Conductor and Insulators (Forester’s; Low and Reed;
Conductors are materials that permit electrons
to flow freely from particle to particle. In these, the electrons in the
outermost shell of an atom are loosely bound and free to move
through the material. Good conductors will permit charge to be transferred across the entire
surface of the object. When any charge is transferred to an object at any given
location, it gets distributed across the entire surface of the object. This
distribution of electric charge is due to electron movement. Since conductors
allow for electrons to be transported from particle to particle, a charged object
will always distribute its charge until the overall repulsive forces between
excess electrons is minimized.
In contrast, insulators are materials that don’t permit free
flow of electrons from particle to particle. In these materials, the electrons
in the outermost shell are tightly bound and hence don’t permit the free flow
of electric current through their surface. Whenever the charge is transferred
to insulated object, the excess charge will remain at the initial location of
charging and the charge is seldom get distributed evenly across the surface.
They play a crucial role in electrostatic experiments and demonstrations since
it prevents the charge to be transferred to the surroundings.
The effect produced by electric charge,
when stationary, is often referred to as static electricity. This refers to an
imbalance of electric charge within or on the surface of a material,
particularly insulators. The charge can be held until it is able to move away
by means of electric discharge of electric current. This is a well-known
phenomenon which occurs as a result of rubbing two insulators together so that
electrons are rubbed off one and on to the other. A homely example is the
crackling resulting from pulling nylon clothing over the head in dry conditions
or the sparking that can sometimes be produced by rubbing clothing on plastic chairs.
If a polythene rod is rubbed with a piece of dry nylon the rod and the nylon
can attract little pieces of paper. This occurs because electrons are rubbed
off the nylon on to the rod which is therefore now negatively charged and thus
attracts and will hold the uncharged pieces of paper.
field (Forester’s; Low and Reed; Prentice)
Electric field is defined as the
electric force per unit charge. Electric lines of force indicate the direction
of the field – the direction in which a charge would move if free to do so. By
convention, the strength of the electric field is denoted by the number of
lines passing through unit area. The closer the lines are together, the
stronger the force in this region of the field.
The direction of the field is taken to
be the direction of the force it would exert on a positive test charge. The
electric field is radially outward from a positive charge and radially in
toward a negative point charge. The electric lines of force are governed by
Coulomb law which states that the force between the two charges are inversely
proportional to the distance between them.
Electric Field (www.en.wikipedia.org)
Electric Lines of Force
from Point Object (www.en.wikipedia.org)
of Electric Lines of Force (brilliant.org/wiki/electric-field-lines)
1. Electric field lines always begin on
a positive charge and end on a negative charge, so they do not form closed
curves. They do not start or stop in midspace.
2. The number of electric field lines
leaving a positive charge or entering a negative charge is proportional to the
magnitude of the charge.
3. Electric field lines never
4. In a uniform electric field, the
field lines are straight, parallel and uniformly spaced.
5. The electric field lines can never
form closed loops, as line can never start and end on the same charge.
6. These field lines always flow from
higher potential to lower potential.
7. If the electric field in a given
region of space is zero, electric field lines do not exist.
8. The tangent to a line at any point
gives the direction of the electric field at the point. Also, this is the path
on which a positive test charge will tend to move if free to do so.
and Capacitance (Prentice)
Capacitors are devices used to store
electric charges, based on the principle of electric induction. They are
basically a pair of conductors separated by an insulator which is the
dielectric. The amount of charge that can be held will depend on the size of
the capacitor, i.e. the area of the opposing plates. It will also depend on the
nature of the dielectric measured as the dielectric constant and the distance
separating the conductors, i.e. the thickness of the dielectric. It is measured
in farads. The capacitors for
use in electrical devices are made to maximize their capacitance in a
conveniently small size.
force (EMF) (Forester’s; Low and Reed; Prentice)
force, abbreviation E or emf, is
defined as energy per unit electric charge, that is imparted by an energy
source, such as an electric generator or a battery. Energy is converted from
one form to another in the generator or battery as the device does work on the
electric charge being transferred within itself. One terminal of the device
becomes positively charged, the other becomes negatively charged. The work done
on a unit of electric charge, or the energy thereby gained per unit electric
charge, is the electromotive force. Electromotive force is the characteristic
of any energy source capable of driving electric charge around a circuit. It is
commonly measured in units of volts (www.britannica.com/science/electromotive-force;
Electromagnetic Spectrum (Forester’s; Low
and Reed; Prentice)
radiation refers to the waves of the electromagnetic field, propagating
(radiating) through space-time, carrying electromagnetic radiant energy.
Electromagnetic radiation spans an enormous range of wavelengths and
frequencies. This range is known as the electromagnetic spectrum. The EM
spectrum is generally divided into seven regions, in order of decreasing
wavelength and increasing energy and frequency. The common designations are:
radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays
and gamma rays. These radiations are characterized by the following:
They may be produced when sufficiently
intense electrical or chemical forces are applied to any material.
They all travel readily through space at
an equal velocity.
Their direction of travel is always in a
They may be reflected, refracted,
absorbed, or transmitted, depending on the specific medium that they strike.
When electromagnetic radiations strike
or come in contact with various objects, several things may happen. Some rays
may be reflected, whereas others are transmitted through the
tissues, where they may be refracted. Still others penetrate to deeper layers where
they may be absorbed. Generally, those radiations that have the longest
wavelengths tend to have the greatest depths of penetration regardless of their
GOVERNING ELECTROMAGNETIC RADIATIONS (Prentice)
The Arndt-Schultz principle states
that no reactions or changes can occur in the body tissues if the amount of
energy absorbed is insufficient to stimulate the absorbing tissues.
The inverse relationship that exists
between energy absorption by a tissue and energy penetration to deeper layers
is described by the Law of Grotthus-Draper. Therefore, only that light
which is absorbed by a system can bring about a photochemical change.
Cosine law states that the amount of
absorption of radiant energy is directly related to cosine of angle between the
propagating ray and normal
The intensity of the radiation striking
a particular surface is known to vary inversely with the square of the distance
from the source.
OF CURRENT (Prentice, 2nd edition)
(galvanic current) has an uninterrupted unidirectional flow of electrons toward
the positive pole. On most modern direct current devices, the polarity and thus
the direction of current flow can be reversed.
Direct Current (Google
the continuous flow of electrons constantly changes direction or, stated
differently, reverses its polarity. Electrons flowing in an alternating current
always move from the negative to positive pole, reversing direction when
polarity is reversed.
(Google images, www.circuitglobe.com)
or Pulsitile currents usually contain three or more pulses
grouped together. These groups of pulses are interrupted for short periods of
time and repeat themselves at regular intervals. Pulsed currents are used in
interferential and so-called Russian currents
(Google images, www.slideplayer.com/slide/5822235)
To convert current
coming from an AC power source to a DC current delivered to the patient is
accomplished by a series of electrical components within the stimulating unit:
a transformer, a rectifier, a filter, a regulator, an amplifier, and an
oscillator. A transformer “steps down” or reduces the amount of voltage
from the power supply. The rectifier converts AC current to pulsating DC
current. The filter changes the pulsating DC current to smooth DC. The regulator
produces a specific controlled voltage output. An output amplifier within
the stimulating unit is used to magnify or increase the amplitude of the
voltage output of the generator and control it at a specific level, regardless
of the electrical impedance of the remainder of the circuit (including the
electrodes and patient). The oscillator is used to produce and output a
specific waveform, which again may be different from that used to power or
drive the stimulating unit (Prentice).
(Prentice, 2nd edition)
The term waveform indicates a
graphic representation of the shape, direction, amplitude, duration,
and pulse frequency of the electrical current being produced by the
electrotherapeutic device, as displayed by an instrument called an
Electrical currents may take on a sinusoidal,
rectangular, square, or spiked waveform configuration,
depending on the capabilities of the generator producing the current
(Prentice, 2nd edition)
On an oscilloscope, an individual
waveform is referred to as a pulse. A pulse may contain either one or
two phases, which is that portion of the pulse that rises above or below
the baseline for some period of time.
that have only a single phase in each pulse. Current flow is unidirectional,
always flowing in the same direction toward either the positive or negative
pole. Direct current is an example of monophasic pulse.
Waveforms that have two separate phases during each individual pulse. Current
flow is bidirectional, reversing direction or polarity once during each pulse.
Biphasic waveforms may be symmetric or asymmetric. If both phases
of the waveform are symmetric, the shape and size of each phase is identical.
Conversely, in asymmetric type, the shape and size of each pulse is not
Pulsed current waveforms are called polyphasic and are representative of
electrical current that is conducted as a series of pulses of short duration
(µsec) followed by a short period of time when current is not flowing called
the inter-pulse interval (msec). Single pulses may be interrupted by an intra-pulse
interval. Pulsed current may flow in one direction as in DC current or may
reverse direction of flow as in AC current. With pulsed currents there is
always some interruption of current flow.
Pulsed Current (google images,
The amplitude of each pulse reflects the
intensity of the current, the maximum amplitude being the tip or highest point
of each phase. The term amplitude is synonymous with the terms voltage and
current intensity. The higher the amplitude, the greater the peak voltage or
Pulse Amplitude (Google
The term pulse charge refers to
the total amount of electricity being delivered to the patient during each
pulse. With monophasic current, the phase charge and the pulse charge are the
same and always greater than zero. With biphasic current, the pulse charge is
equal to the sum of the phase charges. If the pulse is symmetric, the net pulse
charge is zero. In asymmetric pulses the net pulse charge is greater than zero.
Rate of Rise and Decay Times
The rate of rise in amplitude, or
the rise time, refers to how quickly the pulse reaches its maximum amplitude in
each phase. Conversely, decay time refers to the time in which a pulse
goes from peak amplitude to 0 V.
Rise and Decay time
The duration of each pulse indicates
the length of time current is flowing in one cycle. With monophasic current,
the phase duration is the same as the pulse duration and is the time from
initiation of the phase to its end. With biphasic current, the pulse duration
is determined by the combined phase durations.
Pulse frequency indicates the
number of pulses per second. Each individual pulse represents a rise and fall
in amplitude. As the frequency of any waveform is increased, the amplitude
tends to increase and decrease more rapidly.
Pulse Duration (Google
of Heat Transfer (Low and Reed)
The heat can be transferred from one
body to another by the following mechanisms:
Conduction- This is defined as heat
transfer from one point to another through direct contact between the two
bodies and without noticeable movement in the conducting medium. The modalities
working on this principle of heat transfer include paraffin wax bath,
hydrocollator packs, electric heating pads etc.
Convection- This form of heating is
produced by the movement heating medium, usually air or a fluid around an
object. The modalities working on these principle of heat transfer include
Fluidotherapy, whirlpool, moist air baths, and hot air baths.
Radiation- This involves conversion of
one form of energy (usually non-thermal) to heat. In this method, the
intervening medium is not mandatory. The classical example is the heating
effect produced by the sunlight. The energy is transferred from an object at
higher temperature to an object at lower temperature.
Methods of Heat Transfer (Google
PHYSIOLOGICAL EFFECTS OF HEAT (Low
physiological effects produced by superficial heating modalities on human body
are as follows:
1. Effect on blood flow-
Application of heat leads to increased blood flow which is apparent due to
increased redness or erythema of the skin.
2. Effect on metabolism-
Application of heat leads to increased metabolism and metabolic rate.
3. Inflammatory response and edema-
Application of heat leads to early resolution of edema.
4. Effect on musculoskeletal system-
Superficial heat promotes relaxation of muscles and cause increased drainage of
5. Effect on adhesion formation-
Superficial heating modalities leads to softening of soft tissues thereby
prevents the formation of adhesions.
6. Analgesia- Due
to effect of increased circulation, the noxious stimuli gets drained thereby
leading to decrease of pain.
WAX BATH (PWB) UNIT (Forester’s; Low and Reed; Prentice)
Interesting facts about this modality
involves the following:
This modality is an electrically heated
modality and involves application of molten paraffin wax on body.
about 6 times more heat than given by water at same temperature.
The mode of transmission of heat is by
The temperature at which the paraffin
wax is maintained is 40-44ºc.
Melting point of wax is 51-55ºc (54ºc).
The wax at this temp can cause burns if poured directly. So some impurity is
added in the form of mineral oil or liquid paraffin. Combination of impurity
lowers the specific heat which enhances the patient’s ability to tolerate the
heat better than from water. Ratio of wax: paraffin: petroleum jelly is 7:3:1
and wax: liquid paraffin is 7:1.
Paraffin Wax bath Unit
(Google images, indiamart.com)
of PWB Unit
Wax is contained
in a stainless steel or enamelled baths and outer fiberglass shells. The
control panel consists of the following:
Part to be treated is washed with
soap and moisture soaked by towel
Position the patient comfortably
and near to PWB apparatus.
Direct pouring method- Molten wax
is poured directly on the body by a mug or utensil, then wrapped by a
towel. 4-6 layers are made over the body tissue.
Dip and wrap method- This is the
best method of application. The body part is immersed directly into a
container of wax and then taken out. Once wax solidify, the part is again
dipped again let the wax solidify. Repeat this for 4-6 times and then wrap
it with towel.
Brushing method- Wax is applied
with the help of brush. 4-6 coats are made and is covered by towel. Brush
of various sizes (4′ or 6′) can be used.
Toweling or bandaging method- Towel
or roll is immersed in the wax and is then wrapped around body part. This
method is used for treating proximal part of body.
Impaired skin sensation
Defective arterial supply
Cancer and tuberculosis
Lack of comprehension
Deep X-ray therapy
Post immobilization stiffness
of PWB Unit
The PWB must be
maintained to avoid any complications during the treatment. Following points
must be kept in mind:
Sterile the wax by heating it to
Drain the melted wax, filter it out
and replace it for reuse.
Change the wax atleast once in 6
PACK UNIT (Forester’s; Low and Reed; Prentice)
points about the hydrocollator pack unit include:
The unit is made up of
thermostatically controlled electrically heated stainless steel chamber in
which hot packs of different sizes are immersed. The capacities of
different units vary but all the units have insulated bases. The unit
contains wire rack to hang the packs inside the unit.
Provide superficial moist heat to
Packs are stored in a
thermostatically controlled water bath inside equipment. Temperature
inside the unit is 65-80ºc and rise in skin temperature by application of
packs is between 40-45ºc. The packs take 30 minutes to 2 hours to get
The hydrocollator packs are available
in various sizes and shapes. The pack is selected according to part to be
treated. The common sizes are small, large and contoured.
The packs are usually made of silicate
gels such as Bentonite enclosed in a canvas cover. This gel has the
property to absorb large quantity of water. The gel is contained in a set
of separate fabric pockets like a duvet, so that whole of pack is
flexible and the gel is confined.
of Hydrocollator Packs
Packs are taken out with the help
of tong and is wrapped inside towel. 6-8 layers (1-2 cms) of towel is made
around the pack.
It takes some time (8 minutes) for
skin temperature to reach maximum. During this time the pack temperature
is falling but the toweling and pack prevent the skin surface from losing
heat so that the skin and superficial tissues temperature rises. So the
skin response should be checked after 10 minutes of application of pack.
Total treatment time is 15-20
Pack Unit (Google Images, www.indiamart.com)
of Hydrocollator Pack Unit
panel consists of the following:
and Uses of Hydrocollator Packs
Relief of muscle spasm
Local rise in temperature- by
Increase of local circulation
Effect on skin and connective
Relief of pain
Impaired skin sensation