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III. Ohm's Law Using R = rL/A, R can be found using the physical properties ____________________________of a metal wire. In a circuit, R is defined for any device voltage across the device to the as the ratio of __________________ current through ________________________the device: R = V/I A simple circuit: pos. I wire I voltage source circuit element I V 8.0 V Ex. If the potential difference across a resistor is _______ 0.25 A and the current through it is __________, find R. R = V/I = 8.0 V / 0.25 A = 32 V/A units: [R ] = 32 W = 1 ohm = 1 W = [V ]/[ I ] 1 volt/ampere 1 V/A R = V / I Solve this for V=? V = IR I=? I = V/R To remember all 3 equations, use: V I units: [V] = [I][R] 1 V = 1 A·W [I] = 1A = [V]/[R] 1 V/W R Ex: What is the potential difference across a 25-W resistor when it carries a current of 3.0 A? V = IR = (3.0 A) (25 W) = (75 A·W) = 75 V Ex: How much energy is required to make each coulomb of charge pass through the above resistor? V = W/q W = qV = (1.0 C) (75 V) = (75 J) Ex: What is the potential difference across a wire that has no resistance? V = IR = I (0 W) =0 little or no 1. Assume the connecting wires have _________________ much less resistance. (They usually have ________________ R than the circuit elements.) a light or a heater 2. For simple devices such as _______________________ , we often replace the device with the symbol for a resistor __________________ : and assume that resistance it has all of the ______________________. through the wires and the R . 3. I = charge flowing _______________________________ The charge going __________ any circuit element must into equal leaving that element. Assume _________ the charge __________ + side of the source. ____ + charge flows out of the ____ across the R 4. V = potential difference __________________ energy per charge = ____________________________ available to do work heat or motion = energy converted to _______________________ by R = energy is __________________ by passing through R "used up" potential "dropped" = _________________________ across R 0 wire = _______ if there is no resistance, e.g. in a __________ Ex: A simple circuit has 1 loop _______. 1 All of the __________________ is voltage one R dropped across the ___________, V source because it is the only element in the circuit that requires 4 energy ____________ (voltage). 2 R 3 positive Graph the voltage drops as you follow ____________ high charge from the _________ potential side of source, through circuit low side of the source. the _____________ , back to the _______ 1 V 2 no V dropped in wire I·0 b/c V = IR = ________ V = IR 3 distance around the circuit 4 meters A simple circuit with ____________: through R ammeter – measures current passing ____________ R so no ________ V drop across it - Ideally, it has no ____, across R voltmeter – measures potential difference _________ I to enter it - Ideally, it does not allow any ____ A R V The voltmeter must be connected opposite across _______________ sides of R difference to measure potential _____________. Other ____________________ways equivalent to hook up the meters: V A V A V A Remember: causes I So changing V _______________ changes V ____________ I. metallic Ohm’s Law: For __________________ conductors at constant temp., I is ___________ _______________ directly prop. to V. "ohmic" Case A: a device obeys Ohm’s Law _____________ V slope = ΔV/ΔI = constant R so the ratio V/I = ____ is constant ___________ I non-ohmic Case B: ________________ devices V not constant slope = V/I = R is ________________ increases In the case shown, R _______________ I (Traditionally, V is plotted on the ____ y axis) tiny Ex. If R is _____________ , then I is _____________ . huge 0 . This is an ___________ open ∞ I ____ As R ____, circuit. V R = I V No I R= ∞ tiny huge Ex. If R is ___________ , then I is _____________ . 0 I ____ ∞ . This is a ___________ short As R ____, circuit. V R = I V I R=0 extremely dangerous This situation can be _______________________________ . wet Body resistance can be lowered by getting __________ . volts "It's _______________ that jolts, (shocks you) mills But it's ___________ (milliamps of current) that kills." Currents and the harm they can cause: defibrillation which AC tends to send heart nerves into_________________, can be harder to fix than simply__________________________ restarting a stopped heart. Ground fault detection: If there is current through the ground, it is a "fault." Then the current in and the current out will NOT be equal. A difference as small as 5 ma can be detected. With no problem, the currents into and out of device are equal. Ground Fault Current Interruptors, or GFCIs for short test button reset button This works, independent of the device being properly grounded. A__________, short for 'fusible link', is a type of fuse overcurrent protection device. Its essential component metal wire or strip that melts when there is is a __________________________________________________ too much current.. ____________________ Fuses usually are rated in amperes _______________ . If the current exceeds the rating, the opens metal strip melts, and it _________ the circuit. This overheating protects the circuit from __________________ which may start fires damage other circuit parts or ________________ . circuit breaker A _______________________ is an automaticallyswitch operated electrical ______________ . Like a fuse, it is designed to protect an electrical circuit from damage current caused by excess_________. Unlike a fuse, which operates once and then must be replaced, a circuit breaker ________________ once the problem that caused can be reset the excess current is fixed. fuses Circuit breakers act like fuses but can be reset. Two basic types: 1. solenoid (electromagnet) 2. thermal (bimetallic strip) A downed power line circuit can set up a _________ through the ground. Since the cables have little R, most voltage _________ will be dropped along the ground ____________________ . If the distance between the downed line and the source small is___________ , there can be a voltage between significant ___________ two nearby points along the ground, ie, between your 2 feet. stay away or_______________ stand on 1 foot ! _____________