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Chapter 4 – Ohm’s Law, Power and Energy Introductory Circuit Analysis Robert L. Boylestad 4.2 - Ohm’s Law Cause Effect Opposition Every conversion of energy from one form to another can be related to this equation. In electric circuits the effect we are trying to establish is the flow of charge, or current. The potential difference, or voltage between two points is the cause (“pressure”), and resistance is the opposition encountered. Ohm’s Law Simple analogy: Water in a hose Electrons in a copper wire are analogous to water in a hose. Consider the pressure valve as the applied voltage and the size of the hose as the source of resistance. The absence of pressure in the hose, or voltage across the wire will result in a system without motion or reaction. A small diameter hose will limit the rate at which water will flow, just as a small diameter copper wire limits the flow of electrons. Ohm’s Law Developed in 1827 by Georg Simon Ohm For a fixed resistance, the greater the voltage (or pressure) across a resistor, the more the current. The more the resistance for the same voltage, the less the current. Current is proportional to the applied voltage and inversely proportional to the resistance. Ohm’s Law E I R Where: I = current (amperes, A) E = voltage (volts, V) R = resistance (ohms, ) 4.3 - Plotting Ohm’s Law Plotting Ohm’s Law Insert Fig 4.8 4.4 - Power Power is an indication of how much work (the conversion of energy from one form to another) can be done in a specific amount of time; that is, a rate of doing work. Power W P t 1 Watt (W) 1 joule / second Power can be delivered or absorbed as defined by the polarity of the voltage and the direction of the current. 4.5 - Energy Energy (W) lost or gained by any system is determined by: W = Pt Since power is measured in watts (or joules per second) and time in seconds, the unit of energy is the wattsecond (Ws) or joule (J) Energy The watt-second is too small a quantity for most practical purposes, so the watt-hour (Wh) and kilowatthour (kWh) are defined as follows: Energy (Wh) power (W) time (h) power (W) time (h) Energy (kWh) 1000 The killowatt-hour meter is an instrument used for measuring the energy supplied to a residential or commercial user of electricity. 4.6 - Efficiency Efficiency () of a system is determined by the following equation: = Po / Pi Where: = efficiency (decimal number) Po = power output Pi = power input Efficiency The basic components of a generating (voltage) system are depicted below, each component has an associated efficiency, resulting in a loss of power through each stage. Insert Fig 4.19