خدمة تلخيص النصوص العربية أونلاين،قم بتلخيص نصوصك بضغطة واحدة من خلال هذه الخدمة
This laboratory work explores the impedance of biological objects. The basic concepts of alternating currents, including peak, effective values, frequency, and phase, are introduced. The experiment aims to determine the dependence of resistance on the frequency of alternating currents for inductive, capacitive, and biological materials.
Key Concepts:
The Purpose of the Work:
Theoretical Background:
Detailed Explanation of Alternating Current Circuits:
Impedance of Biological Fabrics:
Experimental Procedure:
Control Questions:
Laboratory work №11
Definition of an impedance of biological object
The basic concepts and definitions: the basic characteristics of an alternating current (instant, peak, effective values of pressure and force of an alternating current, the period, frequency and an alternating current phase); impedance, rheography.
The work purpose: to work with electric devices; experimentally to receive dependences of resistance on frequency of an alternating current: inductive- ; capacitor - ; a biological fabric - ; to make corresponding calculations and under the schedule of functional dependence of an impedance of bioobject from frequency of an alternating current to estimate its active resistance.
Brief theory
Studying of alternating currents has a great importance by consideration of physiological processes in a human body and animals. Alternating currents have found the big application at treatment of various diseases. A number of physiotherapeutic methods of treatment and diagnostics are based on use of alternating currents.
Alternating currents can have irritating an effect on organism fabrics. It connected to the short-term displacement of ions under the influence of variable electric field which also can cause change of concentration of fabric ions in cellular membranes. Irritating action of an alternating current appreciably depends on its frequency. The current does not render on any more a fabric of irritating action with frequency increase when displacement of ions in the directed movement becomes commensurable with their displacement at thermal movement. Thus there is a thermal action of a current. This property is used for warming up of fabrics of an organism by high-frequency alternating currents (diathermy).
Other physiotherapeutic methods using high-frequency alternating currents are d'arsonvalization. It is influence by a high-frequency current in the form of the category which is passing between a special electrode and a surface of a skin of the patient (devices of type "Spark", etc.). In comparison with a direct current presence it has a great importance for resistance in a chain of an alternating current besides active loading in a chain of electrocapacity "C" and inductance «L».
Resistance which is rendered by the electric chain containing components R, L, C connected consistently is called as an impedance and is calculated at their consecutive connection under the formula:
As in biological objects inductance is insignificant (L ( 0) and the formula for calculation of their impedance is:
It is known that active ohmic resistance R of a biological fabric practically does not depend on frequency of a current and capacitor - considerably decreases in process of frequency increase that leads to increase in conductivity of all capacitor -ohmic system.
The impedance of organism fabrics depends on their bloodinflation. The method of function research of the blood circulation is based on it. It is named rheography. Thus during a cycle of warm activity are registered impedance changes of a certain part of fabrics on which border are imposed electrodes.
Let's consider the most general laws of an alternating current chains
Picture 1. Picture 2. Picture 3.
Connection in an alternating current chain of resistance «R» (pic. 1), inductance «L» (pic. 2) and the condenser in electrocapacity "C" (pic. 3)
If it is enclosed to the ends of a conductor with resistance R (pic. 1) he alternating voltage which size in time is defined by the equation
(1)
(where - peak value of pressure, - the circular frequency equal =, - a current frequency) the current which size is defined according to the Ohm law by the equation will go to chains:
(2)
where R- active resistance, - peak value of a current.
From the equations (1) and (2) it is visible that the current and pressure on active resistance coincides on a phase.
We will consider a chain of an alternating current with inductance L (pic. 2) without ohmic resistance (R=0). Then the current will go to chains:
(3)
Under the influence of this current in the inductance coil arises а self-induction:
(4)
According to the second rule of Kirchhoff for the closed chain (algebraically the sum of electromotive forces is equal in the closed contour to the algebraic sum of falling of pressure) it is possible to write:
Then:
Calculating from the equation (3) and substituting this value for finding U we have:
but so
(5)
Comparing the equations (3) and (5) we see that pressure on inductance advances a current on a corner.
The size of inductive resistance can be defined from the equation (5) at peak value of pressure, i.e. at
we will receive , (6)
where peak values of pressure and a current. Having divided both parts of equation (6) on we will receive but - inductive resistance. Then, i.e. the size of inductive resistance is directly proportional from inductance of the coil and frequency of an alternating current.
Full resistance (impedance) of organism fabrics
The biological fabric spends not only constant but also an alternating current. There are no systems in a biological fabric which would possess inductive properties, i.e. inductive resistance XL which is closed to zero. The biological fabric possesses basically ohmic and capacitor properties and accordingly the impedance for biological fabrics of an organism is defined only by ohmic resistance.
Let’s taking into account their ohmic and capacitor properties for an explanation of electroconductivity of biological fabrics depending on frequency of a current it have been offered following equivalent electric schemes in pic.6.
Let's consider each scheme separately.
a)at ω→ 0; z →∞;
b)at ω→∞; z→R.
I.e. this scheme explains that at ω =0 (direct current) the general resistance of a biological fabric is equal to infinity. Experience and an expert show that biological fabrics depending on their functional purpose possess certain ohmic resistance.
Graphically dependence z=f (ω) has the following view:
Picture 7. Dependence of impedance of a biological fabric on frequency of a current for the scheme 6а
2)Analyzing the scheme 6b and using a rule of parallel connection of resistance we will receive:
a)at ω→ 0; then and z→R;
b)at ω→∞; then and z→0.
The schedule of this scheme is:
Picture 8. Dependence of an impedance of a biological fabric on frequency of a current for the scheme 6b
3)The scheme analysis 6c and the usage of parallel connection rule of resistance is received:
a)at ω→ 0; then z→R1.
b)at ω→∞; thus .
The schedule of this dependence:
Picture 9. Dependence of an impedance of a biological fabric on current frequency for the scheme 6c
Thus for an explanation of electroconductivity of biological fabrics the equivalent scheme (6c) is most successful because it explains electroconductivity of a biological fabric both on low and on high frequencies of an alternating current.
The impedance of biological fabrics and bodies depends on their physiological condition. At blood filling of vessels the impedance changes depending on a condition of cardiovascular system.
The diagnostic method based on registration of impedance change of biological fabrics in the course of warm activity which is named rheography.
By means of this method it is received brain rheograms (rheoencephalogram), hearts (rheocardiogram), the main vessels, lungs, a liver and extremities.
The order of work performance
Picture 10. The block diagramme of experimental installation
2. Starting work performance you should establish following positions of switches on the oscillograph: the button "input х" - in the pressed position; handles "stability" and "level" - in extreme right position; the handle "volt/division" - for 0,1 V/div.
On the audio-frequency generator (AG): handles "multiplier" and "dial frequencies" (in the form of disk) to establish frequency of 300 Hz (on the dial frequencies should be 3 and a multiplier in the position "100"); the handle "amplitude" (adjustment of target pressure) - in extreme right position. The switch "form" on the audio-frequency generator to establish in position «~». Other switches and buttons on devices are established by the teacher or the laboratorian.
3. On a breadboard model the handle "potentiometer" to put in extreme left position - (minimum).
4. To include an oscillograph and the generator (the toggle-switch "network" is located on a device underside) in a network. Through 1 - 2 minutes to start measurements. On an oscillograph handles "Ò" and “☼”, “↔” and ”↕” to establish not so bright accurate shone point in the screen centre.
5. Smooth rotation of the handle "potentiometer" on a breadboard model and "amplitude" on AG to establish on microampermeter on a breadboard model a current force in 200 mkА.
6. To measure value of pressure on the coil by means of an oscillograph. For this purpose the handle "volt/division" to find such division that the vertical shone line made from 3 to 6 big divisions. Effective pressure Uef is calculated under the formula:
Uef = (dy·Cy)/2,8,
Where: dy - number of divisions on an axis "Y" on an oscillograph to within 0,2 divisions.
Cy - scale interval of one big division is set by the handle "volt/ division." You should bring in table 1the measured values dy and Cy and calculated values Uef.
7. To repeat measurements at all frequencies (the frequency multiplier «1k» corresponds to multiplication on 1000, 10k - 10000), specified in table 1 supporting by means of the handle "potentiometer" on a breadboard model and "amplitude" on AG a current strength equal 200 mkА!!! (If on high frequencies it is not possible to establish a current 200 mkА to take value of a current 100 mkА).
Results of measurements to bring in table 1.
Control questions
تلخيص النصوص العربية والإنجليزية اليا باستخدام الخوارزميات الإحصائية وترتيب وأهمية الجمل في النص
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