Transcript Physics Day27

Physics Day 23
Aim: How much power does a toaster use?
LO: Calculate power use for V, I and R
LO: Calculate energy use from power
LO: Kilowatt-hour is a unit of energy
Do Now:
AGENDA
- How much does electricity cost?
Do Now
- In what units do we measure electricity
Notes
when we buy it?
HW09
- How large is the average monthly
electricity bill?
What is a kilowatt hour?
• Before we see how much electricity costs, we
have to understand how it's measured.
– When you buy electricity they charge you by the
kilowatt-hour (kWh). When you use a 1 kilowatt
appliance for 1 hour, that's a kilowatt-hour. If you
used it for two hours that would be 2 kWh.
• Most things don't draw an even kilowatt,
though.
– A typical central AC draws about 3.5 kilowatts
What’s a Watt?
• The rate of electrical use at any moment is
measured in watts. For example: A 100-watt light
bulb uses 100 watts.
• A typical desktop computer uses 65 watts.
• A central air conditioner uses about 3500 watts.
• If your device lists amps instead of watts, then
just multiply the amps times the voltage to get
the watts. For example:
• 2.5 amps x 120 volts = 300 watts
What’s it cost in 2006
Cost Today in NYC
• 20 cents/ kw.hr
• Average monthly bill is about $100
• Lower in Winter, higher in summer
Electric Power,P
Energy
P
.
time
Since the electrical energy is charge times voltage (QV), the above equation
becomes,
QV
P
.
t
Since the current is charge flow per unit time (Q/t), the above equation
becomes,
QV Q
P
 V  I V .
t
t
Since V = IR, the above equation can also be written as,
2
V
P  IV  I 2 R 
.
R
SI Unit of Power: watt(W)
Killowatt-hour (kWh)
The SI unit of power is watt, after James Watt (1736-1819), who developed
steam engines.
joule
J
watt  W 
 .
sec ond s
Utility companies use the unit kilowatt-hour to measure the electrical energy
used by customers. One kilowatt-hour, kWh is the energy consumed for one
hour at a power rate of 1 kW.
Electrical Energy
•Our daily life depends on electrical energy.
•We use many electrical devices that transform
electrical energy into other forms of energy.
• For example, a light bulb transforms electrical
energy into light and heat.
•Electrical devices have various power requirements.
Electric Power,P
Energy
P
.
time
Since the electrical energy is charge times voltage (QV), the above equation
becomes,
QV
P
.
t
Since the current is charge flow per unit time (Q/t), the above equation
becomes,
QV Q
P
 V  I V .
t
t
Since V = IR, the above equation can also be written as,
2
V
P  IV  I 2 R 
.
R
SI Unit of Power: watt(W)
Killowatt-hour (kWh)
The SI unit of power is watt, after James Watt (1736-1819), who developed
steam engines.
joule
J
watt  W 
 .
sec ond s
Utility companies use the unit kilowatt-hour to measure the electrical energy
used by customers. One kilowatt-hour, kWh is the energy consumed for one
hour at a power rate of 1 kW.