The basics of energy
Energy is best defined as “the capacity for doing work”. Electricity is but one of many forms of energy. Other familiar forms of or descriptions of energy are thermal or heat, light, mechanical, and so on. Energy is also described as kinetic, that energy associated with a moving body, and potential energy, that energy associated with an object’s position.
For centuries mankind has used energy in its various forms to enhance its standard of living. In many cases, ways have been devised to change energy from one form to another to increase its usefulness. An example as old as mankind is the burning of a fuel to produce heat and light.
Electrical energy possesses unique characteristics that made it an extremely valuable form of energy. It has four unique properties //
- It can be produced at one location and transmitted to another instantaneously;
- It can be transformed to other energy forms and thereby used in a variety of ways;
- It can be delivered by a system of wires, and control;
- It cannot be stored.
Consistent terminology has always been an issue when discussing electricity and electric power. Convention is to use a system of measurement based on the MKS (meter, kilogram, second) system. Table below summarizes the terms used to describe various aspects of electricity and shows some of the interrelationships between them.
There is an electric charge associated with electrons. This charge is described by a quantity called a coulomb.
The rate of flow of these charges is called the electric current and is described by a quantity called an ampere. One ampere is equal to the flow of one coulomb of charge during one second across a reference point.
The capital letter I, is used to indicate current and the quantity is sometimes referred to as amps. In many texts, electric current is described as a physical flow of electrons. It is not. The electrons do not flow.
Rather electricity is a flow of energy as a result of electron vibrations. The mechanism is the transfer of energy from one electron to another as they collide, one with another.
Electromagnetic force (EMF), voltage, and difference in potential are different descriptions of the notion of what causes these charges to flow. A physics text would define voltage as the energy per a unit of charge where energy is measured by a quantity called a watt-second.
|Quantity||Name or Unit||Symbol||Relationships|
|Current||Amperes||I||I = q / t = V / R|
|Resistance||Ohms||R||R = V / I|
|Inductive Reactance||Ohms||XL||XL = 2·π·f·L|
|Capacitive Reactance||Ohms||XC||XC = 1 / (2·π·f ·C)|
|Impedance||Ohms||Z||Z = R + j(XL+ XC)|
(EMF), Volts, kilovolts
|E, V, kV||V = I · R
V = J / Q
|Power or Real Power||Watts, kiloWatts,
|P||P = V · I
P = I2 · R
P = V2 / R
|Reactive Power||VArs, kiloVArs,
|Q||Q = I2 · XL
Q = I2 · XC
|Apparent Power||kiloVolt amperes||S||S = P + jQ|
|J||J = V · I · t
J = I2 · R · t
|Frequency||Hertz, cycles per second||f|
An engineering text would say that a difference in potential (or of voltage) of one volt causes a current of one ampere to flow through a circuit that has a resistance of one ohm.
The letter E is used when referring to a voltage source such as a generator or a battery and is often called an electromagnetic force. The letter V is used in all other instances. In both cases the quantity is measured by a quantity called a volt.
Voltage can be thought of as electric potential to deliver energy. Differences in voltage measure the work that would have to be done to move a unit charge from a point of one voltage to that of another voltage. When a source of voltage is applied to a wire, a current will flow. The material in the wire offers some resistance to the flow of current.
This resistance is described by a quantity called an ohm.
One ohm is defined as the resistance of a circuit element when an applied voltage of one volt results in a current of one ampere. The resistance of wire depends on the material it is made of, the cross-sectional area of the wire and its length. For a given material, the larger the cross-sectional area the lower the resistance.
The letter R is used to represent resistance in ohms. The relationship between voltage, current and resistance is known as Ohm’s Law. That is, voltage =current x resistance. This relationship is applicable for direct current conditions.
Electricity is a form of energy which is measured by a quantity called a watt-second. Electric customers usually see their bills keyed to their watt-hour usage.A related but different quantity is electric power. The unit of power is the Watt.
In an electric power system the magnitudes of many quantities are such that larger units are needed to describe them. The larger increments usually encountered are described by the addition of the term kilo or the term mega to the base unit.
For example, a kiloVolt is 1,000 Volts; a kiloWatt is 1,000 Watts, a megaWatt is 1,000,000 Watts or 1,000 kiloWatts, a kiloWatthour is 1,000 Watthours.
Reference // Understanding electric power systems by Jack Casazza/Frank Delea