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Download Ch 1 Electricity Slides 042909
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Chapter 1 Properties of Electricity MElec-Ch1 - 1 Overview • • • • • What is Electricity Ohm’s Law Boat Circuit Problems Multimeters Magnetic Lines of Force MElec-Ch1 - 2 What is Electricity • Electricity consists of electrons Negatively charged Electrons actually flow from negative to positive • Current “flow”, by convention, from positive to negative • Conductors have electrons that readily move e.g. silver, copper, metal and liquids (salt water) • Insulation has tightly bonded electrons Charges do NOT move e.g. plastic, rubber, glass and ceramics MElec-Ch1 - 3 Water Analogy • • • • Height of water = Voltage Valve = Switch Water wheel = Electric motor Small pipe = Resistance MElec-Ch1 - 4 Electrical Circuits • Simple Series Circuit Same current flow through all devices MElec-Ch1 - 5 Complex Series Circuit • If one light bulb burns out… No lights will be on Therefore, not used for light circuits Off Off MElec-Ch1 - 6 Complex Parallel Circuit • If one light bulb burns out… Other will remain on Therefore, used for light circuits On On MElec-Ch1 - 7 Ohm’s Law • • • • E= I = R= P= Voltage (pressure) in volts Current (rate of flow) in amperes Resistance (friction) in ohms E Power (work) in watts I R E=I*R R=E/I I=E/R P=I*E P = I2 * R P = E2 / R MElec-Ch1 - 8 Practice (fig 1-5) • Given 12 volts and 4 amps • What is resistance? E=I*R R=E/I R = 12 / 4 R = 3 ohms (or 3 Ω) 12 VDC 4.0 Amps MElec-Ch1 - 9 Practice (fig 1-6) • Given 12 volts and 8 ohms • What is current? E=I*R I=E/R RT = R1 + R2 I = 12 / 8 I = 1.5 amps 4 ohms (each) 12 VDC MElec-Ch1 - 10 Practice (fig 1-8) • What is current in each device? 6 amps in 2 ohms 2 amps in 6 ohms (each) • What is total current? IT = I 1 + I 2 + I 2 IT = 10 amps 2 Ohms 12 VDC 6 Ohms 6 Ohms MElec-Ch1 - 11 Electrical Power • Watt is unit of electrical power • Given 12 VDC and total resistance of 6 Ω • What is total power? I=E/R I = 12 / 6 P = E * I P = 12 * 2 I = 2 amps P = 24 watts MElec-Ch1 - 12 What We Have Learned So Far • Electricity is the negatively charged electrons that have been freed from the structure of an atom • For electricity to perform some useful function it must flow in a circuit • There are two basic types of electrical circuits The Series Circuit The Parallel Circuit • Using Ohm's Law we can calculate the three electrical values in a circuit, i.e. Voltage, Amperage, and Resistance • The fourth electrical value is Power Power is measured in Watts Using the PIE equation we can calculate Power Using Ohm's Law and PIE equation if we know any two values we can find the other two MElec-Ch1 - 13 Boat Circuit Problems • Open Circuit is “break” in electrical wiring Current flow (path) is interrupted • Sometimes deliberately by switch • Short circuit is electrical wiring shortened from original length Shorts to ground Interwiring short Internal short MElec-Ch1 - 14 Multimeters • Used to measure Voltage Current Resistance • Types of Multimeters Analog Digital • Preferred • Easier to read MElec-Ch1 - 15 Measuring Voltage Voltage to DC Range to 20 volts Red lead to positive Black lead to negative Meter across load • Parallel with load • Battery voltage is 12.6 MElec-Ch1 - 16 Measuring Voltage at Light with good fuse and switch “On” • Should measure approx. 12 VDC at each light MElec-Ch1 - 17 Measuring Voltage Drop • Across a closed switch Should measure zero volts • Across a high resistance “connection” Should measure a voltage drop MElec-Ch1 - 18 Measuring Current Current to DC Range to 10 amps Red lead to current jack on multimeter Insert meter in series with circuit When measuring voltage - meter is across the load MElec-Ch1 - 19 Measuring Resistance Function to resistance Touch test leads together Adjust resistance to zero (0.0) ohms Place test leads across device to be measured • Continuity measurement When test leads are across low resistance (Good circuit or device) Will hear a buzz indicating continuity MElec-Ch1 - 20 Magnetic Lines of Force • Results of Electrical Current Heat (due to resistance in wire) Chemical reaction (in current carrying solution) Magnetic Field (illustrated) MElec-Ch1 - 21 Remote Control Circuit AWG#2 (Large Wire) AWG#16 (Small Wire) Small current through solenoid coil - Controls larger current, through contacts, to Starter Motor MElec-Ch1 - 22 Summary • Electricity is negatively charged electrons • Current flows from positive to negative • Types of circuits Series (one path for current flow) Parallel (multiple paths for current flow) • Ohm’s Law E=I*R P=E*I • Boat Circuit Problems Open Circuits Short Circuits MElec-Ch1 - 23 Summary - 2 • Multimeter Digital preferred Measure voltage across (parallel to) load • Can also measure voltage drop Measure current in series with load • Magnetic Lines of Force Concentrated in coil Solenoids use in remote control circuits • Small current controls large current MElec-Ch1 - 24