Direct current

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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
• Ohms law I ? V or I cV
• Define c 1/R gt I V / R

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Resistance and resistivity

• Resistance R rl/A (unit ohms or W)
• r resistivity (unit W-m)
• Temperature dependency
• r r01a(T-T0)
• a temperature coefficient of resistance
• T0 reference temperature (usually 20 oC)
• R R01a(T-T0)

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Resistivities and temp. coefficients
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Resistivities

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

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Resistivities

• ? vs. temp. Conductor, Semiconductor and
  Superconductor (from left to right)

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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

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Microscopic view of Ohms law
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Resistance applications

• Thermistor
• Strain gauge
• Conductivity cell
• Electrophoresis

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Thermistor (Mn, Ni, Co oxide)
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Thermistor application
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Electric power

• Power loss/gain over an potential V due to
  current, I, in time, ?t
• Electric potential energy UE qV I?tV
• Unit Joul
• 1 cal 4.186 J 1 BTU 1055 J
• Power loss/gain rate of loss/gain of electric
  potential energy
• P UE/?t VI (unit watt) I2R V2/R
• 1 Watt 1 Joul/s

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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?

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Power Battery

• P IV (VIP?)
• P I2R V2/R

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Electrical circuit

• Components
• Resistor
• Capacitor
• Inductor
• Power source

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Kirchhoffs laws

• Kirchhoffs 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

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Simple resistor circuits

• Equivalent resistance
• Resistors in series
• Resistors in parallel

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Circuit response

• Resistor circuit
• R-C circuit
• R-L-C circuit
• Nonlinear component circuit

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RC circuit

• Charging and discharging of a capacitor in an RC
  circuit

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RC circuit applications
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Measurements of circuit
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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

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Power line
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Grounding of electric appliances
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Physiological effects of electricity

• Heating
• Stimulation
• Electrochemical burn

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Equivalent circuit
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Physiological effects of electricity
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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 (gt 10A)

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Important susceptibility parameters

• Sex
• Frequency
• Duration
• Body weight
• Point of entry macroshock vs microshock

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Sex factor
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Frequency factor
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Duration factor
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Body weight factor
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Point of entry
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Distribution of electric power
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Patient electrical environment

• NEC (national electric code) 517-80,81, frequency
  lt1000 Hz, 1000W resistor
• General-care area
• lt 500mV, 4 single or 2 duplex receptacles
• 2 branch electrical circuit
• Equipotential grounding
• Critical-care area
• lt 100mV, 6 single or 3 duplex receptacles
• 2 branch electrical circuit
• Equipotential grounding

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Isolated power system
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Macroshock
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Microshock
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Microshock
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Microshock
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Microshock
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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

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Protection against shock

• Isolating patient from electrical conductor
• Equipotential of all conductor surface
• Equipotential grounding
• Connection resistance lt 0.1W
• Receptacle ground lt 20mV
• Receptacle to conductor lt 100mV

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