- CCConventional
Current+ → -
- EFElectron
Flow- → +
- We useWe use Conventional Current.
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Conventional Current assumes that current flows out of the positive terminal, through the circuit and into the negative terminal of the source. This was the convention chosen during the discovery of electricity. They were wrong!
Electron Flow is what actually happens and electrons flow out of the negative terminal, through the circuit and into the positive terminal of the source.
Both Conventional Current and Electron Flow are used. Many textbooks are available in both formats.
Floyd, 1989, Principles of Electric Circuits, 5th edition, Conventional Current Version.
Floyd, 1990, Principles of Electric Circuits, 4th edition, Electron Flow Version.
In fact, it makes no difference which way current is flowing as long as it is used consistently. The direction of current flow does not affect what the current does.
In general, high school Physics and two year technician programs use Electron Flow.
But three year technologist and university engineering programs use Conventional Current. Certain symbols (ex. diodes and transistors) and rules (ex. Right-hand rules) were created using Conventional Current. Changing from Conventional Current to Electron Flow would cause a degree of confusion for old and new students and errors would occur, so Conventional Current was kept to ensure there was no confusion with those already trained with Conventional Current. Two systems may seem confusing, but as long as usage is consistent, it really is not!
You must realize what convention is being used because the rules change. Ex. Right-Hand rules in Conventional Current become Left-Hand rules in Electron Flow. Example
Throughout this course, Conventional Current is used. Therefore always assume current flows out of the positive terminal of the source.
What is Electric Current?
Electric current is nothing but the rate of flow of electric charge through a conducting medium with respect to time. It is caused by drift of free electrons through a conductor to a particular direction. As we all know, the measuring unit of electric change is Coulomb and the unit of time is second, the measuring unit of current is Coulombs per second and this logical unit of current has a specific name Ampere after the famous French scientist André-Marie Ampere. If total Q Coulomb charge passes through a conductor by time t, then current I = Q/t coulomb par second or Ampere.
Ampere is SI unit of current.
Definition of Electric Current
While a potential difference is applied across a conductor, electrical charge flows through it and electrical current is the measure of the quantity of the electrical charge flowing through the conductor per unit time.
Theory of Electricity
There is an equal number of electrons and protons in an atom. Hence, atom is in general electrically neutral. As the protons in the central nucleus are positive in charge and electrons orbiting the nucleus, are negative in charge, there will be an attraction force acts between the electrons and protons. In an atom various electrons arrange themselves in different orbiting shells situated at different distances from the nucleus.
The force is more active to the electrons nearer to the nucleus, than to the electrons situated at outer shell of the atom. One or more of these loosely bonded electrons may be detached from the atom. The atoms with lack of electrons are called ions. Due to lack of electrons, compared to number of protons, the said ion becomes positively charged. Hence, this ion is referred as positive ion and because of positive electrical charge; this ion can attract other electrons from outside. The electron, which was previously detached from any other atom, may occupy the outer most shell of this ion and hence this ion again becomes neutral atom.
The electrons which move from atom to atom in random manner are called free elections. When a voltage is applied across a conductor, due to presence of electric field, the free electrons start drifting to a particular direction according the direction of voltage and electric field. This phenomenon causes current in the conductor. The movement of electrons, means movement of negative charge and rate of this charge transfer with respect to time is known as current.
The amount of negative electric charge in an electron is 1.602 × 10-19 Coulomb. Hence, one coulomb negative electric charge consists of 1/1.602 × 10-19 = 6.24 × 1018 number of electrons. Hence, during drift of electron to a particular direction, if 6.24 × 10 18 number of electrons cross a specific cross-section of the conductor, in one second, the current is said to be one ampere. Since, we have already seen the unit of current, ampere is coulomb/second.
Measurement of Current
The most common method of measuring current is to connect an ammeter in series with the circuit that’s current to be measured. This is so because; the entire current flowing through the circuit must also flow through the ammeter also. The ideal internal resistance or impedance of an ammeter is zero. Hence, ideally there is no voltage drop across the ammeter connected in the circuit. A conventional analog ammeter consists of a current coil. Whenever current flows through this coil, it deflects from its position depending upon the amount of current flowing through it. A pointer is attached to the coil assembly; hence it points the current reading on the dial of the ammeter. For measuring alternating current, clip on meter or tong tester can also be used instead of conventional ammeter. In this ammeter a current transformer core is attached to the meter which can easily be clipped on the live current carrying conductor. Due to this arrangement, current in the circuit transforms to the secondary of the CT and this secondary current then measured on the dial of clip on meter without disturbing the continuity of the current unlike conventional ammeter.
In the early days, it was thought that the current is, flow of positive charge and hence current always comes out from the positive terminal of the battery, passing through the external circuit and enters in the negative terminal of the battery. This is called conventional flow of current. On the basis of this conception, all the theories of electricity, formulas, and symbols were developed. After the development of atomic nature of matter, we have come to know, that actual cause of current in a conductor is due to movement of free electrons and electrons have negative change. Due to negative charge, electrons move from the negative terminal to the positive terminal of the battery through the external circuit. So the conventional flow of current is always in the opposite direction of electrons flow. But it was impossible to change all the previously discovered subsequent rules, conventions, theories and formulas according to the direction of electrons flow in the conductor. Thus the concept of conventional current flow was adopted. The true electron flow is used only when it is necessary to explain certain effects (as in semiconductor devices such as diodes and transistors). Whenever we consider the basic electrical circuits and devices, we use conventional flow of current i.e. current flowing around the circuit from the positive terminal to the negative terminal. Types of Current
There are only two types of electrical current, direct current and alternating current. We abbreviate them as DC and AC respectively. Concept of DC was developed before AC. But AC becomes most popular means of generating, transmitting and distributing of electric power. The direction of the flow of direct current is unidirectional, means this current does not alter its direction during flowing. Most common examples of DC in our daily life, are the current that we get from all kinds of battery system. But most popular form of electrical current is alternating current or AC. AC does have some advantages over DC for generating, transmitting and distributing and that is why the current we get from our electric supply companies, is normally alternating current.
Alternating Current
The current whose flow is not unidirectional moreover it alternates at a frequency, is called alternating current. In other words, the direction of the current continuously changes from forward to backward and then backward to forward in the circuit. The number of times, this direction changes from forward to backward or from backward to forward per second, is referred as frequency of the current. The current produced in an alternator is always an alternating current. The shape of the waveform of an alternating current is usually sinusoidal. But square, triangular and other types of waveform are also available for attending current.
Conventional Direction of Alternating Current
As direct current, alternating current is denoted with arrow. An AC has both forward and backward direction of flow. The arrow head always indicates the forward direction of the current. In different point of view, when the current has a positive valve, the direction of current is same as the reference arrow and when the current gets negative value; its direction is just opposite of the reference arrow.
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