1. Physics 106 Lesson #18 Energy and Lighting Dr. Andrew Tomasch 2405 Randall Lab atomasch@umich.edu

2. Review: Standard U.S. House Wiring The hot wire is connected to 110V provided by the power company. The hot wire is usually black, occasionally red. Often the corresponding terminal screws are copper-colored. Current returns through the neutral wire, usually white. Often the corresponding terminal screws are silver in color. For safety, a ground wire is connected to all metal components which could accidentally come into contact with the hot lead

3. Review: Switch Placement The switch is placed in the HOT lead. No voltage will reach the appliance when it is off.

4. Review: Ground Wire NOT part of the path for normal current flow Connected to the metal case of the appliance Usually a bare copper conductor Hot wire shorts to case  big current flows  breaker or fuse interrupts current  NO shock hazard at metal case Ground wire = SAFETY! Picture source: hyperphysicshyperphysics Demonstration: GFI Guy

5. Review: Polarized Plug and Receptacle Short Prong : Hot = 120 V Long Prong: Neutral (Return) = 0 V. Round Prong: Ground (NOT a part of the circuit) = SAFETY Current Flow: Hot Wire  Device  Neutral Demo: GFI Guy

6. When a number of possible engineering solutions exist to meet a given need or goal, engineers perform a “trade study”. This is a useful approach whenever a choice is to be made between a number of possible options. The benefits, disadvantages and costs are quantified for each option and decisions are then made by “trading off” advantages against disadvantages for the various options based on a goal such as minimum cost, safety, or maximum reliability. Engineering Trade Studies

7. Concept Test #1 You receive a bill from the electric utility company stating your electrical usage in kilowatt-hours (kw-hr). This is a unit of A.power B.energy C.current D.charge 1 kilowatt = 1000 watts = 1000 Joules/s = energy/time 1 hour = 3600 s = time 1 kilowatt-hour = 1000 Joules/s X 3600s = 3,600,000 Joules of ENERGY

8. Review Calculating Power Electric Potential Energy is transformed into to some other form (heat, light) by the resistor (light bulb). Power: Electric Potential (Energy) decreases across the light bulb (resistor) I R Voltage = V I Voltage = 0 I