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Voltage and Current Basics

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Quick
Voltage is a difference of potential, represented by the symbol v.
Current is a flow of electric charge, represented by the symbol i.


Equations

(Eq1)     v = dw dq	Equation for voltage
(Eq2)     i = dq dt	Equation for current

Nomenclature

v	voltage in volts
w	energy in joules
q	charge in coulombs
i	current in amperes
q	charge in coulombs
t	time in seconds

Details

The concept of electric charge is the basis for describing all electrical phenomena. Some important characteristics of electric charge are:
• The charge is bipolar, meaning that the electrical effects are described in terms of positive and negative charges.
• The electric charge exists in discrete quantities, which are integral multiples of the electronic charge, 1.6022 × 10⁻¹⁹ C.
• Electrical effects are attributed to both the separation of charge and charges in motion.

In circuit theory, the separation of charge creates an electric force (voltage), and the motion of charge creates an electric fluid (current).

The concepts of voltage and current are useful from an engineering point of view because they can be expressed quantitatively. Whenever positive and negative charges are separated, energy is expended. Voltage is expressed as the energy per unit charge created by the separation between positive and negative charges:

(Eq1)

v = dw dq

Electric current, or the rate of charge flow can be expressed as:

(Eq2)

i = dq dt

Electric Current

Electrical and electronic devices work because of an electric current.

An electric current is a flow of electric charge. The electric charge usually consists of negatively charged electrons. However, in semiconductors, there are also positive charge carriers called holes.

The following is a list of the most common ways to generate current:
• Magnetically - The induction of electrons in a wire rotating within a magnetic field. An example of this would be generators turned by water, wind, or steam, or the fan belt in a car.
• Chemically - Involving electrochemical generation of electrons by reactions between chemicals and electrodes (batteries).
• Photovoltaic generation of electrons - When light strikes semiconductor crystals (solar cells).

Less common methods to generate electric current include the following:
• Thermal generation - using temperature differences between thermocouple junctions. Thermal generation is used in generators on spacecraft that are fueled by radioactive material.
• Electrochemical reaction - Occuring between hydrogen, oxygen, and electrodes (fuel cells).
• Piezoelectrical - Involving mechanical deformation of piezoelectric substances. For example, piezoelectric material in the heels of shoes power LEDs that light up when walking.

A battery is a source that provides a potential difference to a circuit that will enable a current to flow. In the case of a battery, electrons are the electric charge, and they flow from the terminal that has excess number of electrons to the terminal that has a deficiency of electrons. This flow takes place in any complete circuit that is connected to battery terminals. It is this difference of charge that creates the potential difference in the battery. The electrons are trying to balance the difference.

Because electrons have a negative charge, they actually flow from the negative terminal and return to the positive terminal. This direction of flow is called electron flow. Most texts, however, use current flow, which is in the opposite direction. It is referred to as conventional current flow or simply current flow.

This figure shows direction of current flow in a circuit with respect to the voltage source.

The battery symbol indicates that a difference potential, also called voltage, is present and is being supplied to the circuit. The potential difference causes current to flow if there is a complete circuit present, as in the figure above. If the battery is reversed, current will flow in the opposite direction.