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