Electric Voltage

For current to flow through a circuit, there must be a "cause"—a kind of force that drives the charges to move. This cause is electric voltage.

Voltage measures the difference in electrical potential between two points in a circuit. It can be compared to pressure in a water circuit:

• if the pressure is high, the water is pushed strongly through the pipes;
• if the pressure is low, the water flows less easily.

Similarly, the higher the voltage between two points, the more it tends to cause a current to flow through a component connected between those two points.

Symbol and unit

Voltage is generally denoted by \(U\). Its unit in the International System of Units is the volt, denoted by \(\mathrm{V}\).

Examples of voltage values:

• approximately \(1.5\ \mathrm{V}\) for an alkaline battery;
• \(5\ \mathrm{V}\) for many electronic circuits or USB ports;
• approximately \(12\ \mathrm{V}\) for a car battery;
• approximately \(230\ \mathrm{V}\) for the household electrical grid in Europe;
• several hundred volts for certain strings of photovoltaic modules.

Voltage = difference between two points

Voltage is not defined "at a single point," but always between two points in a circuit.

• For example, we speak of "voltage across the terminals of a resistor";
• or "voltage between the positive and negative terminals of a battery".

A reference point called ground or \(0\ \mathrm{V}\) is often chosen.

The voltage at other points is then measured relative to this reference point.

Direct Current and Alternating Current (Overview)

Without going into detail (which will follow later):

• A direct current (DC) maintains the same sign and remains more or less constant over time (example: a battery, a photovoltaic panel).
• An alternating voltage (AC) changes sign periodically (example: the \(230\ \mathrm{V}\) from the grid, which alternates 50 times per second).