Using R =  L/A, R can be found using the ____________________________of a metal wire. In a circuit, R is defined for any device as the ratio of __________________ the device to the ________________________the device: R = A simple circuit: V I physical properties voltage across current through V/I voltage source wire I pos. I circuit element III. Ohm's Law

Ex. If the potential difference across a resistor is _______ and the current through it is __________, find R. R = V/I = 8.0 V / 0.25 A = 32 V/A = 32  units:[ ] = [ ]/[ ] = 8.0 V 0.25 A R 1 ohm V I 1 volt/ampere 1 V/A 1 

R = V / I Solve this for V = IR I = V/R To remember all 3 equations, use: V I R V = ? I = ? units: [V] = [I][R] = [I] = [V]/[R] = 1 V 1 A 1 A·  1 V/ 

Ex: What is the potential difference across a 25-  resistor when it carries a current of 3.0 A? Ex: What is the potential difference across a wire that has no resistance? Ex: How much energy is required to make each coulomb of charge pass through the above resistor? V = W/q  W =qV = V = IR = (3.0 A) (25  ) = (75 A·  ) = 75 V (1.0 C) (75 V)= (75 J) V = IR = I (0  ) = 0

3.I = charge flowing _______________________________. The charge going __________ any circuit element must _________ the charge __________ that element. Assume ____ charge flows out of the ____ side of the source. 4.V = potential difference __________________ = ____________________________ available to do work = energy converted to _______________________ by R = energy is __________________ by passing through R = _________________________ across R = _______ if there is no resistance, e.g. in a __________ 1. Assume the connecting wires have _________________ resistance. (They usually have ________________ R than the circuit elements.) 2.For simple devices such as _______________________, we often replace the device with the symbol for __________________ : and assume that it has all of the ______________________. little or no a light or a heater a resistor resistance much less through the wires and the R across the R energy per charge heat or motion "used up" potential "dropped" into + leavingequal + 0 wire

distance around the circuit V 1 2 V = IR 34 V source R Ex: A simple circuit has 1 _______. All of the __________________ is dropped across the ___________, because it is the only element in the circuit that requires ____________ (voltage). Graph the voltage drops as you follow ____________ charge from the _________ potential side of source, through the _____________, back to the _______ side of the source. loop voltage one R energy positive high low no V dropped in wire b/c V = IR = ________ circuit I·0

Other ____________________ways to hook up the meters: ammeter – measures current passing ____________ R - Ideally, it has no ____, so no ________ across it voltmeter – measures potential difference _________ R - Ideally, it does not allow any ____ to enter it R A simple circuit with ____________: R equivalent through across I The voltmeter must be connected across _______________ sides of R to measure potential _____________. A V V drop opposite difference A V A V A V meters

Ohm’s Law: For __________________ conductors at _______________ temp., I is ___________ prop. to V. V I V I Case A: a device obeys Ohm’s Law  _____________ Case B: ________________ devices  slope = ΔV/ΔI = constant so the ratio V/I = ____ is ___________  slope = V/I = R is ________________ Remember: V ____________ I  So changing V _______________ I. (Traditionally, V is plotted on the ____ axis) In the case shown, R _______________ causes changes metallic constant directly "ohmic" R constant non-ohmic not constant increases y

Ex. If R is _____________, then I is _____________. V Ex. If R is ___________, then I is _____________. = V As R  ____, I  ____. This is an ___________ circuit. = V R = V This situation can be _______________________________. Body resistance can be lowered by getting __________. extremely dangerous huge tiny ∞ 0 open ∞ R I tiny huge As R  ____, I  ____. This is a ___________ circuit. ∞0short I R 0 No I I wet

Currents and the harm they can cause: "It's _______________ that jolts, (shocks you) But it's ___________ (milliamps of current) that kills." volts mills AC tends to send heart nerves into_________________, which can be harder to fix than simply__________________________ defibrillation restarting a stopped heart.

Ground fault detection: With no problem, the currents into and out of device are equal. 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.

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 overcurrent protection device. Its essential component is a __________________________________________________ ____________________. Fuses usually are rated in _______________. If the current exceeds the rating, the metal strip melts, and it _________ the circuit. This protects the circuit from __________________ which may damage other circuit parts or ________________. A _______________________ is an automatically- operated electrical ______________. Like a fuse, it is designed to protect an electrical circuit from damage caused by excess_________. Unlike a fuse, which operates once and then must be replaced, a circuit breaker ________________ once the problem that caused the excess current is fixed. fuse metal wire or strip that melts when there is too much current. opens overheating start fires amperes circuit breaker switch current can be reset

fuses

Circuit breakers act like fuses but can be reset. Two basic types: 1.solenoid (electromagnet) 2.thermal (bimetallic strip)

A downed power line can set up a _________ through the ground. Since the cables have _________ R, most voltage will be dropped along ____________________. If the distance between the downed line and the source is___________, there can be a significant ___________ between two nearby points along the ground, ie, between your 2 feet. _____________ or_______________ ! circuit little the ground small voltage stay awaystand on 1 foot