Lec3 1 Electrical Current2002

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Information about Lec3 1 Electrical Current2002

Published on April 8, 2008

Author: Riccardino

Source: authorstream.com

Direct current:  Direct current Electrostatics: charges at rest Electric current: motion of charges +/- currents I = DQ / Dt (unit: C/s = ampere (A)) Lightening: ~ 20,000 A; Home appliances: a few amperes Ohm’s law: I  V or I = cV Define c = 1/R => I = V / R Resistance and resistivity:  Resistance and resistivity Resistance: R = rl/A (unit: ohms or W) r: resistivity (unit: W-m) Temperature dependency: r = r0[1+a(T-T0)] a: temperature coefficient of resistance T0: reference temperature (usually 20 oC) R = R0[1+a(T-T0)] Resistivities and temp. coefficients:  Resistivities and temp. coefficients 點石成金! Resistivities:  Resistivities Conductor Positive vs. negative coefficients Semiconductor Negative coefficients Superconductor Break threshold Ceramic: TIBaCaCuO (125 K) Resistivities:  Resistivities ρ vs. temp.: Conductor, Semiconductor and Superconductor (from left to right) Low temp. superconducting:  Low temp. superconducting Some materials have very low resistance thus become superconductors at extremely low temperature, Mercury: at 4.2 oK Lead: 7.2 oK Microscopic view of Ohm’s law:  Microscopic view of Ohm’s law Resistance applications:  Resistance applications Thermistor Strain gauge Conductivity cell Electrophoresis Thermistor (Mn, Ni, Co oxide):  Thermistor (Mn, Ni, Co oxide) Thermistor application:  Thermistor application Electric power:  Electric power Power loss/gain over an potential V due to current, I, in time, Δt: Electric potential energy: UE = qV = IΔtV Unit: Joul 1 cal = 4.186 J; 1 BTU = 1055 J Power loss/gain: rate of loss/gain of electric potential energy P = UE/Δt = VI (unit: watt) = I2R = V2/R 1 Watt = 1 Joul/s Example:  Example A flashlight powered by 1.5 V battery and used a 3 Watt bulb What is the current flowing through? What is the charges crossing the battery? What is the chemical energy loss for the battery? Power & Battery:  Power & Battery P = IV (VIP?) P = I2R = V2/R Electrical circuit:  Electrical circuit Components Resistor Capacitor Inductor Power source Kirchhoff’s laws:  Kirchhoff’s laws Kirchhoff’s laws The current entering any point is zero, i.e., SI = 0 The potential changes around any closed path is zero, i.e., SV = 0 Examples: galvanometers that make voltmeters and ammeters Simple resistor circuits:  Simple resistor circuits Equivalent resistance: Resistors in series: Resistors in parallel: Circuit response:  Circuit response Resistor circuit R-C circuit R-L-C circuit Nonlinear component circuit RC circuit:  RC circuit Charging and discharging of a capacitor in an RC circuit RC circuit applications:  RC circuit applications Measurements of circuit:  Measurements of circuit Electrical safety:  Electrical safety Fuse (circuit breaker) to prevent overload or short circuit Grounding to prevent electric shock Human can feel 1 mA (max. allowable leakage current) 10 ~ 20 mA will paralyze muscles and is fatal when continuous in a few minutes Power line:  Power line Grounding of electric appliances:  Grounding of electric appliances Physiological effects of electricity:  Physiological effects of electricity Heating Stimulation Electrochemical burn Equivalent circuit:  Equivalent circuit Physiological effects of electricity:  Physiological effects of electricity Physiological effects of electricity:  Physiological effects of electricity Threshold of perception(0.5mA 60Hz; 2-10mA DC) Let-go-current (9.5mA) Respiratory paralysis (18-22mA) Ventricular fibrillation (75-400mA) Sustained myocardial contraction (1-6A) Burn & physical injury (> 10A) Important susceptibility parameters:  Important susceptibility parameters Sex Frequency Duration Body weight Point of entry: macroshock vs microshock Sex factor:  Sex factor Frequency factor:  Frequency factor Duration factor:  Duration factor Body weight factor:  Body weight factor Point of entry:  Point of entry Distribution of electric power:  Distribution of electric power Patient electrical environment:  Patient electrical environment NEC (national electric code) 517-80,81, frequency <1000 Hz, 1000W resistor General-care area < 500mV, 4 single or 2 duplex receptacles 2 branch electrical circuit Equipotential grounding Critical-care area < 100mV, 6 single or 3 duplex receptacles 2 branch electrical circuit Equipotential grounding Isolated power system:  Isolated power system Macroshock:  Macroshock Microshock:  Microshock Microshock:  Microshock Microshock:  Microshock Microshock:  Microshock Electrical safety standard:  Electrical safety standard National Fire Protection Association(NFPA) Association for the Advancement of Medical Instrumentation (AAMI SCL-P 10.75) National Electrical Manufacturer Association (NEMA) FAD-MDS-201-0035 IEEE 711-2 Protection against shock:  Protection against shock Isolating patient from electrical conductor Equipotential of all conductor surface Equipotential grounding Connection resistance < 0.1W Receptacle ground < 20mV Receptacle to conductor < 100mV

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