Electrical Terminology PDF

UNCONTROLLED COPY - FOR TRAINING PURPOSE ONLY Revision Service Will Not Be Provided To The Holder TRAINING MAN UAL For Traini ng Purpose Only ELECTRICAL TERMINOLOGY The following are some common electrical terms, their units and factors affecting them....

UNCONTROLLED COPY - FOR TRAINING PURPOSE ONLY Revision Service Will Not Be Provided To The Holder TRAINING MAN UAL For Traini ng Purpose Only ELECTRICAL TERMINOLOGY The following are some common electrical terms, their units and factors affecting them. WATER ANALOGY The following figure shows an analogy using water to illustrate the electrical terms and their significance. Consider two Tanks A and B with different water levels which are interconnected as shown in the figure. Water will flow from the tank with the higher level to the other tank. The presence of the higher water level in one tank has created a difference in water pressure. Water Analogy Tank A - -ve Charged Body Tank B - +ve Charged Body Water flow- current flow B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 1 TRAINING MAN UAL For Traini ng Purpose Only POTENTIAL DIFFERENCE As seen from the figure the difference in water levels creates a pressure, which makes water flow. Therefore using the same analogy for an electric circuit, when two points are connected by a conductor, there will be a flow of current from the point with a large number of electrons to the other point with a smaller number of electrons. This results in a potential difference between the two points. When there is a potential difference between the two points, it simply means that a field of force is present which tends to move the electrons from one point to another. The unit for potential difference is Volt (V). A potential difference of 1V exists between 2 points of a conductor when it is carrying a constant current of 1A when the power dissipated between these points is equal to 1W. The following figure illustrates the application of potential difference between two points. B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 2 TRAINING MAN UAL For Traini ng Purpose Only ELECTROMOTIVE FORCE (EMF) Electromotive force (emf) or sometimes called electron-moving force is the driving force that causes the current to flow through a conductor. The unit for emf is Volt (V). It is generated by a battery, or by the magnetic force according to Faraday's Law. Emf is commonly generated by electrochemical reaction (e.g., a battery or a fuel cell), absorption of radiant or thermal energy (e.g., a solar cell or a thermocouple), or electromagnetic induction (e.g., a generator or an alternator). Electromagnetic induction is a means of converting mechanical energy, i.e., energy of motion into electrical energy. The emf generated in this way is often referred to as motional emf. Emf can also be considered electrical potential or pressure. The term voltage, which is measured in volts, is typically used instead of emf. Electromotive force is typically symbolised by the letter E and voltage is symbolised by the letter V. VOLTAGE Potential difference, electrical potential and electromotive force are measured in volts, leading to the commonly used term voltage and the symbol V (sometimes E is used for voltage). B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 3 TRAINING MAN UAL For Traini ng Purpose Only DISTINCTION BETWEEN EMF AND POTENTIAL DIFFERENCE The following example illustrates the distinction between emf and potential difference. The circuit has an emf of 4V. The voltmeter reads the potential drop of 3V between A and B. The potential difference between points B and C is 1 volt. Therefore the potential difference is a voltage difference between two points in a circuit. The emf is the voltage generated by the battery. CHARGE The earlier water analogy has shown that water molecules flow from A to B. By using the same analogy in an electrical circuit the electrons flow. These electrons constitute the charge. The unit for electrical charge is coulomb (C). * It is the total charge Q of 6.21 x 1018 electrons. Thus a single electron has a charge of 1.61 x 10-19 C. CURRENT In the water analogy, the rate of flow of water is measured in litres/minute, cm3 /sec etc. Therefore using the same analogy, the current can be defined as the rate of flow of electric charge at a point in a circuit. The unit of current is Ampere (A). B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 4 TRAINING MAN UAL For Traini ng Purpose Only Electric current flow in coulombs/sec = amperes. 1 coulomb of electrical charge flow past any point in a conductor in 1 second constitutes a current of 1 ampere. Current is normally denoted by the letter I. RESISTANCE The resistance of a constriction in a large pipe is so great that essentially all the pressure drop will appear across the resistance. If the Water pipe is constricted (narrowed), the constriction will oppose the flow of water than the remainder of the pipe system. Likewise a resistor in an electric circuit will generally will oppose the flow of current than the wire of the circuit. Resistance is the property of a material to oppose the flow of current and to convert electrical energy into heat. Its magnitude depends on factors such as the nature of conductor material, its physical state, dimensions, temperature and thermal properties. The unit for resistance is Ohms ( ). One ohm is the electrical resistance between two points of a conductor when a constant p.d. of 1V, applied to these points, produces in the conductor a current of 1A, the conductor not being the source of any emf V R I where V is the voltage and I is the current. B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 5 TRAINING MAN UAL For Traini ng Purpose Only CONDUCTANCE Conductance is the ability of a material to conduct electricity. Conductance is the inverse of resistance. A material that has a low * value of conductance will not conduct electricity as well as a material that has a high conductance and vice versa. The unit of electrical conductance is Siemens (S). The following table shows the relative conductance of some common metals. Metal Relative Conductance (Copper = 1) Silver 1.06 Copper (annealed) 1.00 Copper (Hard Drawn) 0.97 Aluminium 0.61 Mild Steel 0.12 Lead 0.08 CONVENTIONAL CURRENT FLOW Conventional current flow is from positive to negative. As the early discoverers had no knowledge of electron flow, they based their laws of electricity on the behaviour of electric circuits i.e. they could only consider the effects. Conventional Current Flow B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 6 TRAINING MAN UAL For Traini ng Purpose Only A discharged to B therefore A is considered to be positively charged with respect to B. In fact B was charge with electrons and more negative than A so electrons flowed from B to A. The convention was too well established to alter when the truth was discovered so it remains the convention today. ELECTRON FLOW Electron current flow is from negative to positive. It shows the “true” direction of current flow. Electron Flow B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 7