Chapter 17 Current and Resistance Name one thing that changed the world more than anything else Electric circuits 17 1 Electric Current Current I Q t Coulomb s or Ampere like water current liters s Two types of currents used dc direct current battery current goes in one direction worked out mostly by Edison ac alternating current wall outlets current goes back and forth worked out mostly by Tesla in Buffalo of course Convention the direction of the current is the same direction of the positive charge flow Voltage what is applied to the device Current what goes through the device as a result of the voltage 2V 2V Positive charges move 2V 2V Negative charges move Effects are the same thus the currents are the same Don t have to worry about what type of charges are moving The direction of positive charges is defined as the direction of current 17 4 Resistance and Ohm s Law Current and voltage Ohm s law V IR R is the resistance SI unit ohm For HW you will see a lot of problems with two out of I R V given all three are known with V IR A voltage of 120 V is applied to a 100 light bulb What is the number of electrons going through the light bulb in 5 seconds 17 5 Resistivity The theory of going down the tube Different materials have different values of Conductors Aluminum 2 82 x 10 8 m Copper 1 72 x 10 8 m Indium 8 x 10 8 m Gold 2 44 x 10 8 m Tungsten 5 6 x 10 8 m Semiconductors Silicon 20 2300 m Insulators Wood 3 x 1010 m For HW make sure you get A and L right Example A gold coin of volume 1 x 10 7 m3 is melted and made into a wire of 20 m long What is its resistance 2 44 x 10 8 m for gold 17 6 Temperature Variation of Resistance In metals the theory of running through the hallway Higher temperature higher resistance 0 1 T T0 temperature coefficient of resistivity at T and 0 at T0 If two temperatures are given take the lower one as T0 For HW some problems are very similar to thermal expansion type of problems with some quantities given at one temperature and asking you find what happens at another temperature 17 8 Electric Energy and Power P energy time power Energy or EPE qV P qV t q t V IV unit Watt used in 101 Example what are you paying for at the end of the month if you have a 100 W light bulb going for 50 hours Energy Pt 100 W x 50 x 60 x 60 18000000 J Niagara Mohawk power company unit kilowatt hour 100 W x 50 hours 5 kW hour they will tell you how much money it takes for 1kW hour V IR P I2R I V R P V2 R Unless it is specified the resistance of conducting wires are considered to be 0 because their resistances are typically much smaller than all other resistances involved except for long distance For superconductors R 0 literally It doesn t happen often in nature for a physical quantity to be exactly 0 Example what happens when you connect the positive end and negative end of a 12 V battery with a conducting wire The resistance of the wire can be considered as 0 17 9 Electrical Activity in the heart The heart runs on an electric voltage not that different from a battery Beating is all controlled by electric pulses Keeping up with the tradition of Tesla two heartrelated electric devices were invented in Buffalo the Electric City of the Future First one pacemaker by Mr Greatbatch Second one Old Sparky electric chair was invented by a UB professor Dr Alfred Southwick William Kemmler from Buffalo was the first to make use of the chair