Using R = r L/A, R can be found using the ____________________________of a metal wire.

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

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

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

5. Ex: What is the potential difference across a 25-W resistor when it carries a current of 3.0 A? V = IR (25 W) = (3.0 A) = (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? (0 W) = 0 = I V = IR

6. little or no 1. Assume the connecting wires have _________________ resistance. (They usually have ________________ R than the circuit elements.) much less a light or a heater • For simple devices such as _______________________ , • we often replace the device with the symbol for • __________________ : and assume that • it has all of the ______________________. a resistor resistance through the wires and the R • I = charge flowing _______________________________ . • The charge going __________ any circuit element must • _________ the charge __________ that element. Assume • ____ charge flows out of the ____ side of the source. into equal leaving + + across the R • 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 __________ energy per charge heat or motion "used up" potential "dropped" 0 wire

7. loop 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). 2 1 voltage one R V source R 4 3 energy positive Graph the voltage drops as you follow ____________ charge from the _________ potential side of source, through the _____________ , back to the _______ side of the source. high circuit low 1 2 no V dropped in wire b/c V = IR = ________ V I·0 V = IR 3 4 distance around the circuit

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

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

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

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

12. Understanding safe circuits: "Grounded" side of circuit is safe to touch pipe driven into ground No shock hazard as long as wires inside toaster do not touch the outside metal case.

13. If the hot wire in the toaster touches the metal case, and you touch the case, current can flow through you to ground. If the neutral wire in the toaster touches the metal case, and you touch the case, no current flows through you to ground.

14. Polarized plugs can only be plugged in one way because one side of the plug is bigger than the other. Then the case can be connected to the neutral wire, and you will not be shocked if you touch the case. Of course, you can make the case out of plastic, so it doesn't conduct…..or…

15. The wide prong on the plug links the threaded base of light bulbs to the neutral terminal (the wider slot) in the receptacle. If the wires are reversed, the hot side of the outlet (the side that can deliver a shock) is wired to the threaded socket. Normally there’s a cardboard insulator isolating the base from the socket. However, if the cardboard insulator is worn out (common on old lamps), the metal parts of the lamp also could become “hot” (charged). You can then get a dangerous shock if you touch the metal, which is easy to do when you’re “grounded” and changing a dead bulb. For example, if you’re standing on damp concrete or in contact with a radiator or other plumbing pipe while you’re touching a miswired lamp, you could get a dangerous jolt of electricity. Buy a “polarized” replacement plug, that is, one that has a normal prong and a wide one. The neutral line on the lamp cord is the one that’s odd; it’ll have ribbing, a sharp ridge or printing on it. If the cord is translucent, the neutral is silver. The wire that goes to the narrower prong has a smooth, plain surface or is gold in color.

16. The third prong on the power cord provides a direct electrical connection from the appliance case to earth ground, making the two points electrically "common" with each other. If they're electrically common, then there cannot be any voltage dropped between them.

17. With no problem, the currents into and out of device are equal. 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.

18. Ground Fault Current Interruptors, or GFCIs for short test button reset button This works, independent of the device being properly grounded.

19. 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. amperes opens overheating start fires circuit breaker switch current can be reset

20. fuses

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

24. A downed power line can set up a _________ through the ground. circuit Since the cables have _________ R, most voltage will be dropped along ____________________ . little the ground 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_______________ ! small voltage stay away stand on 1 foot