Chapter 9. Principles of
Electricity for Electrotherapy
(Part A)
©2008 LWW
Electrical Currents
• Therapeutic value centuries old
– Sold from Sears, Roebuck catalog in 1900
– Outlandish claims
• Discovery of silicon resistors and micro
circuitry during space exploration lead to
new stimulators in 1980s
– Renaissance
Electrical Currents (cont.)
Many types of electrical stimulators followed.
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Many variations
Plethora of current characteristics
Used arbitrary terms to differentiate characteristics
Developed many and varied claims of efficacy in
effort to sell these products
Electrical Currents (cont.)
• The variety of current characteristics and
arbitrary terms lead to chaos.
– Seemed as if manufacturers spoke different
languages
Common Language
• Proposed in 1990 by Section on
Electrophysiology of American Physical
Therapy Association
• Industry, educators, and clinicians use
common terms to decrease confusion.
Basics of Electricity
• Must understand why as well as how to
use electrical stimulation.
Or
• You’ll be a knobologist.
Electricity Defined
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•
A property of certain fundamental particles of all
matter that have a force field associated with
them, manifested by an accumulation of, or
absence of, electrons on an atom or body
A form of energy associated with the existence
and interaction of electrical charge, manifested by
the accumulation of, or absence of, electrons on
an atom or body; exhibits magnetic
(electromagnetic), chemical, mechanical
(electrokinetic), and thermal properties
Electricity Defined (cont.)
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•
A form of energy that exhibits magnetic,
chemical, mechanical, and thermal effects;
formed from the interaction of positive (+)
and negative (−) charges
The physical phenomena associated with
the existence and interaction of electrical
charge, either static charges (electrostatics)
or moving streams of charge (current)
Electricity
• Two types
– Static
– Current
Static Electricity
• Static electricity is frictional electricity.
– One body gains electrons; the other loses
electrons.
– Stored in an insulated conductor in which the
charges are in a state of tension
Current Electricity
• A stream of loose electrons passing along
a conductor
• Current passes in two ways
– Direct current (DC)
– Alternating current (AC)
DC Current
• Steady unidirectional flow of electrons
• Example: between the anode and the
cathode of a battery
• Also called galvanic current
AC Current
• Flow of electrons that rhythmically changes
direction
• Two terminals of generator (source)
alternatively change from positive to
negative
Physics of Electricity
• Matter
– Anything that has weight and occupies space
• Element
– Primary substance of matter (oxygen, copper,
carbon)
• Atom
– Single unit of an element
– Smallest part of an element
– Composed of protons, neutrons, electrons, and
other smaller substances
Physics of Electricity (cont.)
• Molecule
– Two or more atoms held together in
chemical bond.
– May be same (O2) or different (H2O)
• Subunits of atoms
– Protons
– Neutrons
– Electrons
Physics of Electricity (cont.)
• Proton
– Located in nucleus of atom
– Mass of 1 and electrical charge of +1
• Neutron
– Located in the nucleus of atom
– Mass of 1 and electrical charge of 0
• Electron
– Orbits the nucleus
– Mass negligible and electrical charge of −1
Physics of Electricity (cont.)
• Electrical charge
– Net sum of charges of electrons and protons
• Difference between number of protons and
electrons
• In normal state: atom has equal number of
electrons and protons; thus is electrically
neutral
• Chemical, mechanical, solar, or thermal force
causes electrons to be added or removed from
the atom, and it becomes negatively or
positively charged
Current Flow
• Flow of electrical charge from one point
to another
– Higher concentration of electrons to an
area lacking electrons
• Flows from positive pole to negative pole
– Defies logic and reason
Conductor
• Substance that can transport electrical
charge
• Must have free electrons that can be
pushed along
• Metals are the best conductors.
Conductor (cont.)
• Water with minerals or electrolytes is a
good conductor.
– Good electrical conductors also good heat
conductors.
• Electrical conductors don't allow
electricity to flow freely.
– They oppose the flow of electricity, called
resistance.
Conduction
Insulator
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Nonconductor
Resists the flow of electrons.
Has no free electrons to bump
Examples
– Glass, rubber, oil, paraffin, pure distilled water
Conduction (cont.)
Semiconductor
• Substances whose conductivity is poor at low
temperatures
• Increases when
– Small amounts of other substances are added
– Heat, light, or voltage is applied
• Regulate flow of electricity
• Examples
– Carbon, silicone, germanium
Conduction (cont.)
Partial conductor
• Substance that allows some flow of electricity
under certain conditions
• Examples
– Dry wood, paper, tap water, moist air, kerosene
Quantifying Electricity
• Coulomb
– The basic unit of charge, produced by 6.28 × 1018
displaced electrons (6,280 quadtrillion)
• Voltage
– Force created by accumulation of extra electrons
at one point in circuit, usually corresponding to
deficit of electrons at another point in circuit
– If two points are connected, difference in electron
population causes electrons to move from area of
higher concentration to area of deficit
Quantifying Electricity (cont.)
• Voltage (cont.)
– Volt (V) is unit of force
– Force required to push a current of 1 amp (A)
through a resistance of 1 ohm (Ω)
• Different sources
– From storage battery or generator
– Voltage from a generator is called electromagnetic
force (emf)
– Commercial emf either 110 or 220 V
– High-power transmission lines are 20,000 V
– Storm clouds carry several million volts
Quantifying Electricity (cont.)
• Ampere
– Unit of current flow, equal to the passage
of one coulomb per second—that is 6.28 ×
1018 electrons passing per second
• Never a traffic jam.
– Electromedical work requires much less
(milliamperes, mA).
– Some therapeutic devices use 0.1 to 1 mA,
others use 500 to 1500 mA
Review
Elements of current flow
• Force = volt
• Flow = amp
• Resistance = ohm
Quantifying Electricity (cont.)
• Ohm (Ω)
– Unit of resistance or opposition to the flow
of DC
– Equal to resistance of a column of mercury
1 mm2 in cross section and 106 cm high at
0°C
• Ohm = volt/amp
Quantifying Electricity (cont.)
Resistance
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Caused by the conductor
Determined by:
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Type of material
Cross section
Length
Temperature
Quantifying Electricity (cont.)
Impedance
• Opposition (resistance) to flow of AC
– Resistance applies to DC current
• Result of
– Resistance, capacitive reactance, and
inductive reactance
Water/Electricity Analogy
• Water through a pipe similar to electricity
through a conductor
• Volume
– Gallons of water
– Coulombs of electricity (unit of charge)
• Rate of flow
– Gallons/sec flowing past a certain point
– Amps or coulombs/sec
Water/Electricity Analogy (cont.)
• Resistance to flow
– Friction in the pipe
– Ohms in a conductor
– Owing to characteristics of the pipe or
conductor
• Pipe: narrow or wide, long or short,
rough or smooth
• The composition and size of the
conductor determine its resistance,
Water/Electricity Analogy (cont.)
• Requirements for flow
– Water
• Volume of water
• Pressure that is greater than resistance
• Closed system of pipes to carry the
water in (if you want to control flow)
• Water flows by displacing air.
• Valves or faucets to help direct flow
• Different sizes of pipe to provide
resistance and help control volume of
flow
Water/Electricity Analogy (cont.)
• Requirements for flow
– Electricity
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A volume of electrical charge
Voltage greater than resistance
Closed system (circuit) of conductors
Electricity flows by displacing electrons, which
must be accepted by something else; must
have a complete circuit.
• Different sizes of resistors to provide resistance
and control volume of flow
Electrical Equipment
Generator
• A tale of two devices:
– A device that converts various forms of
energy into AC electricity
– A medical device that converts an input
electrical current (AC or DC) into various
output currents (AC, DC, or pulsed)
Electrical Equipment (cont.)
Terminal (pole)
• Output device of a battery or generator
• Attached by wires to electrodes (which are
attached to body
• Positive
– Terminal from which current leaves generator
• Negative
– Terminal into which current returns to generator
Electrical Equipment (cont.)
• Electrical circuit
– System of conductors that allows electrons
to move between poles of a generator
– Closed circuit
• Complete, allowing flow
• No breaks in circuit
– Open circuit
• Interrupted or broken; flow ceases
Medical Devices
•
Muscle stimulator (neuromuscular
electrical stimulation, NEMS)
– A therapeutic device that delivers current
to the body to cause sensory and motor
nerve depolarization
– Causes muscle contraction
Medical Devices (cont.)
• Nerve stimulator (transcutaneous
electrical nerve stimulator; TENS)
– Therapeutic device that delivers
current to the body to cause
sensory nerve depolarization
– Stimulate sensory nerves to
modulate pain
– Muscle contraction may occur, but
this is not the purpose of or
necessary for TENS stimulation.
Safety Devices
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Circuit breaker
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Safety device that
protects equipment
and structures from
excess current
Manufactured to
respond to specific
amounts of current
flow, such as 10 A,
30 A, and so on
When current
exceeds the
specified rating of the
breaker, it trips
Safety Devices (cont.)
•
Ground-fault interrupter (GFI)
– Interrupts current flow
– Senses very small ground-fault currents
such as might flow through the body of a
person standing on damp ground while
touching a hot AC line wire.
– Acts in a little as 0.025 sec to the circuit
breaker
– Typical trip in homes is at 5 mA
Safety Devices (cont.)
• GFI (cont.)
– Used chiefly for wall-outlet circuits into which
potentially dangerous appliances might be
plugged
– Larger versions used in power stations