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In an electrical circuit, voltage is measured in volts (V) and current in amperes (A). The magnitude of a current is the amount of electrical charges which pass through the electrical circuit in one second. Electrical charge is measured in coulombs (C).
The output (P) of an electric circuit is the product of its voltage (U) and current (I).
P = U ∑ I
Electrical resistance, which a conductor or another part of the circuit creates by resisting, in some way, the flow of electrical current, is measured in ohms. The resistance of a conductor is the quotient of voltage and current and is labelled with the symbol R. Within a closed electronic circuit, this relation between voltage (U) and current (I) is referred to as Ohmís law.
The magnitude of a current is directly proportion to the size of its voltage. A good conductor offers only a small amount of resistance and, in such conductors, a small amount of voltage can generate a large amount of current.
The resistance (R) of some wire depends on the type of material it is made from and its shape. A thin wire creates a resistance against the electrical current passing through it indirectly proportional to the size of its cross-section and directly proportional to its length.
It is often necessary to use components within an electrical circuit which limit the size of the current passing through it. Resistors, which are electrical components resisting the flow of electrical current, are used for this purpose.
A good example of the use of a resistor is in regulating the volume in a stereo receiver, where an adjustable/sliding resistor, also called potentiometer, is used in such circuits. By sliding the potentiometer, the size of the current in an electrical circuit changes, where increasing or decreasing the resistance increases or decreases the intensity of the sound coming out of the speakers.
Electrical circuits are classified as being direct and alternating where, in a direct current, a constant current flows in only one direction.
An alternating current periodically changes its direction and size, for which reason its quantity is given in terms of its root-mean-square (double its highest value). The root-mean-square (RMS) of an alternating current is the direct current value which would create the same heat effect as a given amount of alternating current (the RMS of a sinus waveís alternating current is the quotient of its maximum current (Imax) and ÷ 2 ). Because some often-used components such as electrical coils and capacitors store energy in a magnetic or electric field, after which they transfer it into the circuit with certain time-phase delay, the current and voltage in alternating voltage are not "in phase", which means that the voltage and current do not reach their maximum or minimum values at the same time. This effect creates so-called impedance or reactive power.
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