ELECTRIC CURRENT. PHYSIOLOGICAL EFFECT OF CURRENT 1. The electric field E is defined to be E =5 where F is the electrostatic force exerted on a small positive test charge q . E has units of N/C. 2. In uniform electric field the potential difference is AV = Ed, where E is electric field and d'is the distance from A to B, or the distance between the plates. 3. A capacitor is a device used to store charge. The amount of charge q a capacitor can store depends on two major factors - the voltage applied and the capacitor's physical characteristics, such as its size. The capacitance C is the amount of charge stored per volt, or C = , units Farad (F). 4. Capacitance of a parallel plate capacitor C = f, where e, = 8.85 - 10-12 F/m is called the permittivity of free space, e is the dielectric constant of the material, A is area of plates and d is distance between plates. 5. Capacitors are used in a variety of devices, including defibrillators, microelectronics such as calculators, and flash lamps, to supply energy. The energy stored in a capacitor can be expressed in three where q is the charge (C), AV'is the voltage (V), and Cis the capacitance of the ways: E = capacitor(F). 6. Electric current I is the rate at which charge flows, given by / =, units Amperes (A). Here q is the amount of charge passing through an area in time t . 7. Current density is current per unit of cross sectional area / = units A/m'. 8. Ohm's law. One statement of Ohm's law gives the relationship between current I, voltage V , and resistance R in a simple circuit to be I = . Here R is resistance in units of ohms ( 2). 9. Electric power. p=AV ·1 = 1°R = 10. Heat effect of electrical current Q =AV 1·t = 1°R¢ = 11. Resistors connections (a) in series R = Rị + R2 + R3 ..+ RN (b) in parallel = + 20" Av2 R. R, (a) R1 here N total is number of the resistors. R2 R3 RN R, Table 1 - Effects of Electrical Shock as a Function of Current Current Effect Current (mA) 50 Effect (mA) | 1 Onset of pain Ventricular fibrillation possible; often fatal. Threshold of sensation Maximum harmless current 100-300 depending 300 Onset of burns concentration of current on Onset of sustained muscular contraction; cannot let go for duration of shock; contraction of chest muscles may stop breathing | during shock Onset of sustained ventricular contraction and respiratory paralysis; both cease when shock ends; heartbeat may return to normal; used to defibrillate the heart 10-20 6000 ww Table 2 Drd not shockedi Cu ow Resistance, Current path Current, A Person (SHOCKEDN Ohms 1300 1000 650 Hgh Vage Hand-body-hand One hand-body-feet Ac S and Load Hands-body-feet Foot-foot 1300