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Basic Circuit Concepts The figure to the right shows the basic type of electrical circuit, in the form of a block diagram. It consists of a source of electrical energy, some sort of load to make use of that energy, and electrical conductors connecting the source and the load.

The electrical source has two terminals, designated positive (+) and negative (-). As long as there is an unbroken connection from source to load and back again as shown here, electrons will be pushed from the negative terminal of the source, through the load, and then back to the positive terminal of the source. The arrows show the direction of electron current flow through this circuit. Because the electrons are always moving in the same direction through the circuit, their motion is known as a direct current (DC).

The source can be any source of electrical energy. In practice, there are three general possibilities: it can be a battery, an electrical generator, or some sort of electronic power supply.

The load is any device or circuit powered by electricity. It can be as simple as a light bulb or as complex as a modern high-speed computer.

The electricity provided by the source has two basic characteristics, called voltage and current. These are defined as follows:

Voltage

The electrical "pressure" that causes free electrons to travel through an electrical circuit. Also known as electromotive force (emf). It is measured in volts, named after Italian physicist Alessandro Volta.

Current

The amount of electrical charge (the number of free electrons) moving past a given point in an electrical circuit per unit of time. Current is measured in amperes, named after the French mathematician André-Marie Ampére.

The load, in turn, has a characteristic called resistance. By definition:

Resistance

That characteristic of a medium which opposes the flow of electrical current through itself. Resistance is measured in ohms, named after German physicist Georg Simon Ohm.

The relationship between voltage, current, and resistance in an electrical circuit is fundamental to the operation of any circuit or device. Verbally, the amount of current flowing through a circuit is directly proportional to the applied voltage and inversely proportional to the circuit resistance. By explicit definition, one volt of electrical pressure can push one ampere of current through one ohm of resistance. Two volts can either push one ampere through a resistance of two ohms, or can push two amperes through one ohm. Mathematically,

E = I × R,

where
E = The applied voltage, or EMF
I = The circuit current
R = The resistance in the circuit

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