1. The electric field E is defined to be E = 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 Cc = td, 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 ways: E = 94V 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 = R1 + R2 + Rą . + RN (b) in parallel = ; сдит where q is the charge (C), AV is the voltage (V), and C is the capacitance of the AV R. R. R. (a) R, RN here N total is number of the resistors. R2 Table 1 - Effects of Electrical Shock as a Function of Current Effect Effect Current Current (mA) |1 (mA) 50 Threshold of sensation Maximum harmless current Onset of pain Ventricular fibrillation possible; often fatal. 100-300 depending 300 Onset of burns on concentration of current Onset of sustained ventricular contraction and respiratory paralysis; both cease when shock ends; heartbeat may return to normal; used to defibrillate the heart Onset of sustained muscular contraction; cannot let go for duration of shock; contraction of chest muscles may stop breathing during shock |10-20 6000 PROBLEMS 1. Electrical currents through people can produce tremendously varied effects. The major factors upon which the effects of electrical shock depend are the amount of current I, the path taken by the current and the a person, for different current paths. (b) Using Table 1 make conclusion about physiological effect of these currents. (c) What was the power dissipated in his body? Table 2 Drd not shocked Cum Fow Current, A Person (SHOCKEDN Current path Resistance, Ohms 1000 Hgh Vatage and Load Hand-body-hand One hand-body-feet Hands-body-feet Foot-foot 650 1300