Lec3 1 Electrical Current2002

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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 Conductor Positive vs. negative coefficients Semiconductor Negative coefficients Superconductor Break threshold Ceramic: TIBaCaCuO (125 K)


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 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











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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