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Luigi Galvani A charge applied to the spinal cord of a frog could generate muscular (1737-1798) spasms throughout its body. While cutting a frog leg, Galvani's steel scalpel touched a brass hook that was holding the leg in place. The leg twitched. Mary Shelley (1797-1851) wrote “Frankenstein”(1816) Alessandro Giuseppe Antonio Anastasio Volta (1745-1827) In 1799 Volta made his 'Voltaic Cell' by alternating disks of copper and zinc separated by salt-water moistened pads. Zn Zn 2 MORE NEGATIVE MORE POSITIVE Electromotive series Carbon Gold Silver Copper Tin Lead Iron Zinc 2e The zinc electrode collects a positive charge as it dissolves. Both tend to dissolve in even a salt solution! though often dilute sulphuric acid was used… As deposits collect on the copper electrode it develops a positive charge. Cardboard Carbon core ++ +++ bitumen insulating sleeve Zinc can washer Manganese dioxide --Deteriorating zinc canister builds negative charge at base of this dry cell. You probably already know, from replacing batteries in common devices: 1.5 volts 3.0 volts 1.5 volts Batteries chained end-to-end add their potential differences. The word “battery” actually means a set of “cells” lined up in sequence: An automotive accumulator is a battery of 6 2-volt leadacid cells. A plate coated in brown lead dioxide deposits (built while “recharging”) builds a positive charge when dissolved in dilute sulphuric acid. Grey lead plates grow negative as lead sulphate coating develops. - Every e that leaves the zinc electrode and enters the “circuit” enables another ion to dissolve into the acid A constant voltage is maintained (as long as the Zn and Cu lasts). So unlike a single discharge, this source is continuous! +++ --- - -- “Conventional current” or just “current” + I e- The amount of charge passing any given point per second is the current: 1 Coul sec Q I t = 1 Ampere Georg Simon Ohm 1789 - 1854 VI V = IR 1 ohm () allows a current of 1 ampere to pass when a potential difference of 1 volt is applied across it. Ohm’s Law: V = IR An electric toaster element (Nichrome) is 20. How great is the current when it is connected to a 120v line? (1) 1/6 A (2) 1 A (3) 5 A (4) 6 A (5) 120 A (6) 240 A 0.8A runs through a lamp when attached to a 32v source. What is its resistance? (1) 0.025 (2) 25.6 (3) 32 (4) 40 Work is continuously being performed within a battery (or generator) to maintain steady current flow! Through friction /atomic collisions with the crystal lattice of the resistor the supplied energy is converted to heat. (if hot enough may even produce light!) How rapidly is work being done in an electrical circuit? POWER = WORK TIME What electrical unit is most closely associated with work (energy)? What electrical unit involves time? What is the unit of POWER? If an electric iron draws 5 Amperes from a 120v line, at what rate is it using power? (1) 6 Watts (3) 120 Watts (5) 600 Watts (2) 12 Watts (4) 240 Watts (6) 720 Watts Which has a greater resistance, a 1 Watt lightbulb or a 25 Watt lightbulb 1) the 25 W bulb 2) the 100 W bulb 3) the same for both Which carries the greater current? 1) the 25 W bulb 2) the 100 W bulb 3) the same for both A 1440 W microwave oven is designed to operate using a 120 V power source. What is the resistance of this appliance? 1) 0.1 2) 10 3) 72 4) need to know the current 5) none of these Current Effect 0.001 Ampere 0.005 Ampere mild shock painful shock <0.010 Ampere 0.010 Ampere 0.020 Ampere 0.030 Ampere 0.100 Ampere 0.200 Ampere >0.200 Ampere 1.000 Ampere Fatal? no no tingles no paralysis of motor muscles no threshold of pain (cannot let go!) no breathing becomes labored no death due to fibrillation seconds no fibrillation, but severe seconds burns and breathing stops heart stops seconds serious burns instantly Galvani was correct in so far as our central nervous system is in fact a complex electrical circuit: Neurons fire electrical pulses along synapses. Muscle fibers contract in response to these signals. QUESTION 1 QUESTION 2 (4) 6 A (4) 40 32volts / 0.8 Amp QUESTION 3 (5) 600 Watts QUESTION 4 Since P = V2/R, P 1/R. P = IV Greater resistance: (1) the 25 W bulb QUESTION 5 The greater current: (2) 100 W bulb The bulb with the smallest resistance must draw the greater current! Also: since P = IV, P I. QUESTION 6 (2) 10 (120V )2 / 1440W 10