Topic 11.2 Simple Circuits: interpret circuit schematics and explain the role of emf, the complete circuit and the conventions for open and short circuits.

What this topic is asking

The College Board (Topic 11.2) wants you to interpret circuit schematics, explain the role of electromotive force (emf) as the energy per charge a source supplies, and understand complete, open and short circuits, including the effect of internal resistance on terminal voltage.

Schematics and the complete circuit

A circuit only carries current when the loop is unbroken: charge must have a continuous path back to the source. A schematic is the map of that path, stripped of physical detail down to which components connect to which. The first step in any circuit problem is to read the schematic, identify the loop, and see which components are in the current's path.

Electromotive force and terminal voltage

Despite its name, emf is not a force; it is energy per charge, the same kind of quantity as voltage. It measures the "push" a source gives each coulomb. Internal resistance is the small resistance inside the source itself, and some of the emf is spent driving current through it, which is why the terminal voltage drops below the emf as the current rises. This is why a car's headlights dim when the starter motor (a heavy load drawing large current) cranks: the large current causes a big $I r$ drop inside the battery.

Open and short circuits

Two extreme cases are tested directly. An open circuit has a break in the loop (a switch off, a cut wire), so the resistance is effectively infinite and no current flows. A short circuit provides a path of near-zero resistance across the source, so by $I = V/R$ a very large current flows, limited mainly by the internal resistance; this can overheat wires and is what fuses protect against. The strategic role of this topic is to set up the analysis tools for the rest of the unit: you must read a schematic, recognize the source's emf as the energy supply, account for internal resistance when present, and identify whether the circuit is complete, open or shorted. With the current found from $I = \mathcal{E}/(R + r)$, the resistance rules (Topic 11.3), power (Topic 11.4) and Kirchhoff's laws (Topics 11.6, 11.7) all follow.

Try this

Q1. State what emf measures and give its unit. [2 points]

• Cue. The energy supplied per unit charge by a source, measured in volts.

Q2. State what happens to the current in a circuit if the loop is broken (open circuit). [1 point]

• Cue. The current falls to zero (no complete path for charge to flow).