Electric Power
Voltage and current are two basic parameters of an electric circuit. But, only voltage and current are not sufficient to express the behavior of an electric circuit element. We essentially need to know, how much electric power, a circuit element can handle. All of us have seen that a 60 watts electric lamp gives less light than a 100 watts electric lamp. When we pay electric bill for electricity consumption, we are actually paying the charges for electric power for a specified period of time. Thus electric power calculation is quite essential for analyzing an electric circuit or network.
Power is the rate of energy supplied or consumed by an electric element with respect to time
Suppose, an element supplies or consumes an energy of dw joules for a time of dt second, then power of the element can be represented as,
This equation can also be rewritten as,
Hence, as the expression of voltage and current in the equation are instantaneous, the power is also instantaneous. The expressed power is time-varying.
So, the power of a circuit element is the product of voltage across the element and current through it.
As we have already told that a circuit element can either absorb or deliver power. We represent the absorption of power by putting a positive sign (+) in the expression of power. Likewise, we put a negative sign (-) when we represent the power delivered by the circuit element.
Passive Sign Convention
There is a simple relationship between the direction of current, polarity of voltage and sign of the power of a circuit element. We call this simple relationship as passive sign convention. When a current enters in an element through its terminal of positive voltage polarity, we put a positive sign (+) before the product of the voltage and current. This implies that the element absorbs or consumes power from the electric circuit. On the other hand, when the current through the element leaves its terminal of positive voltage polarity, we put a negative sign (-) before the product of the voltage and current. This implies that the element delivers or supplies power to the electric circuit.
Let us take a resistor connected across two circuit terminals. Although, the rest of the circuit is not shown here in the figure. The polarity of the voltage drop across the resistor and the direction of current through the resistor are shown in the figure below. The resistor is consuming power of vi watts as current i ampere enters in the resistor though its positive side of the dropped voltage v volt, as shown.
Let us take a battery connected across two circuit terminals. Although, the rest of the circuit is not shown here in the figure. The polarity of the voltage drop across the battery and the direction of current through the battery are shown in the figure below. The battery is delivering a power of vi watts as current i ampere enters in the battery of v volt through its positive polarity terminal as shown.
Electricity
There are some inventions which charged the human civilization. The first invention was the wheel, the second invention was electricity, the third invention was telecommunication, and the fourth invention was the computer. We will discuss here the basic introduction of electricity. Each substance in this universe is made of plenty of atoms and each atom has the same number of negative electrons and positive protons. As a result, we can say that each neutral substance has the same number of electrons and protons in it. The protons are immovable and strongly attached to the nucleus of the atoms. Electrons are also bounded to atoms and orbiting around the nucleus in different distinct levels. But some of the electrons can move freely or can come out from their orbit due to external influences. These free and as well as loosely bonded electrons cause electricity.
In a neutral condition, the number of electrons and protons is the same in any piece of substance. But if somehow the number of electrons in a substance becomes more than the number of protons, the substance becomes negatively charged as the net charge of each electron is negative. If the number of electrons in a substance becomes less than the number of protons, the substance becomes positively charged.The concentration of free electrons always tries to be uniform. This is the only reason for electricity. Let us explain in detail. If two dissimilarly charged conductive bodies come in contact, the electrons from the body of higher electron concentration will move to the body of lower electron concentration to balance the electron concentration of both bodies. This movement of charge (as electrons are charged particles) is electricity.
The related terms in electricity
Electric Charge:
As we told earlier that the number of electrons and number of protons are equal in a neutral body. The amount of negative charge and positive change is also equal in a neutral body since the electric charge of an electron and a proton is numerically equal but their polarity is opposite. But for any reason, the balance of the number of electrons and protons in a body gets distributed the body becomes electrically charged. If the number of electrons more than that of protons the body becomes negatively charged and the amount of charge depends on the number of excess electrons in the body. In the same manner, we can explain the positive change of a body. Here the number of electrons becomes lesser than that of protons. The positivity of the body depends on the difference between protons and electrons in the body.
Electric Current:
When charge flows from one point to another to make uniform charge distribution then the rate at which the charge is flowing called electric current. This rate mainly depends on the difference between the charged condition of two points and the conditions of the pathway through which the charge is flowing. The unit of electric current is Ampere and it is nothing but coulomb per second.
Electric Potential:
The level of the charged condition of a body is known as electric potential. When a body is charged it gets the ability to do some work. Electric potential is the measurement of the ability of a charged body to do work. The current flowing through a conductor is directly proportional to the difference of electric potential between at two ends of the conductor. The electric potential can be visualized as the difference of water level in two water tanks linked with a pipeline. The speed of water flowing from the higher headed tank to lower headed tank depends on the level difference or head difference of the water in the tanks not on the quantity of water stored in the tanks. In the same way, the electric current between two bodies depends on the potential difference between two bodies not on the quantity of charge stored in the bodies.
Electric Field:
There is always a force between two nearly placed charged bodies. The force may be either attractive or repulsive depending on the nature of the charge of two bodies. When a charged body enters the nearby zone of another charged body the force is practically experienced. Space surrounds a charged body where another charged body can experience a force is called the electric field of the former body.
These above mentioned four terms are the main parameters of electricity.
How is Electricity Generated
There are three basic ways by which we generally produce electricity.
Electromechanical Process:
When a conductor moves in a magnetic field and the conductor cuts the field flux lines electricity is produced in the conductor. Depending on this principle all electrical generators work such as DC generators, alternators, and all kinds of dynamos.
Electrochemical Process:
In all types of battery electricity is produced due to chemical reactions. Here chemical energy gets converted to electrical energy.
Solid State Electric Generation:
This is the most modern process of electricity generation. Here, free electrons and holes are generated at a PN junction and distribution of charge carriers gets imbalanced across the PN junction when the junction is exposed in the light. These free electrons and holes and their imbalanced distribution across the junction cause electricity in an external circuit. On this principle, PV solar cells work.
Types of Electricity
When electricity produced in the armature of a generator it is always alternating. That means polarity of electricity alters in a periodic interval. In DC generators the produced electricity in armature gets rectified through commutator. In alternators, the AC produced in the armature supplied to the external circuit through slip rings.
When electricity does not change its direction it is called DC electricity. Batteries and solar cells produce DC electricity.
Generation Transmission and Distribution of Electricity
After electricity gets generated in an electrical power plant it gets stepped up by step up transformer for transmitting purpose. Generation of electricity at low voltage level is practical and economical. But low voltage transmission is not economical. But for electrical transmission, the generated electricity first gets stepped up and then after transmission it is stepped down by step down transformers for electrical distribution purpose. Generation of electricity, transmission of electricity and distribution of electricity are normally with three phase system. Very ultra high voltage ac transmission is not economical always and that is why dc transmission is sometimes used. The supply system of domestic houses may be a single phase AC but all commercial, industrial and bigger house supplies are of three phase system.
Nature of Electricity and Concept of Electricity
Electricity is the most common form of energy. Electricity is used for various applications such as lighting, transportation, cooking, communication, production of various goods in factories and much more. None of us exactly know that what is electricity. The concept of electricity and theories behind it, can be developed by observing its different behaviors. For observing nature of electricity, it is necessary to study the structure of matters. Every substance in this universe is made up of extremely small particles known as molecules. The molecule is the smallest particle of a substance into which all the identities of that substance are present. The molecules are made up of further smaller particles known as atoms. An atom is the smallest particle of an element that can exist.
There are two types of substances. The substance, that’s molecules are made of similar atoms is known as an element. The matter whose molecules consisting dissimilar atoms, is called a compound. The concept of electricity can be achieved from the atomic structures of substances.An atom consists of one central nucleus. The nucleus is made up of positive protons and charge less neutrons. This nucleus is surrounded by numbers of orbital electrons. Each electron has a negative charge of – 1.602 × 10 ^ – 19 Coulomb and each proton in the nucleus has a positive charge of +1.602 × 10 ^ – 19 Coulomb. Because of the opposite charge there is some attraction force between the nucleus and orbiting electrons. Electrons have relatively negligible mass compared to the mass of the nucleus. The mass of each proton and neutrons is 1840 times the mass of an electron.
As the modulus value of each electron and each proton are same, the number of electrons is equal to the number protons in an electrically neutral atom. An atom becomes positively charged ion when it loses electrons and similarly an atom becomes negative ion when it gains electrons.
Atoms may have loosely bonded electrons in their outermost orbits. These electrons require a very small amount of energy to detach themselves from their parent atoms. These electrons are referred as free electrons which move randomly inside the substance and transferred from one atom to another. Any piece of substances which as a whole contains an unequal number of electrons and protons is referred as electrically charged. When there is more number of electrons compared to its protons, the substance is said to be negatively charged and when there is more number of protons compared to electrons, the substance is said to be positively charged.
The basic nature of electricity is, whenever a negatively charged body is connected to a positively charged body by means of a conductor, the excess electrons of negative body starts flowing towards the positive body to compensate the lack of electrons in that positive body.Hope you got the very basic concept of electricity from the above explanation. There are some materials which have plenty of free electrons at normal room temperature. Very well known examples of this type of materials are, silver, copper, aluminium, zinc etc. The movement of these free electrons can easily be directed to a particular direction if the electrical potential difference is applied across the piece of these materials. Because of plenty of free electrons these materials have good electrical conductivity. These materials are referred as good conductor. The drift of electrons in a conductor in one direction is known as the current. Actually electrons flow from lower potential (-Ve) to higher potential (+Ve) but the general conventional direction of current has been considered as the highest potential point to lower potential point, so the conventional direction of current has been just opposite of the direction of flow of electrons. In non-metallic materials, such as glass, mica, slate, porcelain, the outermost orbit is completed and there is almost no chance of loosing electrons from its outermost shell. Hence there is hardly any free electron present in this type of material.
Hence, these materials cannot conduct electricity in other words electrical conductivity of these materials is very poor. Such material are known as non-conductor or electrical insulator. The nature of electricity is to flow through a conductor while an electrical potential difference applied across it, but not to flow through insulator even high electrical potential difference applied across them.
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