Transcript Power and Energy Calculations

Power and Energy Calculations
Power Ratings
• Power is the rate that
an electrical device
converts electrical
energy into another
form (ex. light or heat)
• At the first lamp…
V = 120 V
R = 240 Ω
I = V/R
I = 120V / 240Ω
I = 0.5A
What is the power of the bulb?
Power (Watt) = Voltage (Volts) x Current (Amperage)
Or
P=VxI
P = 120V x 0.5A
P = 60 W (watts)
Example
A TV uses 100 V of electricity and has a resistance
of 35 Ω. Calculate the power of the TV.
I = V/R
I = 100V / 35Ω
I = 2.9A
P = VI
P = 100V x 2.9A
P = 286 W
Energy Used
If the same TV is watched for 3 hours, how much
energy does it use?
Energy (Watt*hours) = Power (Watt) x Time (hours)
E = Pt
E = 286W x 3h
E = 858Wh or 0.86kWh
1000Wh = 1kWh
Cost of Operating Devices
How much energy does a 120 W bulb use in 1
week, if it is left on 12 hours a day?
E = Pt
E = (120W)(12 hours)(7 days)
E = 10080Wh
E = 10.08kWh
If electricity costs $0.08 per kWh, how much
would it have cost to keep the light bulb on?
Cost = Rate x Energy
Cost = $0.08 x 10.08kWh
Cost = $0.81
How much energy does a 0.56kW fridge use in one year?
How much does it cost to operate the fridge if energy
costs $0.12 per kWh?
E = Pt
E = (0.56kW)(24 hours)(365 days)
E = 4905.6 kWh
Cost = Rate x Energy
Cost = ($0.12)(4905.6kWh)
Cost = $588.67
Calculating Efficiency of a Device
Efficiency of a device is a comparison (or a ratio) of
the useful energy that comes out of the device to
the total energy that went in.
Percent Efficiency = (energy out / energy in) x 100%
A oven uses 490J of input energy to produce 59J of heat
energy. What is its percent efficiency?
Input energy = 490J
Output energy = 59J
Percent Efficiency = ?
Percent Efficiency = (energy out / energy in) x 100%
Percent Efficiency = (59J / 490J) x 100%
Percent Efficiency = 12%
Therefore, the efficiency of the oven is 12%
Practice Problems
1. Page 492 #1
2. Page 493 # 1 – 3
3. Worksheet
4. If you finish this, read pages 490 -491, 494 –
495 and then do:
5. Page 498 # 1 – 12, 15