Fundamentals of Electrotherapy and Biomedical Physics Ashish Kakkad
INDEX
ABCDEFGHIJLMNOPRSTUVW
A
Absorption 69
Active components 1
Alternating current 25
Ammeter 11
Amplitude 26
Artificial magnets 57
Atomic theory of magnetism 59
Attenuation 70
Attenuation coefficient 71
Autotransformer 88
B
Bias voltage 35
Biogas 29
Biphasic pulsed current 25
C
Capacitance 95
Capacitive reactance 100
Capacitors
in parallel 99
in series 98
Causes for pain 104
Causes of electric shock 48
Cell
hazard 34
Characteristics of electric lines of force 4
Capacitor 95
charging 99
discharging 99
inventor of 95
types of 97
Choke coil 38
Classification of current according to frequency 27
Classification of electrode 42
Comparison between electricity and magnetism 61
Components of pain 102
Conductors 16
Continuous alternating current 27
Cosine law 72
Coulomb's inverse-square law 9
Coulomb's law 9
Counter irritation 107
Current electricity 24
Cylindrical conductors 97
D
Decay time 26
Dermatomal rule 105
Descending pain suppression system 106
Diamagnetism 57
Dielectric 95
Diode 35
Direct current 25
Disposable cell 34
E
Earth shock 52
Earthing 31
Eddy current 82
Electric cell 34
Electric charge 2
Electric conductance 7
Electric current 3
Electric field 3
Electric field of capacitor 96
Electric potential energy 8
Electric resistance 5
Electric shock 48
first aid 51
medical uses 51
prevention 51
self-care 51
signs and symptoms 49
torture by 51
treatment 51
types of 48
Electric tension 4
Electrical energy 8
Electrical potential difference 4
Electrical power 8
Electricity generation 27
Electricity transmission and distribution 29
Electricity types 24
Electrochemical cells 34
Electrode gel 45
characteristics of 46
Electrodes 42
Electrolytic cells 34
Electromagnetic induction
inventor of 79
Electromagnetic spectrum 64
history of 65
types of 66
Electromagnets 58
Electromotive force 4
Electronics 1
Electrostatic potential energy 8
Energy loss from transformer 89
Equatorial line 57
Even ratio transformer 87
Extinction 70
Extrinsic semiconductor 17
F
Factors affecting attenuation 70
Factors affecting pain perception 103
Factors affecting skin resistance 41
Factors for electromagnetic induction 79
Factors in lethality of electric shock 50
Faraday's law 81
Fast pain 102
Ferromagnetism 57
Field effect transistors 92
Fixed capacitor 97
Forward bias 36
Foucault current 82
Frequency 27
Frequency 64
Fuel cells 34
Full-wave rectification 38
Fuse 21
G
Galvanic cell 34
Galvanic skin response 41
Geometric length 56
Geothermal energy 29
Grothus law 72
Grotthus Draper law 72
Grouping of capacitors 98
Grouping of resistance 6
H
Half-wave rectification 37
History of transistor 91
Hydropower 28
I
Inductive reactance 81
Insulators 16
Intensity 26
Interpulse interval 26
Interrupted alternating current 25
Intrinsic semiconductor 17
Inverse square law 69
Inverter 37
J
Joule's effect 9
Joule–Lenz law 9
Joule's law 9
Junction field effect transistor 92
Junction transistors 92
L
Lambert-Cosine law 72
Lateral spinothalamic tract 104
Law of refraction 68
Lenz's law 82
M
Macroshock 48
Magnet 56
shapes 59
types of 58
Magnetic effect of electric current 59
Magnetic flux 79
Magnetic length 57
Magnetic meridian 57
Magnitude 26
Mains supply 30
Metal electrode 43
Metal oxide semiconductor field effect transistor 92
Microshock 49
Molecular theory of magnetism 59
Monophasic pulsed current 25
Mutual induction 80
N
Natural magnets 57
NPN transistor 92
N-type semiconductor 18
Nuclear power 28
O
Ohm's law 6
P
Pathway of pain 103
Pain regulation 105
Pain-gate theory 106
Parallel plate capacitors 97
Paramagnetism 57
Parameters of electrodes 44
Passive components 1
Permanent magnets 58
Phase 25
Phase duration 26
Photoelectric cells 34
Photovoltaic cells 34
Placebo 107
PNP transistor 92
Potential difference 4
Potential drop 4
Potentiometer 11
Power plugs and sockets 20
Precautions against earth shock 53
Primary cell 34
Properties of a magnet 60
Properties of magnetic lines of force 60
Psychogalvanic reflex 41
P-type semiconductor 17
Pulsatile alternating current 25
Pulse 25
Pulse duration 26
Pulsed alternating current 27
R
Rechargeable cells 34
Reciprocal engines 29
Rectifier 37
Referred pain 105
Reflection 67
Refraction 67
Renewable energy 28
Resistance in parallel 7
Resistance in series 6
Reverse bias 36
Rheostat
variable resistance 10
Rise time 26
Rubber electrodes 42
S
Secondary cell 34
Self induction 80
Self-adhesive electrodes 44
Semiconductors 16
Series rheostat 10
Severity of shock 49
Shunt rheostat 11
Skin resistance 41
Slow pain 102
Snell–Descartes law 68
Snell's law 68
Solar cells 34
Solar power 28
Spherical conductors 97
Static electricity 24
Static transformer 86
Step down transformer 87
Step up transformer 87
Storage cell 34
Switches 21
T
Temporary magnets 58
Thermal power 28
Thermionic valves 35
Tidal wave energy 29
Transformer
construction of 86
principle of 86
uses 86
Transistor
history of 91
types of 92
uses 91
Triode 37
U
Uninterrupted alternating current 25
Uses of eddy current 83
Uses of electromagnetic waves 73
Uses of variable capacitor 98
V
Vacuum electrodes 44
Variable capacitor 98
Variable transformer 88
Velocity 65
Voltage 4
Voltaic cell 34
Voltmeter 12
W
Waveform 26
Wavelength 64
Wind power 28
×
Chapter Notes

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Fundamentals of Electric Circuit1

 
THEORIES OF ELECTRICITY
 
Electronics
Electronics is the field of manipulating electrical currents and voltages using passive and active components that are connected together to create circuits. Electronic circuits range from a simple load resistor that converts a current to a voltage, to computer central processing units that can contain millions of transistors. Electronic devices operate by the movement of electrons through conductors.
 
Active Components
They change their resistance or impedance when varying voltages are applied to them and as a result can amplify, rectify, modify or distort alternating current waveforms. For example vacuum tubes, diodes, transistors.
 
Passive Components
They don't alter their resistance, impedance or reactance when alternating currents are applied to them. They normally don't distort waveforms. For example Resistors, inductors, transformers and capacitors.
 
Matter and Electricity
All matter consists of molecules. A molecule can be defined as the smallest particle, which shows all the characteristics of a particular matter. For example, molecule of water is obtained by dividing a drop of water again and again until it can be divided and still is water. Further division of this water molecule will yield three particles which are not water. Molecule of water contains two atoms of hydrogen and one atom of oxygen.
zoom view
Fig. 1.1: Atom with protons, neutrons and electrons
2
Chemical combination of different atoms makes a molecule. An atom can be further divided into three particles known as protons, electrons and neutrons (Fig. 1.1). These particles cannot be divided further.
 
ELECTRIC CHARGE
As we know, protons and electrons are the particles possessing electrical properties whereas neutron is electrically neutral. Charge is amount of electrons or protons.
Electrons are the negatively charged particles, which revolve around the positively charged protons which are located in the nucleus of an atom along with neutrons. Proton is about 1800 times heavier than electron. There is always attraction between unlike charges. Because electron is much lighter than proton, hence it is pulled towards the proton. If the force of attraction is enough, then the electron comes too closer to the proton and both the particles together form a neutral particle to be known as neutron.
The electrical charge of an electron can be explained with the help of an imagination that there exist lines of forces, which are outward pointing. Though the size and weight of electron and proton varies significantly, the negative field of an electron is just as strong as the positive field of a proton. Though it is small physically, the field near the electron is quite strong. The strength of the field varies inversely with the distance squared.
Though electrons and protons have different kind of charge in them, both have charges of equal magnitude. An electron (negatively charged) repels another electron, while a proton (positively charged) repels another proton. But the proton and electron have attracting force between them. As electron is lighter, electron will be attracted towards the proton. So the basic physical law states that:
“Like charges repel each other; unlike charges attract each other” (Fig. 1.2).
zoom view
Fig. 1.2: Attraction between like charges and repulsion between like charges
 
Unit
  • Coulomb (C)
  • One coulomb is equivalent to 6 × 1018 electrons3
 
Symbol
  • q, –q or Q.
 
ELECTRIC CURRENT
Electric current is the rate of flow of electric charge. Electric current is a flow of electric charge, mainly the electron, from one point in unit time, through a medium such as wire in unit time. It can also be carried by ions.
 
Direction
Before some years, it was believed that flow of positive charge occurs from anode to cathode so direction of conventional current was given from anode to cathode. In later years, it was found that electric current was due to negatively charged particles electrons. Electrons move from cathode to anode. But still we use to indicate direction of current from anode to cathode only.
 
Formula
  • I = Q / t
  • Where I is amount of current (in ampere)
  • Q is electric charge (in coulomb) passing through the cross-sectional area
  • t is time to pass from given surface area (in seconds).
 
Measurement
By ammeter
 
Unit
  • Ampere (A)
  • 1 ampere means one coulomb charge flowing through cross-sectional area of conductor in one second.
 
Symbol
I
Most commonly in liquids, electric currents are because of movement of electrolytes. Electrolytes are electrically charged particles (ions). For example if a current is passed in a solution of sodium chloride, the sodium ions move towards the negative electrode, while the chloride ions move towards the positive electrode.
In air and other ordinary gases electrical conductivity is low due to few mobile ions. Mostly air and gases are insulators. Vacuum contains no charged particles so it behaves as an insulator.
 
ELECTRIC FIELD
Michael Faraday gave the concept of electric field. It is the region around the charged particle or timely changing magnetic field, where if another 4charged particle comes; it will feel either attracting or repelling force depending on its polarity (Fig. 1.3).
zoom view
Fig. 1.3: Electric field and electric lines of force
 
Characteristics of Electric Lines of Force
  • Electric lines of force are just imaginary lines which show spreading of electric field.
  • The direction of electric lines of force is from positive charge to negative charge.
  • Electric lines of force reduced longitudinally because of attraction between unlike charges.
  • Electric lines of force expand laterally because of repulsion between like charges.
  • Electric lines of forces do not intersect each other.
  • Electric lines of force enter or leave any conductor at right angle to surface of conductor.
  • Electric lines of force are open loops unlike magnetic lines of force.
  • The concentration of electric lines of force will increase if radius of conductor in which it enters is less.
 
VOLTAGE OR POTENTIAL DROP OR ELECTRIC TENSION OR POTENTIAL DIFFERENCE OR ELECTROMOTIVE FORCE OR ELECTRICAL POTENTIAL DIFFERENCE
Electric charge is location dependent property of circuit. In two different points of circuit, amount of electric charge will be different. In simpler words, we can assume that on particular location, electric charge has its potential energy (Fig. 1.4).
So just for example, there are two points A and B in the circuit. Point A is of high potential energy means containing high amount electric charge as compared to point B. So if charge has to be moved from A to B, it will 5be without energy. But if electric charge is to be moved from B to A, work has to be done. Potential difference is the difference in electric potential energy per unit charge between two points of circuit. A voltage may represent either a source of energy, or it may represent lost or stored energy.
zoom view
Fig. 1.4: Electric circuit where I is electric current, R is resistance, V is potential difference
 
Production
Voltage can be caused:
  • By static electric fields
  • By electric current through a magnetic field
  • By time-varying magnetic fields
  • A combination of all three.
 
Measurement
  • By voltmeter.
 
Unit
  • Volts (V), or Joules per coulomb
  • One volt is the potential difference that requires one Joule of energy to move one coulomb of charge.
 
Symbol
  • ΔV.
 
ELECTRIC RESISTANCE
The electrical resistance of circuit is the opposition to the passage of an electric current through conductor. More the electric resistance, less the current passes.
The resistance of an object is defined as the ratio of voltage to electric current through the circuit.
 
Formula
  • R = V / I
  • Where R is electric resistance of object (in ohm)6
  • V is potential difference between two points of circuit (in volt)
  • I is current passing from conductor (in ampere).
 
Units
  • Ohm (Ω).
 
Symbol
  • R.
 
OHM'S LAW
It states that the current through a conductor between two points is directly proportional to the potential difference across the two points and inversely proportional to resistance. The law was named after the German Physicist Georg Ohm, and Published in 1827.
 
Formula
  • I = V / R.
Where I is the current through the conductor in defined cross-sectional area (in ampere), V is the potential difference between two points of circuit (in volt) and R is the constant electric resistance of conductor (in ohm).
Here resistance R is independent of current, while voltage and current are interdependent.
 
GROUPING OF RESISTANCE
Grouping of resistance can be done in two types:
 
Resistance in Series
zoom view
Fig. 1.5: Resistance in series
7
From Figure, 1.5.
  • V = V1 + V2 + V3……………………………………(1)
According to Ohm's law,
  • V = IR
From formula (1)
  • V1 + V2 + V3 = IR
  • V1 / I + V2 / I + V3 / I = R
  • R1 + R2 + R3 = R
  • In general, R = R1 + R2 + R3 + ‒‒‒ + Rn
  • Where n is number of resistance in series.
 
Resistance in Parallel
zoom view
Fig.1.6: Resistance in parallel
From Figure, 1.6.
  • I = I1 + I2 + I3……………………………………….(2)
According to Ohm's law,
  • V = IR
From formula (2),
  • V = (I1 + I2 + I3) R
  • 1/R = (I1 + I2 + I3) / V
  • 1/R = I1/V + I2/V + I3/V
  • 1/R = 1/R1 + 1/R2 + 1/R3
  • In general, 1/R = 1/R1 + 1/R2 + 1/R3 + ‒‒‒ + Rn
Where, n is number of resistances in parallel.
 
ELECTRIC CONDUCTANCE
The inverse of electric resistance is called electrical conductance. More the conductivity, more the current passes.
The conductance of an object is defined as the ratio of electric current through it to voltage.8
 
Formula
  • G = I / V
  • Where G is electric conductance of object (in mho)
  • V is potential difference between two points of circuit (in potential difference)
  • I is current passing from conductor (in ampere).
 
Unit
  • Siemens (S) or mho (℧).
 
Symbol
  • G.
 
ELECTRICAL ENERGY OR ELECTRIC POTENTIAL ENERGY OR ELECTROSTATIC POTENTIAL ENERGY
Electrical energy is the energy gained from any form of electricity.
It can be related to following:
  • The energy stored in an electric field
  • The potential energy of a charged particle in an electric field
  • The energy provided by electricity.
This form of energy is not well known as other types of energy like kinetic energy, potential energy, etc. Electrical energy is the presence and flow of an electric charge. The energy portion of electricity is found in a variety of phenomena such as static electricity, electromagnetic fields and lightning.
We are not getting electrical energy directly in ready to use form. Electrical energy can be gained by transformation of other energies into it. For example, by rotating turbines from waterfall, kinetic energy can be converted into electrical energy. From battery, chemical energy can be converted into electrical energy. From sunlight, solar energy can be converted into electrical energy and so on.
 
Unit
  • Joule (J).
 
ELECTRIC POWER
Electric power is the amount of work done by an electric current in a unit time. Electric power is the rate at which electric energy is transferred by an electric circuit. When a current flows in a circuit against resistance, work is done. Instruments can be made those convert this work into heat (electric heaters), light (light bulbs and neon lamps), or motion, i.e. kinetic energy (electric motors).
 
Formula
  • 9
(Replace I or V in P = IV according to Ohm's law V = IR),
Where, P is power (in watts),
  • I is current (in ampere)
  • V is potential difference (in volt)
  • R is electrical resistance (in ohm).
 
Unit
  • Watt (W) or joule per second.
 
Symbol
  • P.
 
JOULE'S LAW OR JOULE'S EFFECT OR JOULE–LENZ LAW
Joule's laws are a pair of laws about amount of heat production in electric circuit and internal energy of ideal gas. They are named after James Prescott Joule.
 
First Law
When electric current passes through any conductor, heat energy is produced in it. It is due to the collision of electrons with the atoms. Joule gave relationship between heat produced in the conductor, intensity of current, resistance of conductor and time for current to pass. It was given by Joule in 1840. This heating effect is known as Joule heating. It is also called the Joule–Lenz law since it was later independently discovered by Heinrich Lenz.
 
Formula
  • Q = I2 Rt
Where Q is the heat generated by a constant current (joule)
I is intensity of electric current (ampere)
  • R is resistance of conductor (ohm)
  • t is time for current to pass in conductor (second).
 
Second Law
It states that the internal energy of an ideal gas is independent of its volume and pressure, depending only on its temperature.
 
COULOMB'S LAW OR COULOMB'S INVERSE-SQUARE LAW
This law describes interaction between electrically charged particles. It was first published in 1785 by Charles Augustin de Coulomb and was essential to the development of the theory of electromagnetism.
Force between two charges is directly proportional to multiplication of two charges values and inversely proportional to square of distance between two charges.10
 
Formula
  • F = kq1q2/r2
  • Where F is force between two electric charges (in newton)
  • k is coulomb force constant or coulomb constant
  • q1 and q2 are two different electric charges (in coulomb)
  • r is distance between two electric charges (in meter).
 
RHEOSTAT
The most common way to vary the resistance in a circuit is to use a rheostat.
 
Construction
t is made up of coil of a resistance wire wound around a semicircular insulator, with the wiper sliding from one turn of the wire to the next. Each turn of wire is insulated from successive turn of wire.
 
Variable Resistance or Series Rheostat (Fig. 1.7)
zoom view
Fig.1.7: Series rheostat
In this wires are in arranged series, so more number of wires when involved in circuit, amount of resistance increases and amount of current decreases. When number of wires in for offering resistance in circuit decreases, amount of current increases. These types of apparatus are not advisable for patients to apply current to their body as current cannot be made zero in these apparatus. They can be used in other apparatus where there is no placement on patients’ body. 11For example, paraffin wax bath where no contact of machine with patients is needed.
 
Potentiometer or Shunt Rheostat (Fig. 1.8)
zoom view
Fig 1.8: Shunt rheostat
It is wired across a course of potential difference and any other circuit has to be taken off in parallel to it. This acts by altering the potential difference between the ends of circuit. When potential difference is more, amount of current will be produced more. When potential difference is less, amount of current will be produced less. This type of apparatus used where direct placement on patient's body as current can be made from zero upto maximum.
 
Other Uses
Wire-wound rheostats made with ratings up to several thousand watts are used in applications such as DC motor drives, electric welding controls, or in the controls for generators.
 
Symbol
 
AMMETER
An ammeter (Fig. 1.9) is the device used to measure the electric current in a circuit. Electric currents are measured in amperes the name given is ammeter.
Most of them are used to measure electric current in milliampere or microampere so they are also known as milliammeters or microammeters. By the late 19th century, better instruments have been designed for more precised measurement.12
zoom view
Fig 1.9: Ammeter
 
Symbol
 
VOLTMETER (FIG. 1.10)
It is an instrument used for measuring electrical potential difference between two points in an electric circuit.
zoom view
Fig. 1.10: Voltmeter
There are two types of voltmeters:
  • Analog voltmeter
  • Digital voltmeter.
Analog voltmeters move a pointer in proportion to the voltage of the circuit, while digital voltmeters give a numerical value in numbers of voltage by use of an analog to digital converter. The first digital voltmeter was invented and produced by Andrew Kay in 1954.13
Voltmeter accuracy is affected by following factors:
  • Temperature
  • Supply voltage variations.
An ideal voltmeter has infinite resistance. It is not possible to make a voltmeter with infinite resistance so a well-designed voltmeter should contain high resistance.
 
Symbol
 
MULTIPLE CHOICE QUESTIONS
1. Electric field lines pass…
  1. From positive charge to negative charge
  2. From positive charge to positive charge
  3. From negative charge to positive charge
  4. From negative charge to negative charge
2. Comment about statement “electric field lines do not cross each other.”
  1. True
  2. False
  3. Cannot be commented
  4. Depends on polarity of charge
3. The electric lines of force enter into any surface at
  1. 180° to surface
  2. 90° to surface
  3. 45° to surface
  4. 0° to surface
4. As the radius of curvature of surface, where lines of force enters increases …
  1. Concentration of lines of force increases
  2. Concentration of lines of force is not affected by radius of curvature of surface
  3. Concentration of lines of force decreases
  4. None of the above
5. Following is not unit of potential difference…
  1. Volt
  2. Joules per coulomb
  3. Both of the above
  4. None of the above
6. Potential difference is measured with…
  1. Ammeter
  2. Voltmeter
  3. Capacitor
  4. Transistor
7. Identify one that is not matching
  1. Potential difference14
  2. Electromotive force
  3. Volt
  4. Potential drop
8. Force between two charges “q1” and “q2” separated by distance “r” will be…
  1. Directly proportional to product of q1, q2 and r2
  2. Directly proportional to product of q1 and q2 and inversely proportional to r
  3. Directly proportional to product of q1 and q2 and inversely proportional to r2
  4. Inversely proportional to product of q1 and q2 and directly proportional to r2
9. The function of the rheostat is …
  1. To alter the voltage in the circuit
  2. To alter the electric current in the circuit
  3. To alter the resistance of the circuit
  4. To alter the power of the circuit
10. Following type of rheostat is used where machine is used for application of current to patient's body directly
  1. Series rheostat
  2. Shunt rheostat
  3. Both of the above
  4. None of the above
11. Pick up correct statement
  1. An ideal voltmeter has infinite resistance
  2. An ideal voltmeter has zero resistance
  3. An ideal voltmeter has ability to alter voltage in the circuit
  4. Ideal voltmeter accuracy is not affected by external factors
12. Function of the ammeter is…
  1. Measurement of resistance of circuit
  2. Measurement of voltage between two points of circuit
  3. Measurement of electric current in the circuit
  4. Measurement of electric power in the circuit
Answers:
1—a
5—d
9—c
2—a
6—b
10—b
3—b
7—c
11—a
4—c
8—c
12—c
Bibliography:
  1. Clayton's Electrotherapy. AITBS Publishers,  2000.
  1. Kr Khokhar. Helpline Electrotherapy for Physiotherapists, 2nd ed. Bharat Bharti Prakashana Co.,  2005.
  1. 15 Singh J. Textbook of Electrotherapy, 1st ed. Jaypee Brothers Medical Publishers (P) Ltd.  2007.
Reference websites:
  1. http://en.wikipedia.org
  1. http://ww.answers.yahoo.com
  1. http://www.circuitstoday.com
  1. http://www.citycollegiate.com
  1. http://www.ee.duke.edu
  1. http://www.hyperphysics.phy-astr.gsu.edu
  1. http://www.thebigger.com
  1. http://www.wisegeek.com