Electricity,.pdf
- 1. STATIC ELECTRICITY AND ELECTRICAL POTENTIAL : Electricity >> elektron (Greek word for amber). Rubbing amber against another material >> transfer of electrons>>static electricity (source of ignition). Static electricity = one object having an excess of electrons and another a deficit. It can also occur as a result of chemical reactions in batteries, and in biological tissues. CONDUCTORS, INSULATORS AND SEMICONDUCTORS: A potential difference >> electrons can pass from one atom to another. Substances may be classified into conductors, insulators or semiconductors. Conductors are usually metals, and carbon (electrones are loosely bound and can readily move through the metal under the influence of an electric potential),and liquids also such as saline can also conduct electricity. In insulators the electrons are firmly bound (not normally able to move) . Semiconductors such as thermistors, transistors and diodes (conductivity is intermediate between that of conductors and insulators)>> electron need extra-energy to escape from the atom ,and conduct electricity (i.e as tempera-ture increases more electrons escape and hence the conductivity increases in thermistors). A photodetector is a special type of semiconductor in the form of a resistor, diode or transistor current). . (electrons which have absorbed the light energy cause an increase in MAGNETIC FIELD: 1-A conductor with a current flowing through it can exert a force on another conductor carrying a current>> magnetism . Though some substances as iron can exhibit magnetism although it appears that no current is flowing through them. 2-The region throughout which a magnet exerts its effects >> magnetic field, and a changing magnetic field induces a flow of electrons in a conductor >> 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 can be increased if a piece of a suitable magnetic material such as iron is placed into the core of the coil (ferromagnetic materials). 3- The term magnetic field strength is used to describe the power of the field in a vacuum, the term magnetic flux is increased greatly over magnetic field strength in ferromagnetic materials.The unit of magnetic flux is the Weber (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. In (MRI) equipment the flux density = 0.2-4 T. DIRECT AND ALTERNATING CURRENT : The term direct current (d.c.) >> steady flow of electrons along a wire in one direction only ,as thermocouples and batteries .In a non-rechargeable battery ( primary cell), the chemical reaction is not reversible but in a rechargeable battery ( seconary cell), the chemical reaction is reversible by the passage of current in a direction opposite to that in which it is supplied by the battery. Primary cells should never be used to replace rechargeable batteries, as an explosion may occur. The term alternating current (a.c.) describes a flow of electrons first in one direction and then in the opposite direction along a wire. alternating current against time may appear as a sine wave .A current which does not have a steady value may be described in terms of an a.c. added to d. c. N.B Magnetic flux gives the “number” of magnetic field lines passing through the area. THE AMPERE AND CURRENT MEASUREMENT: The ampere (A) is the unit of current in the SI system. It represents a flow of 6.24 x 10-18 electrons per second past some point .Whenever a wire carrying an electric current is placed in a magnetic field there is a force which tends to move it in a direction perpendicular to both the electric current and the magnetic field,which is the working principle of the the galvanometer (A coil of wire is suspended on jewelled bearings in a magnetic field). The current to be measured >>coil >> interaction between the electric current and the magnetic field >> coil to rotate ,which is balanced by a hair spring. The deflection of the coil is proportional to the electric current (A pointer over a scale) .Many devices used in anaesthetic equipments are based on the principle of the galvanometer.
- 2. THE ELECTROMAGNETIC FLOWMETER: (An electromagnetic effects). A conductor is moved through a magnetic field>> electric potential develops >>magnitude is proportional to the rate at which the conductor is moving . As blood is a good conductor of electricity>> used to measure blood flow. which lies 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 >> moving conductor >> potential developed as it flows perpendicular the magnetic field, measured by two electrodes, C and D (at sides of the artery). The flowmeter measures an average velocity, and is calibrated with the same type of flow. An alternating rather than a steady magnetic field is used >>improves the stability of the measured value. CURRENT FLOW, ENERGY PRODUCTION AND THE VOLT: As current flows, heat and sometimes light energy is produced it could be 96 watts or 96 joules /second. 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. Potential difference in volts (V) = Power in watts (W) /Current flow in amperes (A) ,so if Potential difference = 96 W and current = 4 A >> potential = 24 V (d.c. powered operating theatre light). HEAT PRODUCTION AND A.C. UNITS: 1-An alternating current with a maximum amplitude of one ampere has a smaller heating effect than a constant direct current of one ampere because, as the peak flow occurs for only a small fraction of the time (commercially it is the heating and energy production that are of interest). So, to it is necessary to relate the current and voltage values of a.c. electricity to the d.c. units so root mean square (r.m.s.) values are used. 2-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. (the sine waves are squared, all the amplitudes are converted to positive numbers,than >>mean of these, a value is achieved which is related to the amplitude of the wave. Finally>> taking the square root of this figure, the equivalent d.c. value is obtained. HEAT PRODUCTION AND FUSES: The rapid increase of heat production by current flow is utilized >> fuses (a correct sized . Diameter) chosen so that if the current exceeds the rating of the fuse the wire rapidly heats and then melts>>stopping the current. ELECTRICAL BURNS: If a high voltage source is touched, burns could occur at the finger tip where the current flow is concentrated through a small area. The current flow per unit area (current density). SURGICAL DIATHERMY: 1-Current above 1 kHz >>minimal effect on humens ,and negligible above 1 MHz ( in diathermy), but but>> heating effect , and coagulation in all frequencies( depends on the current density).2-The diathermy has two connections to the patient, the neutral or patient plate and the active or cutting electrode used by the surgeon, in the cutting electrode the current >> a very small area>> local heating and burning. No burning should occur at the patient plate (large area).If for any reason the area (reduced)>> a risk of burns. 3-If the patient plate becomes completely detached, or is not attached correctly, the current may flow to earth through any point >>patient is touching an earthed metal object >>possible for a burn >> a patient's hand touches the operating table>> isolated type of diathermy circuit increase safety. It can also, go to the ecg leads>> burn there,so,better to be away from the active electrode. 4-Even without contact the electromagnetic waves can pass through capacitor linkage (by their high frequency). 5-The thin insulator draps may lead to burns,also,areas with small cross area (digits,ear) can be affected, also the implanted metal with less resistance can make the current pass. 5- The pace maker !! 6-Another type of diathermy circuit does not require a patient plate(bipolar diathermy)>> sides of the forceps>>used in applications such as neurosurgery >> localizes the current and prevent excessive tissue damage. Howerver, monopolar diathermy must be used in many required cutting situations. Electrical symbols
- 3. CAPACITANCE AND ELECTRIC CHARGE: Capacitance = ability of an object to hold electric charge, 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 second. Coulombs (C) = Amperes (A) x Seconds (s). (C) = 6.24 x 10-18 electrons The Defibrillator: 1- An instrument in which electric charge is stored and then released (The key component for storing the charge is a 2- potential of 5000 V is applied across the two plates of a capacitor>> electrons equivalent to 160 mC of charge. 3-Potential (V) =Power (W)/Current (A)----V = J s-1/C s-1---V=J/C >> J = CV.The energy needed to move of an electrical charge through a potential difference (E==VXQ) .If the potential difference between the capacitor plates increases>>more E . is needed to add extra charge>>. capacitor ) >>The total E stored by integration>> Energy = 1/2 x 160 mC x 5000 V = 400 J 4-This current pulse>>35 A for 3 ms, gives a synchronous contraction of the myocardium . The inductor makes the pulse in an optimum shape and duration. When the defibrillator is discharged, the inductor absorbs some of the energy stored (not all of it is delivered to the patient)>> 360 J. and 100 J, for use with internal cardiac electrodes. Synchronized mode in supraventriclular dysrhathmias >>the energy delivered at a correct time in the cardiac cycle, i.e. during the R wave( if the pulse is mistimed >> ventricular fibrillation). 5- In biphasic ventilators>>two waves in two different directions>> lower energy>>less damage. plates but will die away ). A capacitor will however allow alternating current to pass as it is continuously being charged and discharged ,so allowing current to flow. ECG with a.c. interference from capacitance (an alternating current pass across an air gap ) explains why electrical interference may appear on an ECG trace. (an operating theatre light is separated from a patient(patient acts as a plate ) by an air gap. Mains electricity at a frequency of 50 Hz is passing through the light. A small mains frequency current passes from the lamp to the patient causes a 50 Hz voltage to appear on the ECG trace. This interference may be of sufficient amplitude to obscure the recording. Electrostatic Charge: The ability to store a charge is not limited to a capacitor and a charge, usually called an electrostatic charge, can build up the bobbin in a variable also, insulators may develop charges of this type on their surfaces with risks of sparks. CAPACITANCE AND INTERFERENCE : A capacitor cannot directly conduct direct current (current will flow at first to charge up the capacitor INDUCTANCE AND INTERFERENCE: Many components of electrical apparatus have coils of wire with strong magnetic fields (the transformer ),so ECG leads and this source of interference >> inductive coupling, may lead to interference appearing on the tracing, which can be reduced by a system known as screening. The monitoring leads are covered by a sheath of woven metal which is earthed, so that interference currents are induced in this metal screen and not in the signal leads. Signal to Noise Ratio: Interference from capacitance and inductance are given the term 'noise' (unwanted sound) The signal to noise ratio is improved with an electronic filter (by diathermy signals) .The signal to noise ratio cannot be improved by amplifying the waveform because both the signal and the noise will be increased by the same factor, but the ratio can be improved by averaging a repetitive waveform, e.g. the signal, over several cycles (the noise is averaged to zero )as of its random nature ,whereas the signal remains constant if it is repetitive.
- 4. RESISTANCE : 1-The unit of electrical resistance is the ohm (Ω), which will allow one ampere of current to flow under the influence of a potential of one volt. 2-As temperature rises it increases the resistance (temperature measurement ) of a wire resistor but reduces that of many semiconductors. 3-If a wire is stretched it becomes longer and thinner and therefore its resistance increases. Such resistors are known as strain gauges (movements of the diaphragm with changes of pressure alter the tension in the resistance wire). and displayed as a measure of pressure change. 4-To monitor or measure such changes of resistance, a circuit known as a Wheatstone bridge is often used.( set of four resistors, a battery,and a galvanometer). R4 could be a strain gauge transducer or a resistance thermometer, while R3 is a variable resistance which can be adjusted until the galvanometer reads zero. In this condition the bridge is said to be balanced (a null deflection system, i.e. no Current flows). The output is amplified and connected to a recorder or oscilloscope in place of the galvanometer. The bridge is said to be balanced when = 5-Most pressure transducers contain four strain gauges which form the four resistances in the Wheatstone bridge. 6-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. Sensitivity switches in apparatus often employ a pattern of resistors of this type and in the position shown on the right of the figure potential V3 is attenuated to 80% of V1. IMPEDANCE :The resistance of a resistor to the flow of alternating current does not vary with the frequency of the current. A capacitor can transmit alternating current as mentioned earlier, but in this case high frequency current passes through it more easily. With an inductor the reverse is true. The term impedance is used in preference to resistance when there is a dependence on frequency. The unit of impedance is the same as that of resistance, i.e. the ohm. However, impedance is often indicated by the symbol Z. A practical example of the use of the variation of impedance with the frequency of the current is the isolating capacitor present in some diathermy circuits. It offers a high impedance to mains frequency current and so protects the patient from electrocution. Skin Impedance and Attenuation: 1-Example of impedance >>conduction of a biological electrical signal such as the ECG through the patient's skin to the electrode as, in addition to the resistance at the point of contact, capacitance and inductance may affect the signal. Impedance is less if good contact is maintained at the point where the electrode touches the skin. 2-Electrode consists of a metal disc covered with conductive electrode gel to reduces the electrical impedance at the point of contact with the skin .The degree of attenuation of the signal depends not only on the skin impedance but also on the input impedance of the monitoring apparatus used to detect it. 3-An example in which there is a high impedance ZE from the skin and electrodes. This could arise from dried electrode gel or a loose electrode contact. In addition, there is a relatively low amplifier input impedance ZA. The signal is greatly attenuated .The diagram on the right shows an amplifier with a high input impedance used with electrodes with good skin contact to give negligible attenuation of the input signal. Most amplifiers now have a very high input impedance to allow satisfactory ECG recordings even with the simpler and smaller skin electrodes. A high amplifier input impedance also protects the patient against electrocution .Skin impedance is lowered when the skin is moist>> sympathatic activity>> a test is known as the galvanic skin response, or GSR.