A resistor is an element, which offers a specific electrical resistance to current flow. Resistors are usually made from a carbon composite or from high resistivity metal wire. Carbon resistors are used in low current applications (e.g. electronic circuits) and wire resistors for high currents (e.g. heating elements). The impedance is the same for AC and DC currents.
The simplest form of capacitor consists of two metal plates separated by an electrical insulator. When charges of opposite sign are established on the plates the mutual attraction between unlike charges holds the charge on the plates. Thus a capacitor has the capacity for storing charge. The amount of charge Q stored on one plate is proportional to the potential difference V between the plates:
Q = CV,
where the constant of proportionality С is called the capacitance. The unit of capacitance is the farad [F] = [С×V-1]. Since the metal plates of a capacitor are separated by an insulator the impedance to DC current flow is infinite. For sinusoidal AC currents the magnitude of the impedance is |ZC| = 1/2pfC, (f is frequency AC) so that a capacitor offers little impedance to the flow of high frequency current.
An inductor consists of a number of turns of wire in the form of a coil. The inductor is constructed of high conductivity wire. Its impedance to DC current flow is zero. For a sinusoidal AC current, the magnitude of the impedance is ½ZL½ = 2pfL, L is self-inductance of the coil.
Equivalent electrical circuit of living tissue is showed in Fig.4. Rs, Rt are resistances of skin and tissue, C1, C2, C3 are capacitances of skin or tissue, a, b are electrodes.
When DC passes through this circuit the
equivalent resistance Re is 
.
The value of resistance Rt very small and Rt»0, therefore 
.
When ACpasses through this circuit the impedance Z is:
.
Formula is showing that the impedance depends on the frequency f. For high frequencies, when f®¥, ZC®0 and Z= Rt.
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The AC passes through C1, C2, Rt and C3.
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Fig.5. Dependence of impedance Z on frequency f.
7. Effects of electrical current on tissue
Electrical current is responsible for some changes at the cellular, tissue, segmental and systemic levels of the biological system. The direct effects of electrical current are those, which occur in the area where the electrical current is flowing. The current causes the movement of charged particles and the accumulation them at membrane that creates polarized fields. The direct effects of current be recognized as: electrothermal; electrochemical; electrophysical.
ELECTROTHERMAL: The movement of charged particles in the conductive medium may be said to cause a micro vibration of the particles. This vibration causes minute frictional forces eventually leading to the production of heat. The amount of electrical to heat conversion is described by Joules law, which states that: The amount of heat production (H) is proportional to the square of the total current intensity (I2), the resistance, and the time for which the current flows.
H = 0.24 I2 Rt
The production of heat is rarely the goal of any electrotherapeutic treatment but can occur especially with DC. If DC treatment is used both treatment time and intensity must be minimized to avoid thermal effects.
ELECTROCHEMICAL: Changes in the chemical steady state of the tissue are most commonly associated with the application of DC. The unidirectional flow creates redistribution of the sodium and chlorine followed by the formation of new chemical compounds such as NaOH (sodium hydroxide) and HCl (hydrochloric acid) in the tissues under the electrodes. To minimize the formation of such compounds, one can decrease treatment time, or reverse polarity every few seconds or decrease intensity. The AC current enhances succinate dehydrogenase (SDH) and ATPase activity. This can lead to increased formation of ATP for cell metabolism.
ELECTROPHYSICAL: This effect is connected to movement of the ions, proteins or lipoproteins. The best known physiological consequence of such ionic movement include the excitation of nerves, where in the presence of electrostimulation the sodium and potassium ions move across the cell membrane.
8. Clinical methods in which direct current is used.
1. Galvanization (galvanotherapy). In this method low level direct current is passed through human body. U=60-80V.
2. Electrophoresis. Electrophoresis is a method of percutaneous introduction of charged drug into human body under the action of potential difference.
9. ALTERNATING ELECTRICAL AND MAGNETIC FIELDS in medicine
Similar to alternating electric current, the alternating electric field (or alternating magnetic field) changes its direction with time. The alternating electric field causes the change in the motion of ions both in extracellular and intercellular fluids. The effect of alternating electric field depends on frequency. At low frequencies (50-150 Hz) the electrical field changes ion concentration inside and outside of cell. This is an irritant (or stimulative) effect of alternating electrical field on tissue. At high frequencies (more than 500 kHz) the displacements of ions under the action of alternating electrical field become equal to their displacements at internal thermal motion. So, high- frequency electric field produces only thermal effect.
At UHF (ultra high frequency) therapy the frequency of alternating electric field is 40-50 MHz. Thus, the insulators (dielectric tissues) are heated better, than conducting tissues (electrolytes).
The magnetic field is also used in medicine for tissue heating. The alternating magnetic field generates vortical currents in conducting tissues.
At inductothermy (the application of alternating magnetic field in medicine) conductors (electrolytes) are heated better than insulators. For such a therapy an alternating magnetic field with frequency of 10-15MHz is used.
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