Electricity Theory PDF

# Chapter One: Electricity Theory ## 1.1 Introduction Electricity is a form of energy tied to the existence of electrical charge and, as a result, is related to magnetism. It plays a fundamental role in all the technologies we use today. Everyday work and play activities through manufacturing and...

# Chapter One: Electricity Theory ## 1.1 Introduction Electricity is a form of energy tied to the existence of electrical charge and, as a result, is related to magnetism. It plays a fundamental role in all the technologies we use today. Everyday work and play activities through manufacturing and scientific research use electricity as a source of energy. In this chapter, the theory of electricity, fundamental units, and costs are introduced. Devices, equipment, and materials used to distribute electricity from the power utility to points of use in the building and building electrical system design methods are discussed in Chapters 2 and 3. ## History of Electricity - In 1660, a German experimenter named Otto von Guericke built the first electric generating machine. It was constructed of a ball of sulfur, rotated by a crank with one hand and rubbed with the other. - About 1746, Ewald Georg von Kleist, a German inventor, and Dutch physicist Pieter van Musschenbroek of the University of Leyden, working independently, invented an electrical storage device called a Leyden jar, a glass jar coated inside and outside with tin foil. - In 1747, American inventor and statesman, Benjamin Franklin, suggested the existence of an electrical fluid and surmised that an electric charge was made up of two types of electric forces, an attractive force and a repulsive force. - In 1786, an Italian anatomy professor, Luigi Galvani, observed that a discharge of static electricity made a dead frog's leg twitch. - In 1820, H. C. Oersted, a Danish physicist, discovered that a magnetic field surrounds a current-carrying wire, by observing that electrical currents affected the needle on a compass. - In 1831, American Samuel Morse conceived the idea of sending coded messages over wires using the electromagnetic telegraph and a code of electrical impulses identified as dots and dashes that eventually became known as "Morse Code." - Charles de Coulomb was the first person to measure the amount of electricity and magnetism generated in a circuit. - G. S. Ohm, a German college teacher, formulated a law showing the relationship between volts, amps, and resistance. - In the late 1800s, electric lighting was viewed as an ideal use of electrical energy. - In 1882, the Edison Electric Light Company, later known as General Electric, successfully demonstrated the use of artificial lighting by powering incandescent streetlights and lamps in London and New York City. - American Nikola Tesla of Croatian decent, one of Edison's former employees and a rival of Edison at the end of the 19th century, is the inventor of 3-phase power distribution, the alternating current motor, wireless transmission. He began experimenting on generators in 1883, and discovered the rotating magnetic field. - In 1885, George Westinghouse, head of the Westinghouse Electric Company, bought the patent rights to Tesla's alternating current system. ## 1.2 Electrical Theory ### The Phenomenon of Electricity Electricity is a physical phenomenon tied to the behavior of positively and negatively charged elementary particles of an atom. An introduction on the elementary particles of an atom is necessary to develop a sense of what electricity is and how it behaves. Two theories exist: the classical theory and the modern theory. Both are briefly introduced in the sections that follow. ### Classical Theory: Flow of Electrons The Law of Charges states that opposite charges attract each other and like charges repel each other. Thus, negatively charged electrons are attracted to positively charged protons. Conversely, negatively charged electrons tend to repel one another. In classical theory, electrical current is electron flow. Electrons in an orbital shell near the nucleus have a strong attraction to the protons in the nucleus and thus are difficult to free. Electrons in outer orbital shells experience a weaker attraction and are more easily freed. Energy can be added to an electron to move it to the next higher orbital shell. If sufficient additional energy is added, a valence electron can be forced out of the atom. Such an electron is said to be free. These free electrons make up electrical current flow. ### Modern Theory: Flow of Charged Particles In modern theory, electricity is tied to even smaller subatomic particles that possess either a positive or negative electromagnetic charge. ## Electric Current A flow of electric charge through a conductor is an electrical current or, simply current. When opposite charges are placed across a conductor, negatively charged subatomic particles move from the negative charge to the positive charge. ### Conductors, Insulators, and Semiconductor - *A conductor* carries electrical current without providing too much resistance to current flow. - *Insulators* are materials that resist the flow of electricity. They have electrons that tend to retain electrons on their original atoms, making it difficult for electrons to move and conduct electricity. - *Semiconductors* are materials that are neither good conductors nor good insulators. ## Producing Current Flow Electricity is the flow of current through a conductor. Current must be forced to flow in a conductor by the presence of a charge. - **Static electricity from friction**: Simply rubbing two materials together produces a charge of static electricity. - **Thermoelectricity** is electricity from heat. When two dissimilar metals are joined, a thermoelectric charge is created when the joined metals are heated. - **Piezoelectricity** is electricity from pressure. Certain crystalline materials produce a piezoelectric charge when a force deforms or strains the material. - **Electrochemistry** is electricity from a chemical reaction. A galvanic reaction produces opposite electrical charges in two dissimilar metals when they are placed in certain chemical solutions. - **Photoelectricity** is electricity from light. When small particles of light called photons strike a material, they release energy that can cause atoms to release electrons. - **Magnetoelectricity** is electricity from magnetism. The force of a magnetic field can drive electron flow. ## Symbols Used in Lighting and Electrical System Design In electrical design calculations, the symbol E is commonly used for voltage and the symbol I for amperage. In lighting design, *E* is used as a symbol for illuminance and *I* is the symbol for luminous intensity. The reader is cautioned that the lighting and electrical design professions use common symbols with different meanings. ## 1.3 Units of Electricity ### Fundamental Units of Electricity Units used to describe electricity are voltage, amperage and ohms. These are defined in the sections that follow. - **Voltage or electromotive force (E or EMF)** This is the driving force behind current flow. A difference in charge creates an electrical pressure, which moves current in one direction. The unit of electrical pressure is the volt (V). - **Amperage or Inductive Flow (I)** The rate of current flow in a closed electrical system is measured in a unit called the ampere, frequently called the amp. - **Resistance (R)** The length of a conductor (wire), the diameter of the conductor, type of conductor material, and temperature of the conductor affect the resistance to flow of current. The unit used to measure electrical resistance is the ohm (Ω). ### Ohm's Law Current flow is caused by electromotive force or voltage. Amperage is the rate of current flow and may be referred to as inductive flow. Resistance (R) refers to the ability of a conductor to resist current flow and is measured in ohms. Voltage (E), amperage (I), and resistance (Ω) in an active electrical circuit are related through Ohm's law: $E = IR$ Ohms' Law makes it possible to determine one of these values, if the other two are known. ### Water System/Electrical System Analogy Forces that influence flow of current in an electrical system resemble those found in a water system. It is helpful to relate electron flow to water flow to assist in developing a good understanding of how the fundamental units of electricity relate. Voltage is similar in nature to water pressure. In a water system, a greater system pressure results in a greater flow of water. With electrical systems, voltage is the electrical pressure. A higher voltage level produces a higher level of electron flow. Voltage is the driving force of current flow. Amperage describes the rate of electrical current flow. In water systems, rate of flow is described in a flow rate, usually expressed in number of gallons of water per minute (gpm) that flow past a given point in a pipe. In an electrical system, amperage describes the number of electrons that flow past a given point. Pressure losses reduce pressure available at a given point in a water system. A greater resistance to water flow results when pipe diameter is decreased or pipe length is increased. In an electrical system, resistance to electron flow increases as conductor diameter decreases or length increases. ## Power Power is the rate at which work is accomplished; it is work or energy released divided by time. The electrical unit of power is watt. In theory, the watt can be related to other measures of power: - 1 horsepower (hp) = 746 watts - 1 watts = 3.413 Btu/hr - 1,000 watt = 1 kilowatt (kW) On a direct current circuit, voltage (E) and amperage (I) are related to wattage through the DC power equation, Also known as Joule's Law: $P = EI$ ## Energy If power used by an appliance is multiplied by the amount of time that the unit operates, the energy consumption value or amount of work accomplished is determined. The standard billing for energy consumption is the kilowatt-hour (kWh), which is equivalent to 1000 watt-hours. $q = Pt$ ## Electrocution Humans are conductors of electricity and have electrical resistance similar to any other material. When a person comes in contact with electricity, that person can feel the current flow through his or her body, ranging from faint tingling sensations to death. The lowest level at which people can perceive electrical current is about 0.001 A (1 milliamp). Slightly above this level, a mild tingling sensation is felt. At currents higher than 0.05 A (50 milliamps), heat produced by electrical current is enough to burn human skin and tissue. At levels of current flow exceeding 0.1 A (100 milliamps), the heart stops. A person may survive an electrocution if his or her heart can be started again. Care should be exercised when working with electricity. ## 1.4 Electrical Circuits ### The Basic Electrical Circuit An electric circuit is a continuous path along which an electric current can flow. A simple circuit is composed of a power source (e.g., battery or generator); the load, an electrical component or group of components that consume electricity (e.g., a lamp or appliance); and a set of conductors that carry current from the source to the load (e.g., wires). If the circuit is broken at any point, current will not flow. ### Basic Electrical Circuit (Drawing) (A diagram of a basic electrical circuit with a conductor, resistor (light bulb), switch, and battery). ### Closed Circuit A closed circuit is an uninterrupted path that allows a continuous flow of current through an electrical conductor. ### Open Circuit If the path of current flow in a circuit is interrupted or opened (turned off), an open circuit results. ### Switched Circuit A switch is installed in a circuit to allow the circuit to open or close to control operation of the lamp. (A diagram showing a closed circuit, an open circuit, and a switched circuit) ### Control Device and Protective Device A circuit may also have a control device and/or a protective device, but these are optional. A control device either opens or closes the path of the circuit. Light switches, thermostats, and time clocks are examples of common control devices found in circuits. ## Circuiting Configuration There are two basic types of circuiting configurations used in electrical systems: series and parallel. ### A Series Circuit A SERIES circuit is connected so that current passes through each component in the circuit without branching off to individual components in the circuit. Although a series circuit requires fewer connections, if one lamp fails the circuit becomes open and all lamps go out (like a string of low-cost Christmas lamps). The equivalent resistance of any number of resistors connected in series is the sum of the individual resistances. For N resistors in series then, $\displaystyle{R_{eq} = R_1 + R_2 + ... + R_n = \sum_{n=1}^N R_n}$ (A diagram of a single-loop circuit with two resistors in series) ### A Parallel Circuit In a parallel circuit, current branches off to individual components in the circuit. In this circuiting configuration, if one lamp fails, the circuit remains closed and all other lamps remain lit. As a result, parallel circuiting is the most frequently used circuiting technique. **NOTE:** In parallel connection, the components have the same voltages. (A diagram of two resistors in parallel) The equivalent resistance of resistors in parallel is the sum of the inverse of each resistance in parallel $\displaystyle{\frac{1}{R_{eq}} = \frac{1}{R_1} + \frac{1}{R_2} + ... + \frac{1}{R_n}}$ Or the equivalent resistance of two parallel resistors is equal to the product of their resistances divided by their sum. $\displaystyle{R_{eq} = \frac{R_1 * R_2}{R_1 + R_2}}$ And unlike series connection where voltage is divided between resistors. In Parallel connections, current is divided. $\displaystyle{i_1 = \frac{R_2 * i}{R_1 + R_2}}$ $\displaystyle{i_2 = \frac{R_1 * i}{R_1 + R_2}}$ Also note that more current flows on the least value of resistance ## Current and Voltage Laws In 1857, German physicist Gustav Kirchhoff's established two laws known today as Kirchhoff's Laws. These laws state the general restrictions on the current and voltage in an electric circuit. These laws are paraphrased as follows: - **Law No. 1:** The sum of the potential differences (voltages) in a complete circuit must be zero. This law is also known as Kirchhoff's Voltage Law (KVL). - **Law No. 2:** At any specific instant at any junction (e.g., connection) in an electric circuit, the total current (amperage) flowing into the junction is the same as the total current leaving the junction. This law is also known as Kirchhoff's Current Law (KCL). ### Kirchhoff's Voltage Law (KVL) (A diagram of a single-loop circuit illustrating KVL) The Voltage Law states the relationship between voltage drops and voltage sources in a complete circuit. By convention, with voltage drops are assumed to be negative and voltage gains positive, and according to this law the sum of these two quantities in a complete electrical circuit is equal to zero. This means that the voltage drops around any closed loop in a circuit must equal the voltages applied. ### Kirchhoff's Current Law (KCL) (A diagram showing currents flowing in a circuit with a junction) According to the Current Law, when a charge enters a junction, it has no place to go except to leave. By convention, currents flowing into a junction are assumed to be negative and currents flowing out of the junction positive, and by this law the sum of these two quantities is equal to zero. So, no matter how many paths into and out of a single junction, all the current leaving that junction must equal the current arriving at that junction. ## 1.5 The Relationship Between Magnetism and Electrical Current Magnetism is a force of attraction between ferromagnetic metals such as iron, nickel and cobalt and a force of repulsion between diamagnetic materials such as antimony and bismuth. A magnet displays the properties of magnetism. A simple magnet has two poles: a north pole and a south pole. A free-hanging magnet within the earth's magnetic field will orient itself longitudinally between the earth's poles. The magnet's north pole will face the earth's north pole and the magnet's south pole will face south. A magnetic field radiates out from the two poles of a single magnet or between the poles of two magnets. A strong link exists between electricity and a magnetic field. The force of a magnetic field can produce electrical current flow in a conductor. On the other hand, electrical current flow in a conductor produces a magnetic field. This relationship is introduced in the following paragraph. When a conductor is moved through a magnetic field or a magnetic field is moved across a fixed conductor, a voltage is produced in the conductor. The voltage causes current to flow through the conductor. When this happens, current flow is induced in the conductor and the phenomenon is called induction. Moving the conductor in one direction across the magnetic field causes current to flow in one direction. Reversing direction of conductor movement reverses direction of current flow. When the conductor is no longer moved through a magnetic field, current flow stops. When a constant voltage is applied in a closed circuit, the voltage forces current to move in one direction through the conductor. As current flows in one direction, the magnetic fields of the electrons (or charged particles) align and combine to produce a strong magnetic field that extends around the conductor. Increasing voltage, and thus increasing current flow, produces a stronger magnetic field. Decreasing the voltage, and thus decreasing current flow, reduces the magnetic field. When the circuit is opened, current flow through the conductor stops and the electrons (or charged particles) again move in random paths. Their magnetic fields cancel. With no current flow, there is no magnetic field surrounding the conductor. Finally, if the connections of the conductor to the power source were switched, the polarity of the circuit would change. Current would flow in the opposite direction and the polarity of the magnetic field around the conductor would reverse. ## 1.6 Direct and Alternating Current ### Direct Current Direct current (DC) is current flow in one direction in an electrical circuit. It is always from the negative to the positive terminals of the power source such as a battery. Flashlights and automobile electrical installations are designed to operate on a DC power. A graph of DC voltage versus time is shown in Figure. ### Alternating Current Alternating current (AC) is a continuous reversal of the direction of current flow such that at a point in time the current flow is in one direction and at another point in time it is in the reverse direction. ### Single-Phase Alternating Current Power A single-phase (10) alternating current distribution system refers to a system in which all the voltages of the supply vary in unison. A basic system typically has two conductors: one is neutral and the other carries current (the hot or live conductor). ### Three-Phase Alternating Current Power Three-phase (30) alternating current distribution system consists of three separate lines of single-phase power with each line out of phase by 120° (1/3 of a cycle). ## Transforming Voltage and Current Transformers a transformer is an electrical device that transfers an alternating current and voltage from one circuit to another using the induction phenomenon. Transformers serve as an efficient way of converting power at a primary voltage and amperage to the equivalent power at a different secondary voltage and amperage. Thus, the theoretical relationship between primary ($E_p$) and secondary ($E_s$) voltages is proportional to the number of windings in the primary ($N_p$) and secondary ($N_s$) windings are expressed as: $\displaystyle{\frac{E_p}{{N_p}} \approx \frac{E_s}{{N_s}}}$ ### Inductors An inductor is a coil of wire that creates an electromagnetic field. On AC circuits, inductive loads are created as current flows through coils or windings found in motors, transformers, and light fixture ballasts (fluorescent and high-intensity discharge fixtures). ### Capacitors A capacitor is composed of metal plates separated by air or a dielectric material such as paper, ceramic, or mica. Capacitors store electrical energy in an electrostatic field and release it later. ## Effect of Capacitive and Inductive Loads - The *inductive effect* on a series AC circuit causes the phase of the current to lag behind the phase of the voltage -- that is, peak amperage lags peak voltage. - The *capacitive effect* on a series AC circuit causes the phase of the current to lead the phase of the voltage -- that is, peak voltage lags peak current. Although there are no inductive and capacitive effects on a DC circuit, current flow on an AC circuit is impeded by inductance and capacitance. ### Impedance *Impedance* ($Z$) is a measure of resistance to current flow on an AC circuit due to the combined effect of resistance, inductance and capacitance. Impedance is measured in ohms (Ω). Ohm's Law for AC circuits is: ## Power Factor DC and AC circuits perform differently with respect to power use. On a DC circuit, the product of measured voltage and measured amperage equals wattage (VA = W). In contrast, on most AC circuits the computed volt-amperage is different than power consumed (wattage); that is, the product of the measured voltage and amperage (VA) does not equal wattage (VA≠W). This phenomenon is directly related to the inductive effects in circuits powering motors, transformers, and magnetic ballasts as described earlier. ### Three Components of AC Power: - *Real power* is the "working power" that performs useful effort in a circuit (e.g., creating heat, light, and motion); it is expressed in watts (W) or kilowatts (kW). - *Reactive power* is the power that generates the magnetic field required for inductive devices to operate. It dissipates no energy in the load but which returns to the source on each alternating current cycle; - The *apparent power* is the "power available to use." It is expressed in volt-amperes (VA) or kilovolt-ampere (kVA), because it is the simple product of voltage and current. - The *power factor* (PF or cos) for a single-phase circuit is the ratio between real power and apparent power in a circuit: $PF = (real \ power / apparent \ power)$ $PF = watts / (Volts * Amps) = W/ VA$ The power factor is a number between 0 and 1 (frequently expressed as a percentage, e.g., a 0.7 PF =70% PF). ## Power Factor Correction PF is important in the design of AC systems, because if PF is less than 1.0, the current carrying wire, the transformers being use must accommodate the total apparent power. Some consumers install PF correction devices (e.g., a capacitor) to cut down on higher costs associated with a low PF. Some industrial sites will have large banks of capacitors, called power factor correction capacitors, specifically for the purpose of correcting the PF back toward 1 to save on power company charges. The main advantages of the PF correction are as follows: - A high PF reduces the load currents, resulting in a considerable saving in hardware costs (i.e., conductors, switchgear, substation transformers, and so on). - Power companies typically impose low power factor penalties, so by correcting the PF, this penalty can be avoided. - The electrical load on the power company is reduced, which allows the power company to supply the surplus power to other consumers without increasing its generation capacity. ## Energy Charge - The energy charge is simply the cost of electrical energy consumed ($energy). This may be computed by the following equation, where energy consumption (q) is expressed in kilowatt-hours and unit cost of electricity ($/kWh) is expressed in dollars per kilowatt-hours: $ \displaystyle{\$energy = q. \$kWh} $ - The energy charge is based on energy consumed by the customer during a billing period, say once a month or every 30 days. Energy consumed is determined by reading the electric meter. ## Power "Demand" Charge - *Maximum demand* is the user's highest rate at which energy is consumed in kilowatts (kW) over a small time interval (usually 15 min but sometimes30 or 60 min) that is measured by the electric meter during a billing period. - *Demand charge* is the billing fee related to maximum demand. Depending on the billing rate, a high demand charge may remain at that rate for 12 months even though the demand for succeeding months is significantly lower. ## Demand Limiting and Load Shedding - *Demand limiting* is accomplished by disconnecting loads that are not needed during periods of high demand. - *Load shedding* is a method by which nonessential equipment and appliances are deliberately switched off to maintain a uniform load and thus limit demand. - *Load shifting* moves nonessential loads to periods of low demand. - The *time-of-use (TOU) rate* rewards the user for reducing power consumption during periods when electrical demand is highest and a lower rate for the remainder of the year. - *Additional surcharges* these includes the service or billing charge covers the cost of metering and bill collecting activities such as meter reading and preparing and mailing billing statements and. Etc.