Electrical Quantities

[Kipa Crawn]

Electricalmeasurements are the methods, devices and calculations used to measure electrical quantities. Measurement of electrical quantities may be done to measure electrical parameters of a system. Using transducers, physical properties such as temperature, pressure, flow, force, and many others can be converted into electrical signals, which can then be conveniently measured and recorded. High-precision laboratory measurements of electrical quantities are used in experiments to determine fundamental physical properties such as the charge of the electron or the speed of light, and in the definition of the units for electrical measurements, with precision in some cases on the order of a few parts per million. Less precise measurements are required every day in industrial practice. Electrical measurements are a branch of the science of metrology.

In electrical and electronic circuits, there are five major electrical quantities that used to analyze circuits. These quantities are electric charge, electric current, voltage, electric power and electrical energy. In this article, we shall learn about these quantities in detail.

Technically, electric charge is the concept which explains the electrical behavior of materials. Therefore, it is the electric charge that forms the basis of existence of the electricity. The electric charge is the most elementary quantity in an electric circuit.

As per the electron theory of matter, we know that every material consists of tiny particles called molecules, and molecules in turn made up of atoms. An atoms consists of three fundamental particles namely electrons, protons and neutrons. Where, electrons bear a negative charge, protons carry a positive charge and neutron is neutral, i.e. there is no charge on the neutron. These three particles are called subatomic particles.

In the nature, the smallest amount of charge that exists is the charge carried by an electrons, denoted by e, where it is equal to $\mathrm1.6 \times 10^-19C$. Though, a proton also carries a charge of $\mathrm1.6 \times 10^-19C$, but it is positive.

The conventional direction of the electric current is from the point of higher potential (positive terminal) to the point of lower potential (negative terminal). However, the actual direction of electric current is the direction of flow of electrons which is from the negative terminal to the positive terminal.

As from the definition of the electric current, it is clear that the electric current is due to the flow of electric charge or electrons. Therefore, it shows two fundamental effects viz. heating effect and magnetic effect. Which means, when the electric current flows through a conductor, it produced a magnetic field around the conductor and generates heat in the conductor.

Voltage, also known as potential difference, is the electric pressure which makes the electric charges to flow in a conductor. The voltage is defined as the work or energy required to move a unit charge from one point to another in an electric circuit. It is denoted by the symbol V (constant voltage) and v (time-varying voltage).

Most important fact about voltage is that it does not exists at a point by itself, which means it is always measured with respect to some other point. Because of this, voltage is also known as "potential difference". In any electric circuit, the voltage is the factor which entirely responsible for flow of current in the circuit.

In any electric circuit, if the electric current enters the circuit element at the positive terminal and leaves the element at the negative terminal, then the power is said to be absorbed by the element. On the other hand, if the current enters the element through the negative terminal and exits through the positive terminal, the element delivers the electric power.

The ability to do work in the electric circuit is known as electrical energy. The electrical energy is defined as the total amount of work done or total energy expended over a certain period of time in an electric circuit.

In this article, we discussed that electric charge is the most fundamental electrical quantity in an electric circuit. When this charge flows through a conducting material, it constitutes an electric current in the conductor. The factor which pressures the electric charge to flow is the voltage or potential difference. In addition to charge, current and voltage, we require two more quantities namely "power" and "energy" for practical calculations. Power is the rate at which energy expended in the circuit, while Energy is the total amount of work done over a certain period of time in the circuit.

To see the analogies more clearly, examine the following table that shows the constitutive relationships for the various analogous quantities. The entries for the mechanical analogs are formed by substituting the analogous quantities into the equations for the electrical elements. For example the electrical version of Ohm's law is e=iR. The Mechanical I analog stipulates that e is replaced by v, i by f and R by 1/B, which yields v=f/B.

To apply this analogy, every node in the electrical circuit becomes a point in the mechanical system. Ground becomes a fixed location, resistor become friction elements, capacitors become masses and inductors become springs. Sources must also be transformed. A current source becomes a force generator, and a voltage source becomes an input velocity. This is best illustrated with an example.

The conversion from an electrical circuit to a mechanical 1 analog is easily accomplished if capacitors in the circuit are grounded. If they are not,the process results in a mechanical system where positions must be chosen very carefully, and the process can be much more difficult.

This circuit was drawn with the capacitor grounded. If the capacitor is grounded the position of the mass can be chosen as an absolute position (relative to the fixed reference). If the capacitor is not grounded we must use relative positions and the result is much more complicated.

The procedure to go from Mechanical 1 to Electrical is simply the reverse of Electrical to Mechanical 1. Either a mathematical method can be used (refer to previous example, Electrical to Mechanical 1, and read the table from bottom to top, or a simple visual method can be used where force generators are replaced by current sources, friction elements by resistors, springs by inductors, and masses by capacitors (which are grounded). Each position becomes a node in the circuit.

One deficiency in this analogy is that it only works easily for inductors with only one current defined through them. This can be seen by the analogies between energy in an inductor and energy in a mass.

Since the energy of the mass in a Mechanical 2 analogy is measured relative to a fixed reference (i.e., a single velocity=v=0) the energy of the inductance must be measured relative to a single current.

To apply this analogy, every loop in the electrical circuit becomes a point in the mechanical system. Resistors become friction elements, capacitors become springs and inductors become masses. Sources must also be transformed. A current source becomes an input velocity, and a voltage source becomes a force generator. This is best illustrated with an example.

Converting a circuit diagram to a mechanical 2 analog uses a similar procedure as electrical to mechanical 1 except that the voltages around a loop summed to zero (instead of the sum of currents at a node) is analogous to the sum of forces at a point being summed to zero .

The reason for choosing the currents so that only one current flows through the inductor now becomes apparent -- if chosen in this way, the position of the mass can be chosen as an absolute position (relative to the fixed reference). It needn't be done this way but, if not, the result is much more complex.

In general, to draw a mechanical 2 analog of an electrical circuit, simply sum voltages around each loop, and equate these to the forces being applied at a point. If possible, draw currents such that only one current flow through inductors (so that the velocity of the mass can be defined in absolute terms relative to a fixed reference).

A visual method can be done, but will not be discussed here. It is similar to the method for drawing dual circuits (i.e., mechanical elements are drawn perpendicular to the electrical elements so the each loop in the electrical circuit becomes a position in the mechanical circuit).

The procedure to go from Mechanical 2 to Electrical is simply the reverse of Electrical to Mechanical 1. Refer to theprevious example, Electrical to Mechanical 2, and read the table from bottom to top. A visual method can be done, but will not be discussed here.

This PPT is useful to all the students who study in electrical engineering and also for those students whose know about basic information of electrical quantities like charge, voltage, current, electrical power and energy.Read less

Many people dedicate their entire lives to electrical engineering and measurement technology. This illustrates once again how broad and complex this subject is. Because of this, it is not surprising that there is a multitude of different parameters for the characterization of current, voltage or power.

There is AC voltage and there is DC voltage. One can recognize the difference between the two voltage types best when looking at the instantaneous value, also called momentary value. This parameter indicates the value of the voltage (or current) at this exact moment. To identify instantaneous values, small letters such as u(t) or i(t) are often used for voltage and current.

In contrast, the instantaneous value of an AC voltage changes continuously. In this case, the time course of the instantaneous value represents a periodic signal. This periodic signal can e.g. be a sine wave but may also be a square wave signal.

In order to characterize AC voltages more precisely, other parameters are used in addition to the instantaneous value. These parameters include the peak and peak-to-peak value. To indicate these one adds a subscript character to the quantity like Vp or Vpp.

3a8082e126