Final answer:
To measure and define a current, you need the electric current and cross-sectional area. The ampere is the unit for current, measuring the charge flow per second. An ammeter is used in series with the circuit or component to measure the current.
Step-by-step explanation:
To measure and define a current, you need to know the water's speed and cross-sectional area. In the context of electricity, the analogous variables needed are the electric current and the cross-sectional area of the wires through which the current is flowing. The standard unit of electric current is the ampere, and one ampere is one coulomb of charge moving in one second (C·s⁻¹).
In electrical circuits, to measure the total current, an ammeter is placed in series with the circuit. To measure the current through a specific resistor, such as R1, R2, or R3, the ammeter would replace the wire section immediately preceding or following that resistor. This ensures the accurate measurement of the current flowing through the selected component. In circuits where the current is the same throughout, like that shown in Figure 19.9, any point can be chosen to measure the total circuit current.
The concept of current applies to various types of circuits, including those that are analogized to water flowing through pipes. In these scenarios, the total current in a circuit and current through specific components, like resistors, can be represented by different models but the measurement method using an ammeter remains consistent.