©2008 LWW

Chapter 9. Principles of Electricity for Electrotherapy

(Part A)

Electrical CurrentsElectrical 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.)Electrical Currents (cont.)

Many types of electrical stimulators followed.• 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.)Electrical Currents (cont.)

• The variety of current characteristics and arbitrary terms lead to chaos.– Seemed as if manufacturers spoke different

languages

Common LanguageCommon 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 ElectricityBasics of Electricity• Must understand why as well as how to

use electrical stimulation.Or• You’ll be a knobologist.

Electricity DefinedElectricity Defined• 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.)Electricity Defined (cont.)

• 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 Electricity

• Two types– Static– Current

Static Electricity 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 Current Electricity

• A stream of loose electrons passing along a conductor

• Current passes in two ways– Direct current (DC) – Alternating current (AC)

DC CurrentDC Current

• Steady unidirectional flow of electrons• Example: between the anode and the

cathode of a battery • Also called galvanic current

AC CurrentAC Current

• Flow of electrons that rhythmically changes direction

• Two terminals of generator (source) alternatively change from positive to negative

Physics of ElectricityPhysics 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.)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.)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.)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 FlowCurrent 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

ConductorConductor

• Substance that can transport electrical charge

• Must have free electrons that can be pushed along

• Metals are the best conductors.

Conductor (cont.)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.

ConductionConduction

Insulator • Nonconductor• Resists the flow of electrons. • Has no free electrons to bump • Examples

– Glass, rubber, oil, paraffin, pure distilled water

Conduction (cont.)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.)Conduction (cont.)

Partial conductor• Substance that allows some flow of electricity

under certain conditions • Examples

– Dry wood, paper, tap water, moist air, kerosene

Quantifying ElectricityQuantifying 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.)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.)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

ReviewReview

Elements of current flow• Force = volt• Flow = amp• Resistance = ohm

Quantifying Electricity (cont.)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.)Quantifying Electricity (cont.)

Resistance• Caused by the conductor• Determined by:

– Type of material– Cross section– Length– Temperature

Quantifying Electricity (cont.)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/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.)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.)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.)Water/Electricity Analogy (cont.)

• Requirements for flow– Electricity

• 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 EquipmentElectrical 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.)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 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 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.)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 Safety Devices • Circuit breaker

– 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.)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.)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