Transcript Energy and Lighting_ppt_RevW10

Physics 106 Lesson #18
Energy and Lighting
Dr. Andrew Tomasch
2405 Randall Lab
[email protected]
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!
Demonstration: GFI Guy
Picture source: hyperphysics
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
Engineering Trade Studies
• 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.
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
1 kilowatt = 1000 watts = 1000 Joules/s
B. energy = energy/time
C. current 1 hour = 3600 s = time
D. charge 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:
V  IR
I V /R
2
V
P  IV  I R 
R
2
I
Voltage = V
R
Voltage = 0
I
Electric Potential
(Energy) decreases
across the light bulb
(resistor)
I