# Lec3 1 Electrical Current2002

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## Presentation Transcript

### 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

### 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

Frequency factor

Duration factor

### Body weight factor:

Body weight factor

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

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