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Electricity Theory Whenever an abundance of electrons (-) develops on one end of a material and a scarcity of electrons (+) is present on the on either end, electrons will flow from atom to atom from the abundant endto the scarce end, if the two areas are connected by conducting material.
Electrical Theory—cont. Either current, a magnetic field or motion can be produced as long as the other two are present.
Electrical Theory--cont. Four common methods of developing a surplus of electrons: • Electromechanical • Electrochemical • Thermoelectrical • Photoelectrical
1. Electromechanical • Generators and alternators are electromechanical devices. • An electromechanical device produces electricity when it rotates. • Three common sources of power to rotate generators and alternators: • Wind • Water • Engine • Generators/alternators produce electricity through electromagnetic induction.
2. Electrochemical Electrochemical reactions can either produce electricity, or use electricity. Chemical reaction produces a voltage A voltage causes a chemical reaction
3. Thermoelectrical Thermoelectrical devices can either uses electricity to produce heat, or use heat to produce electricity. An electric heater produces heat using electricity. A thermocouple uses a heat to produce electricity.
4. Photoelectrical Photoelectricity is the emission of electrons from matter upon absorption of electromagnetic radiation. Photovoltaic Cell (PV Cell)
Electrical Theory-cont. • Electricity is a form of energy that can produce light, heat, magnetism, or chemical changes. • Light occurs when electricity passes through a filament. • Heat is produced when electricity flows through a resistance. • A magnet field forms around any conductor carrying electricity. • Electricity passing through water causes the hydrogen and oxygen to split.
Introduction • Electricity is the primary source of power for stationary equipment. • A basic understanding of the principles of electricity is a requirement for using electrical powered equipment efficiently and safely.
Electrical Terms • Understanding electricity is dependent upon understanding the following electrical terms: • Resistance • Amperes • Volts • Ohms law • Watts • Conductor • Insulator • AC current • DC current • Power Factor • Electrical power • Electrical energy
Electrical Terms--Resistance • Resistance: a measure of the difficulty encountered by the electrons as they flow through a conductor. • Resistance is a characteristic of all materials. • Electricity passing through a resistance causes heat. • Resistance is measured in units of Ohms () An Ohm is defined as the resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces a current of 1 ampere.
Electrical Terms-Conductor • Conductor: any material that has a low resistance to the flow of electricity. • Metals are good conductors. • The resistivity of conductors increases with temperature.
Electrical Terms-Insulator • An insulator is any material that provides a high resistance to the flow of electricity. • Glass, plastics and Bakelite are common electrical insulators. • The resistivity of insulator decreases with temperature.
Electrical Terms-Amperes • Amperes: the measure of the rate of current flow. • “Rate” is the amount of something divided by time. • 1 amp = 6.24 × 1018 electrons per second • A current (amperage) occurs whenever there is a source of electricity, conductors and a complete circuit. An Amp meter is wired in series with the load to measure current (amps).
Electrical Terms-Volts • Voltage (E or V): the electromotive force (potential) available to cause electrons to flow. • Measured in units of volts (V). • A volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. • Always measured by comparing the difference (potential) between two points. A volt meter is attached parallel to the load measure voltage.
Ohm’s Law • The relationship of resistance, voltage and amperage is explained by an equation called Ohm’s Law. • The flow of electricity through a conductor is directly proportional to the electromotive force that produces it. • As an equation:
Ohm’s Law Example What is the current flow in a circuit with a voltage of 120 volts and a resistance of 0.23 ?
Electrical Terms-Watts • Watts: the measure of electrical energy (power). Note: 1 hp = 746 W
Electrical Terms-Watts-cont. • Electrical Power example • Determine the power consumed by a resistor in a 12 volt system when the current is 2.1 amps.
Electrical wheel • The electrical wheel Illustrates Ohm’s law and the electrical power equation. • The value at the point of the 4 pie slices can be found using any one of the three equations on the rim of the pie slice. • Example: E (Volts) can be determined by
Electrical Terms-Watt hour • Watt Hour: the measure of electrical energy use. • A combination of electrical power and time. • Units used to determine electrical charges,
Electrical Terms-Watt hour-cont. • Electrical cost example: • Determine the amount of energy a 100 Watt light bulb will use when operated for 8 hours. • -- What will it cost to operate the light bulb if the electrical energy costs 0.12 $/kWh?
Electrical Terms-AC & DC • Electrical circuits can be classified according to how the current voltage varies with time. • The two common currents are called: • Direct current (DC) • Alternating current (AC)
Electrical Terms-Direct Current-cont. • Direct current • The electrons move in one direction only. • Amperage is constant. • Voltage is constant • The type of current used in batteries.
Electrical Terms-Alternating Current-cont. • The amperage and voltage varies over time and periodically reverses direction (cycle). • Standard domestic current. • Standard domestic electrical service is 60 cycle.
Electrical Loads • Electrical loads can be divided into two categories: • Resistive loads • Non resistive loads (reactant Loads)
AC Electrical Loads-Resistive • Resistive loads convert electrical energy to heat. • Electric heaters • Arc welders • Electric furnaces • Toasters • Air dryers
AC Electrical Loads-Non resistance (Reactance) • Examples of reactance loads are motors and fluorescent lights. • To determine the power and energy of an AC circuit with reactance loads the power factor must be included. • Power Factor • Reactance loads do not use all of the electricity that is sent to them. They store part of it for a short period of time and then pass it back to the generator. • The result is that they do not use the theoretical energy that is available. • Power Factor will always be between 0 and 1. • In AC circuits with reactance loads, the power is determined by:
Power Factor - Resistive Loads • In AC both voltage and current vary with time. • If the load is resistive, the voltage and current peaks occur at the same time. • Since volts X amps = Watts, the power also peaks at the same time.
Power Factor - Reactive Loads • If the load is reactive, the current lags the voltage. • The current and voltage peaks do not occur at the same time. • During the time of the phase shift in the cycle the current is negative. -I x V = -W • Negative Watts means that for this portion of the cycle power power is flowing back to the source.
Determining Power Factor Example • What is the power factor for a reactive load which consumes 1,100 watts at 15.0 amps when connected to a 120 volt circuit? • A power factor less than one means that more current is flowing to the load than is required to supply the actual power used by the load. • If the power factor of a load is improved, with no other changes, the power used by the load stays the same, but the current to the load is reduced.
