Flow & Potential

 Electric Current

In a circuit, energy is transferred via flow of charged particles. In a copper wire, the electrons at the outer shell of atoms are free to move. Even with no external power connected, these free electrons do move around. However since their movement is random, no net directional flow is realized. Once a battery is connected across the same wire, these electrons drift from one end to the other end. (In practice, we never connect a wire across a battery, as it discharges it quickly and heats up the wire. We always connect a load to the power source via wires). 

The definition of electric current is: The net flow of electric charge carriers past a certain point in an electric circuit in a given period of time. The question might arise here. Are the electric charge and current the same? the answer is no. The unit of electric charge is Coulomb (named after Charles A. Coulomb) and the unit of current is Ampere. Even though they represent two quantities but in fact they are closely related. 

One Coulomb is represented by the number of electrons in a circuit divided by a fix number of 6.24x10^18 (6.24 times ten to the power of 18). When we talk about current we need to have an external power (battery, or a voltage source) in a circuit. In case of coulomb, we don't even need a power source to give meaning to coulumb. For example, imagine a piece of aluminum picture frame with 12.48X10^18 free electrons. it can be stated that the frame  represents 2 coulomb of electric charge (12.48X10^18 divided by 6.24x10^18) .

Now the coulomb is understood, we define one ampere as the rate of flow of electric charge when one coulumb of charge carriers passes a certain point in an electric circuit in one second. An analogy: speed of a car in kilometer per hour; ampere is coulomb per second.

Symbol representing current is often capital letter I, and the symbol A is designated for ampere. 

An electric conductor must have enough free electrons per unit volume. Copper as an inexpensive material has 8.5 x 10^22 free electrons per cubic centimetre. Comparing aluminium to copper reveals that aluminium has 60% free electrons as copper. Since Al is less expensive, it is economically desirable. However to match its current carrying capability to Cu, a larger diameter cable is needed. 

Insulators on the other hand are materials with very little free electrons. Covalent bonded materials are electric insulators and are called polymers. The bonding between two hydrogen and one carbon atoms are so strong that there are no free electrons. In reality there are still free electrons in a polymer, but not enough to make it a conductor. For example polystyrene has 60000000000 free electrons per cm^3. It seems a lot but when compared with copper, it is very small.

Potential Difference

As we mentioned earlier, there would be no flow of currents till we add a power source such as a battery.  Source of this type is also referred to as emf which stands for electromotive force. As an analogy, it is the same principle as falling water. If we stand underneath a small water fall say 2 feet high we experience less impact than if we stand underneath a water fall 10 feet above. In this example former has less potential difference. Basically a higher potential difference (Volts in an electronic or electric circuit) is capable of moving more electrons. we say that voltage and current have a direct relation; the higher the voltage, the higher the current. The basic formula of V = I times R represents this concept. Electromotive force term is becoming depreciated because it is not really a force. Force is measured in newtons, however emf is measured in joules per coulomb. Joule is unit of work or energy ( More about joule at http://en.wikipedia.org/wiki/Joule ).

Current Direction

In a circuit, flow of electrons are from negative terminal to positive terminal. This is called Mathematical Current Direction

At the time the world of electronics were started by works of Michael Faraday and other researchers, atomic structure knowledge was limited. They defined flow of current from positive to negative terminals; based on their observations. all the rules for electronic circuits assumed positive to negative direction through an external load. This is called Conventional Direction

Today, we still adapt the conventional current direction in our circuit analysis.

Above information gathered by Jay Kajavi from other sources.

April 14, 2012