Physics 106 Lesson #18

Energy and Lighting

Dr. Andrew Tomasch

2405 Randall Lab

atomasch@umich.edu

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

Review: Switch Placement

The switch is placed in the HOT lead.

No voltage will reach the appliance when it is off.

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: hyperphysicsDemonstration: GFI Guy

Review: PolarizedPlug 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

• 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

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

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)

22

/V IR I V R

VP IV I R

R

I

RVoltage = V

I

Voltage = 0 I