Electrical potential is measured in units called Volts (V). Voltage is the amount of work (energy, measured in Joules) necessary to move one unit of charge (Coulombs) through an electric field a fixed distance, divided by the unit charge.
In a circuit diagram, we assume the electric field E between nodes is uniform and scales with distance, so we do not draw it explicitly. Instead, we consider the voltage between nodes across an element (like a resistor or capacitor). The electric field and physical distances are abstracted away.
The rate of flow of charge is current (I), measured in Amperes (A). Wires in a circuit connect nodes, and elements sit between nodes. The animation shows charge carriers moving along the wire; their speed is proportional to the current.
A wire (node) has the same voltage throughout — it is an equipotential. In this example, the entire top node from the positive terminal of the source to the left side of the resistor is at the same higher potential (colored red), and the entire return path from the right side of the resistor down to ground and back to the source’s negative terminal is at the same lower potential (colored blue).
Voltage only has meaning as a difference between two places. For example, point A might be at +10 V with respect to point B. To make this easier, we include a common reference point called Ground and measure voltages with respect to it.
Examples: the potential between the lugs on a car battery is about 12 V; between the two rectangular holes in a wall outlet about 120 V; between clouds during lightning can be millions of Volts. An electron at 2 V (with respect to ground) would require 1 electron‑Volt (eV) of energy to move to 3 V: the potential difference is 1 V.