Download Home Wiring_ppt_RevW10

Physics 106 Lesson #17
Home Wiring
Dr. Andrew Tomasch
2405 Randall Lab
[email protected]
Review: Capacitance
• Most objects
can be charged
and therefore
store charge.
• Capacitance is
charge stored
per electric
potential: Unit:
Q
C
V
Coulombs/Volt = Farads
(for Michael Faraday)
Review: Capacitance of a Parallel-Plate Capacitor
Q
C
V
A
C 
d
For a parallel-plate
capacitor, the
capacitance is
proportional only
to geometric
factors (the area of
the plates A and
the separation
distance between
them d) and the
natural constant 
 C is fixed at the
factory!
Review: Dielectrics
• Capacitance is
increased by adding a
dielectric (increase ε):
C
A
d
• More charge is stored
per volt with a larger ε
compared to air
   0
ε is a measure of how
readily the dielectric
material polarizes in
the presence of electric
charge.
Vacuum
Review: Energy
Stored in a Capacitor
2
Units: Joules
1
1
Q
2
Energy  QV  CV 
2
2
2C
Q  CV
Q
V
C
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.
• Danger (!!!):
Electrocution can occur because a
current can flow between the hot wire
and the ground (e.g. water pipes…)
Ground Fault Interrupters (GFI)
protect against electrocution due
to currents flowing to ground.
Parallel or Series Circuits?
• All appliances need the
same voltage (110V)
so they need to be
connected in parallel.
• Switches are connected
in series with the
appliances they control
on the hot lead side.
• Important:
Avoid short circuits!
Dangerous!
110 V
Hot
Neutral
Where to Place the Switch?
The switch is placed in
the HOT lead.
No voltage will reach the
appliance when it is off.
Ground Wire
• NOT part of the path that
current normally flows
• 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: hyperphysics
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
The Effects of Electric Shock
Electric
current
Physiological Effect
1 mA
Threshold of feeling, tingling sensation.
5 mA
Accepted as maximum harmless current.
10-20 mA
100-300 mA
6A
Beginning of sustained muscular contraction
("Can't let go" current.)
Ventricular fibrillation, fatal if continued.
Respiratory function continues.
Sustained ventricular contraction followed by
normal heart rhythm. (defibrillation).
Temporary respiratory paralysis and possibly
burns.
A typical home circuit can deliver 15 to 20 A !
In a Nutshell…
• Most household circuits operate at
110V→ Dangerous!
• Appliances are connected in parallel from hot to
neutral, switches in series with the hot lead.
• Three wires: Hot (black)
Neutral (white)
Ground (bare copper)
• The current flows from hot to neutral – NEVER
through ground.
• Metal parts near electric circuits must be
grounded for safety.