Lec3 1 Electrical Current2002

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