1. Electricity can be produced through static electricity via rubbing materials like amber, or through chemical reactions in batteries and biological tissues, producing an electric potential difference. 2. Substances are classified as conductors, insulators, or semiconductors based on their ability to conduct electrons. Conductors like metals readily conduct, while insulators tightly bind electrons. Semiconductors conduct under certain conditions like increased temperature. 3. A current flowing through a conductor or a magnet produces a magnetic field, and a changing magnetic field induces a current. The magnetic field strength and flux are measured in units of tesla and weber. Direct current flows in one direction while alternating current reverses direction in a sine wave

1. Electricity
STATIC ELECTRICITY AND ELECTRICAL POTENTIAL
The word electricity >>'elektron', the Greek word for amber. Rubbing amber against another material
can lead to a transfer of electrons >>one of the substances will have an excess of them (plastics and
rubber)>>static electricity. Its a source of ignition in anaesthetic explosions. This phenomenon also
occurs as a result of chemical reactions in batteries, and in biological tissues >> electric potential
difference.
CONDUCTORS, INSULATORS AND SEMICONDUCTORS Substances may be classified into conductors, insulators or semiconductors according to their ability to
conduct electrons. Conductors are usually metals, and carbon is also a good conductors (outer
electrons of atoms of a
metal are loosely bound >> readily move by an electric potential). Some liquids such as saline and body fluids can also conduct electricity. In insulators the electrons are firmly
bound (not able to move and form an electric current). Semiconductors (thermistors, transistors and diodes). >> the outer electrons are bound to atoms less firmly >> extra
energy >> escape from the atom>> conduct electricity. In a thermistor >>as the temperature increases more electrons escape and hence the conductivity increases
>>measure temperature- Excessive heat will adversely affect the performance of monitoring equipment containing semiconductors-A photodetector is a special type
of semiconductor in the form of a resistor, diode or transistor. When radiation falls on the detector, electrons in the material absorb some of the energy of radiation
and are able to move>> increase in current .
MAGNETIC FIELD A conductor with a current flowing through >>exert a force on another conductor carrying a current; >>magnetism. Some substances as iron alloys
>>exhibit magnetism although it appears that no current is flowing through them. Magnetism is due to the sum of the many minute currents formed
by the motion of
electrons orbiting their nuclei. The region throughout which a magnet or a current-carrying conductor exerts its effects >> magnetic field, and a changing magnetic field induces a
flow of electrons in a conductor to produce an electric current. If a conductor is wound into a coil, a current flowing in it produces a magnetic field which is strongest within the core
of the coil. The strength of this magnetic field >>increased if a piece of a suitable magnetic material such as iron is placed into the core of the coil. These ferromagnetic materials
are strongly attracted into a magnetic field.
The term magnetic field strength is used to describe the power of the field in a vacuum>>magnetic flux (may be greater than the original magnetic field strength). Magnetic flux is
increased greatly over magnetic field strength in ferromagnetic materials, The unit isWeber (Wb),The unit of magnetic flux density is the Tesla (T), being Wb /m2. The magnetic flux
density produced in air by the earth's magnetic field is approximately 60 Μτ.
DIRECT AND ALTERNATING CURRENT
The term direct current (d.c.) >>flow of electrons along a wire in one direction only, as thermocouples and batteries
(chemical energy is converted into electrical energy) >>primary cell >>non-rechargeable, but in a rechargeable battery (
secondary cell), the chemical
reaction is reversible >>passage of current in a opposite direction. An attempt to recharge a primary cell >>generation of gas >>destruction of or
explosion .The term alternating current (a.c.) >> flow of electrons first in one direction and then in the opposite direction along a wire >>a sine wave.
Current which does not have a steady value ( an a.c. component added to a d.c).are also,present.
THE AMPERE AND CURRENT MEASUREMENT The ampere (A) is the unit of current in the SI system>> flow of 6.24 x 10 18
electrons per second past some poin (electromagnetic force which is associated
with an electric
current). whenever a wire carrying an electric current is placed in a magnetic field >>force which tends to move it in a direction perpendicular to
both the electric current and the magnetic field>> the working principle of the galvanometer(a coil of wire is suspended on jewelled bearings in a
magnetic field-Current to be measured passes through this coil>>rotate>> force on the coil is balanced by a hair spring-Pointer which moves over a
scale-Some anaesthetic equipment are based on the principle of the galvanometer.
THE ELECTROMAGNETIC FLOWMETER If a conductor is moved through a magnetic field>>electric potential >>magnitude is proportional to
the rate at which the conductor is moved .As blood (conductor of electricity)>> measure blood flow.
Consider the
artery lying in a magnetic field produced by the current-carrying coils A and B, the direction of the field being perpendicular to the flow of blood. The blood >> a moving
conductor and the potential developed as it flows through the magnetic field is perpendicular to both the direction of flow and the magnetic field-Potential can be measured by two
electrodes, C and D, touching opposite sides of the artery. An electromagnetic flowmeter (C-shaped probe, which contains coils to generate the magnetic field, and two electrodes
to measure the potential induced as the blood flows) >>potential is proportional to the rate at which the blood is flowing, the velocity of the blood varies across the diameter of the

2. flowmeter measures an average velocity)- The flowmeter is calibrated -An alternating rather than a steady magnetic field is used and
the alternating component of the potential is measured>> stability of the measured value.
CURRENT FLOW, ENERGY PRODUCTION AND THE
VOLT
When a current flows>> heat and sometimes light energy is produced and
energy is also produced. In an emergency d.c. powered operating theatre
light >>energy
production of heat and light could be 96 watts or 96 joules of energy per second. In order to drive the electric current a potential
difference must be present across the bulb. The unit of potential difference is the volt, and this is based on the (energy production and
current flow).
The volt is defined as that potential difference which produces a current of one ampere in a substance when the rate of energy
dissipation is one watt. volts (V) = Power in watts (W) /Current flow in amperes (A)>>In the example shown>> Potential difference = 96
W/4A = 24 V
HEAT PRODUCTION AND A.C. UNITS Heat production from electric current flow is also used as a basis for comparable a.c.
units of measurement. An alternating current with a maximum amplitude of one
ampere has a smaller
heating effect than a constant direct current of one ampere because, in the case of alternating current, the peak flow occurs for only a
small fraction of the time. commercially it is the heating and energy production that are of interest. It is necessary to relate the current
and voltage values of a.c. electricity to the d.c. units so that the heating effects are the same>>root mean square (r.m.s.) values. An
r.m.s. voltage of 240 V (the mains potential in the United Kingdom) has an identical heating effect to that of a d.c. voltage of 240 V,
when applied across an identical resistor, but its peak voltage is 340 V.- All the amplitudes are converted to positive numbers-Finally,
by taking the square root of this figure, the equivalent d.c. value is obtained.
HEAT PRODUCTION AND FUSES Increase of heat production with increasing current flow >>fuses melt-Fuse is a wire of a correct
size chosen diameter >>current exceeds the rating of the fuse >> melts>>stopping the current, for
safety .
ELECTRICAL BURNS Shock from mains electricity >>risk of ventricular fibrillation or if only passes through a limb >> local heating and
burns (finger tip) >>flow is concentrated through a small area)>> current density.
SURGICAL DIATHERMY Current density >>heating effect of surgical diathermy.The passage of direct current or low frequency alternating
current>>stimulate muscular contraction and give a risk of ventricular fibrillation.Its less as the frequency of the
current
increases, being small above 1 kHz and negligible above 1 MHz ,but the heating effect and burning can occur at all frequencies.
Diathermy equipment >>high frequency (1 MHz) >>cutting and coagulation ,depending on the current density-Two connections: 1-to
the patient ( the neutral plate) and 2-the (active) or cutting electrode used by the surgeon >>local heating and burning occurs, and no
burning should occur at the patient plate (current flows through a large area).If for any reason the area of contact is reduced
>>current density is increased with a risk of burns. If the patient plate becomes completely detached , or is not attached correctly,
current may flow to earth through any point at which the patient is touching an earthed metal object ( burn produced where a
patient's hand touches the operating table)-Isolated type of diathermy circuit increase safety-In bipolar diathermy (no need for neutral
plate) >> electric current travels down one side of a special forceps through the tissues and back through the other side of the
forceps>> but using lower power as in neurosurgery (localize the current!!)-The current may take another route when the active electrode is not
in contact with the patient>>capacitances (have low impedance to high frquency current) ,also to the ECG electrodes>>burns- Insulator drapes>>burns-
Current density>>small cross sections as digits -low resistance of implants>>current to flow-The pace- makers !! . PROPERTIES OF ELECTRICAL
COMPONENTS (SEE LAST SLIDE)
CAPACITANCE AND ELECTRIC CHARGE
Capacitance (ability of an object to hold electric charge),charge being a measure of the amount of electricity. The SI unit of charge is the
coulomb (C).The coulomb is the quantity of electric charge which passes some point when a current of one ampere flows for a period of one

3. The
Defibrillator
Electric charge which is stored and then released in a controlled fashion- The key component (capacitor) >> two plates separated by an insulator- Its
maximum setting of a potential of 5000V (across the two plates of a capacitor)>> 160 mC of charge. Power (W) = Energy (J) per second -Current
(A) = Charge (C) per second
1- V=W/A 2-V = j s-1 / C s-1 3- J = CV **By integration
Stored energy (J) = 1/2 Stored charge (C) x Potential (V) In the example >>Energy = 1/2 160 mC x 5000 V = 400 J
Electrodes (the sternum and over the apex of the heart)>>current pulse=35 A for 3 ms>> normal beats may follow-An inductor is included
>> electric pulse has an optimum shape and duration. It absorbs some of the energy stored in the capacitor so not all of it is delivered to
the patient-The maximum delivered energy is normally 360 J,and 100 J, for use with internal cardiac electrodes in a patient with an open
chest-Treating certain dysrhythmias >>synchronized mode >> E.supplied at the correct time in the cardiac cycle, i.e. during the R wave ( risk of
causing ventricular fibrillation). This monophasic type of defibrillators is replaced with a biphasic type (lower energy).
Electrostatic
Charge
Stored charge is not limited to a capacitor- A charge (electrostatic charge) can build up on the surface of any object insulated from its
surroundings (bobbin in a variable orifice flowmeter) as friction of the bobbin with the wall >>bobbin sticking. Similarly, insulators may also,
develop charges>> sparks (flammability). agents) .
CAPACITANCE AND INTERFERENCE
When direct current is applied to an uncharged capacitor, some current will flow at first to charge up the capacitor plates but will
die away as the capacitor becomes fully charged. Operating theatre light is separated from a patient by an air gap, mains
electricity>> a frequency of 50
Hz is passing through the light-The patient acts as one plate of an capacitor with the electric light acting as the other plate. A small current>> lamp to
the patient (50 Hz voltage ) appears on the ECG trace (interference)>>obscure the recording. Capacitance effects are also used as the working
principle of some pressure transducers>>displaced diaphragm>>changes in the capacitor.
INDUCTANCE AND INTERFERENCE An inductor (coil of wire) -Many components of electrical apparatus have coils of wire >> strong
magnetic fields around them>>cause interference in the ECG leads>> inductive coupling -Magnetic
field formed by the
transformer in near by electrical apparatus>> reduced by a system known as screening>>leads are covered by a sheath of woven metal which is
earthed>>interference currents are induced in this metal screen and not in the signal leads.
Interference from capacitance and inductance )'noise'( - ECG waveform with a high signal to noise ratio, whereas on the
right is a similar waveform with a low signal to noise ratio- Unwanted distortion of a biological signal, such as an ECG by
diathermy, can be
Signal to Noise
Ratio
avoided by using an electronic filter .The signal to noise ratio cannot be improved by amplifying )signal and the noise will be increased
similarly( -Improved by averaging a repetitive waveform, e.g. the signal, over several cycles ) electronically(. The noise is averaged to
zero because ) its random nature(, whereas the signal remains constant >>(repetitive).
RESISTANCE The unit of electrical resistance is the ohm (Ω) The ohm is that resistance which will allow one ampere of current
to flow under the influence of a potential of one volt. Resistance(Ω) =Potential(V) / current (A) >> Ohm's law.
Temperature rise increases the resistance wire resistor but reduces that of many semiconductors>>Thermometers .If a wire is stretched
>>longer and thinner >>resistance increases (strain gauges) >> pressure transducers ( movements of the diaphragm with pressure >>
changing wire resistance)>>Pr. Wheatstone bridge (measure such changes of resistance)>>set of four resistors-battery-galvanometer-RA
could be a strain gauge transducer or a resistance thermometer-R3 is a variable resistance>>adjusted until the galvanometer reads zero-
The bridge is said to be balanced (A null deflection system)_ R1/R2=R3/R4 -the output is amplified and connected to a recorder or
oscilloscope in place of the galvanometer-Another arrangement of resistors R1 to R5 with a potential V1 applied across them. If the five resistors
are of equal value, by tapping off from point A to one of the other points B, C, D, or E, a reduced potential may be obtained and in the example
potential V2 is 20% of V1 i.e. it is attenuated.

4. IMPEDANCE
The resistance of a resistor by alternating current does not vary with the frequency of the current. A capacitor can transmit alternating current ,but high
frequency current passes more easily-With an inductor the reverse is true circuits. Monitors (resistors, capacitors and inductors) possess a particular
resistance to the flow of electric
current>> frequency of the current- The unit of impedance is the same as that of resistance, i.e. the ohm- Impedance is often indicated by the symbol Z, its preference to resistance
when there is a dependence on frequency-Isolated capacitor present in some diathermy>> high impedance to mains frequency current>>electrocution.
Skin Impedance and
Attenuation
Impedance is less if good contact is maintained at the point where the electrode touches the skin.
ECG electrode (metal disc covered with conductive electrode gel)>>reduces the impedance -The
degree of
attenuation of the signal depends not only on the skin impedance but also on the input impedance of the monitoring apparatus. 1-A high
impedance ZE from the skin and electrodes (less gel or a loose electrode contact),with low amplifier input impedance ZA >>signal is greatly
attenuated>>ratio of the two impedances ZE and ZA, 2- An amplifier with a high input impedance used with electrodes with good skin
contact>> negligible attenuation of the input signal, also protects the patient against electrocution- Skin impedance is lowered when the
skin is moist>>sympathetic tone>> measure of the autonomic activity(galvanic skin response).
12-ELECTRICITY: (QUESTIONS)
A-( G.REASONS ) THE IMPORTANCE OF THE NEUTRAL PLATE
OF THE DIATHERMY.
B-HOW DOES THE DEFIBRILLATOR WORK?
PROPERTIES OF ELECTRICAL COMPONENTS