voltage of the battery, and the power dissipated in the external resistor. What fraction of the total power is wasted in the internal resistance?