Practical circuits — common mistakes

Pay close attention to keywords like 'open circuit' (no current, voltmeter reads e.m.f.) versus 'closed circuit' (current flows, voltmeter reads terminal p.d.). Also, remember to read carefully what voltage is being asked for: total, across a specific component, or lost volts!

When current flows, some voltage is lost internally across the battery's own internal resistance. A voltmeter connected across the external terminals will measure this reduced 'terminal potential difference', not the full EMF, as some energy is dissipated as heat within the source itself.

Kirchhoff's laws provide a systematic way to set up equations for current and voltage in complex circuits. The First Law ensures charge conservation at junctions, while the Second Law ensures energy conservation around loops, allowing us to solve for unknown currents and voltages even in intricate networks.

LDRs and thermistors are variable resistors whose resistance changes significantly with light intensity or temperature, respectively. By incorporating them into a potential divider, their resistance changes cause a corresponding change in the output voltage, enabling the circuit to detect and respond to environmental changes, forming the basis of many automatic control systems.

Matching the load resistance to the internal resistance (R=r) is desirable when the goal is to transfer the maximum possible power to the load, for example in audio systems connecting an amplifier to a speaker. However, this is not always the goal, as it results in low efficiency (50%), with half the power being wasted in the source. For power supplies, a much lower internal resistance is preferred to maximize efficiency and maintain a stable terminal voltage.