ammeters

Meaning: An ammeter measures electric current, the charge passing a point per unit time. It is placed in series with the element of interest so that all of that branch's current flows through it. An ideal ammeter has zero resistance, so it causes no voltage drop and does not change the current. A real one has a small resistance and lowers the current slightly.

Equation: I = ΔQ/Δt, where I is current in amperes (A), ΔQ is charge in coulombs (C), and Δt is time in seconds (s).

On the MCAT: Circuit diagrams show a circle marked A. Questions ask where to place it, or what it reads. Treat it as a plain wire, then use V = IR and the series and parallel rules to find the current.

Common confusion: Students swap it with the voltmeter. A voltmeter goes in parallel and ideally has very high resistance. An ammeter goes in series and ideally has very low resistance. Placing an ammeter in parallel across a component gives the current a near-zero-resistance path and shorts that component.