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Transcript
Electrical Circuits ALESSANDRO VOLTA (1745-1827) ANDRE MARIE AMPERE (1775-1836) GEORG SIMON OHM (1789-1854) Sources of Voltage (Energy) Voltage, also known as “electromotive force” can be create by many sources of energy solar geothermal wind biomass nuclear hydroelectric chemical piezoelectric ALLESANDRO VOLTA AND THE ELECTRIC BATTERY A VOLTAGE SOURCE IS LIKE A WATER PUMP Electrical Current Current is the rate of flow of charge through a conductor. charge current time I q t Current flow is defined in the direction of positive charge; since electrons flow in wires, current is opposite the flow of electrons. SI units 1coulomb 1 amp 1 second C A s Small microamp (A) currents flow through your body, but larger amounts of current are dangerous, even deadly. CURRENT EFFECT 0.0001 A threshold of feeling 0.001 A mild shock felt 0.005 A shock is painful 0.015 A muscle control is lost 0.100 A death can occur Electrical Current Potential difference creates an electric field in a circuit. The electric field induce charge to flow. Moving electrons collide with vibrating atoms, so they zigzag in a random path, with a slow click for forward drift velocity. animation Although the electric field travels at near the speed of light, the drift velocity of electrons is less than a millimeter per second. Electrons do not race around a circuit! Electrical Resistance Resistance is a measurement of a material’s ability to resist the flow of electrical charge. voltage resistance current V R I SI 1 ohm 1 volt units 1 amp Resistivity depends on the nature of a material. Conductors have low resistivity and insulators have high resistivity. Resistance depends on the material’s type, length, cross- section, and temperature. click for resistance codes resistance applet V A Alternating versus Direct Current Thomas Edison brought direct current to the masses. Nikola Tesla proved alternating current to be more efficient. Alternating current (ac) switches the direction of the current back and forth at 60 times per second (60 Hertz). DIRECT CURRENT North America uses ac for its power grid because it was safer and more reliable when infrastructure began. Batteries provide direct current (to increase battery life). Most devices use direct current so a transformer converts ac to dc often at lower voltage. Direct energy conversion devices like light bulbs, heaters, can run off ac or dc ALTERNATING CURRENT click for animation Ohm’s Law Electrical circuit versus a water circuit click for animation click for animation slope = 1/R voltage OHMIC cu rr en t cu rr en t cu rr en t When a device shows a linear relationship between voltage and current, it is said to be “ohmic” slope = 1/R voltage NONOHMIC slope = 1/R voltage ONE-WAY (DIODE) Energy, Power, and Cost in Circuits POWER LAW power energy charge voltage current voltage time time P I V Combine Power Law with Resistance equation P I V I (IR) PI R 2 V P I V V R (V )2 P R Cost of Electrical Power energy power time Example - Find the cost of a 1500 watt hair dryer run for 12 minutes, using the rate of $0.10 per kilowatt hour. cost rate energy energy in kilowatt hours (kW h) cost in dollars ($) time in hours (h) $ rate in dollars per kilowatt hours kW h 1 kW 1.5 kW 10 3 W 1h time 12 min 0.2 h 60 min power 1500 W cost 0.10 $ kWh (1.5 kW) (0.2 h) cost $0.03 or 3 cents! Kirchoff’s Rules Rule #1 - The Loop Rule A statement of conservation of energy -4 v The sum of the potential differences (voltages) around any closed loop in a circuit must be zero -2 v example +12 v Rule #2 - The Junction Rule A statement of conservation of charge The current entering a junction in a circuit equals the current leaving the junction 2A 10 A 3A 5A example -6 v Series Circuits A series circuit has only one pathway around the circuit Rule # 1 means that the voltage across all resistors in series must add up to the source voltage V V11 V22 V33 Rule # 2 means that the current through all resistors in series must equal the source current I I1 I 2 I 3 Combine the two equations: V V1 V2 V3 I I1 I2 I3 Req R1 R2 R3 click for animation Series Christmas Tree Lights Series wiring was often used for Christmas tree lights One bulb burns out, they all go out! Who’s to say which one burned out! Modern bulbs use a “jumper” wire. This wire has insulation around it. When the bulb burns out, the jumper now has 120 volts across it. The voltage produces a spark, and the insulation burns off. The circuit is now complete, with the one bulb not lit but easily replaced. Parallel Circuits A parallel circuit has multiple pathways around the circuit Rule # 1 means that the voltage across all resistors in parallel must equal the source voltage V V1 V2 V3 Rule # 2 means that the current through all resistors in parallel must add up to the source current I I1 I2 I3 Combine the two equations: I I1 I2 I3 V V1 V2 V3 1 1 1 1 Req R1 R2 R3 click for animation click for animation click for animation Combination Circuits click for animation A combination circuit must be simplified into groups of series and parallel resistors, and then the equivalent for each group is then found. Req 10 2 12 1 1 1 = + Req = 3 Req 12 4 Req =1+2 +3 + 4 =10 Combination Circuits The total current in the combination circuit is determined and used to work “backwards” to find other branch currents and resistor voltages. V 120 v I 12 A Req 10 Find current through circuit I V / R 36 /12 3 A Find current through 12 equivalent V IR 12 3 36 v Find voltage across 3 equivalent V IR 32 6 v Find voltage across 2 resistor