1. Chapter 9.2
Power

2. What is Power?

3. What is Power? Energy is what costs money
James Prescott Joule
James Watt
Energy is what costs money
Power is the rate at which energy is being transformed (or consumed)
Energy is measured in Joules (J)
Power is measured in Watts (W = J/s)

4. of heat and light energy
Electrical Power
The rate of change in electrical energy
A 60W light bulb uses 60J of electrical energy every second and produces 60J
of heat and light energy

5. Power
Power is equal to voltage multiplied by current
P = IV

6. P I V= P V I=
P=IV P I V

7. Example 1 P = ? I = 8A V = 120V P = 960W P = IV P = 8A x 120V P I V
Household sockets supply 120V. If the current drawn by the toaster is 8A, what is the power rating of the toaster? I V
P = ?
I = 8A
V = 120V
P = 960W
P = IV
P = 8A x 120V

8. Various Power Ratings Electronics Watts Appliance Watts
Desktop Computer
120
Laptop 35
PS3
184
XBOX
102
iPhone (charging) 5
40” LCD
400
Electronic Clock 3
Fan 25
Blow Dryer
1000
Shaver 15
CD Player
Appliance
Watts
Washing Machine
500
Electric Clothes Dryer
4000
Air Conditioner (room)
1000
Dishwasher
1400
Microwave
1100
Toaster
950
Kettle
1500
Vacuum
Iron
Refrigerator
Sewing Machine
100
Note: Yours may be different

9. A Comparison on Light Bulbs
Incandescent
Compact Fluorescent VS
40W
11W
60W
16W
75W
20W
100W
30W

10. E P = t Energy Consumption Energy costs money
So, we are interested in how much energy we use powering our different appliances and electronics
We know Power has units (J/s) and Energy has units (J)
Therefore… E t
P =

11. Energy Consumption
Re-work that equation… E t
P = t t
E = Pt

12. E t P= E P t=
E=Pt E P t

13. Example 2 E = ? P = 960W t = 90s t = 1.5min E = 86400J E = Pt
Using the same toaster from before (960W), it takes 1 and a half minutes to turn bread into delicious toast. How much energy is used? P t
E = ?
P = 960W
t = 90s
t = 1.5min
E = 86400J
E = Pt
E = 960W x 90s

14. Do BLM 3-38 Electrical Power & Energy
Do BLM 3-39 Power Problems

15. A Larger Unit for Energy
With the units we are using (Watts and seconds), our calculated values for energy can get fairly large
Instead of using…
The following is often used:
1.0 joule = 1.0 watt x 1.0 second
1.0 kW•h = 1.0 kilowatt x 1.0 hour

16. A Larger Unit for Energy
Appliances and energy companies like BC Hydro work with large quantities of energy and will therefore tend to show energy consumption using kilowatt hours instead of joules
kW•h

17. Example 3 E P t There are 1000W in 1kW. Therefore, divide by 1000
Refrigerators are turned ON every single moment of the year. A refrigerator has a power rating of 500W. How much energy, in kW•h, is used by the refrigerator in 1 year?
P = 500W
= 0.5kW
There are 1000W in 1kW. Therefore, divide by 1000
There are 365 days in 1 year and 24 hours in 1 day. Therefore multiply
365 d x 24h/d = 8760h
t = 1year
= 8760h E
E = ?
E = Pt
= 0.5kW x 8760h P t
E = 4380 kW•h

18. The Cost of Energy
Yes, every month or so your parents have to pay the electricity bill =

19. BC Hydro Residential Rates (2011)
Under the Residential Conservation Rate, customers pay 6.67 cents per kWh for the first 1,350 kWh they use over an average two-month billing period. Above that amount, customers pay 9.62 cents per kWh for the balance of the electricity used during the billing period.
TWO-STEP CONSERVATION RATE (2 MONTH PERIOD)
9.62 cents
per kWh
6.67 cents per kWh
STEP 2
STEP 1
1,350 kWh – Step 1 threshold

20. Cost = Rate x E Calculating Cost BC Hydro charges 6.67 cents per kW•h
Steps
Determine the amount of energy in kW•h
Multiply energy by the rate (6.67 cents/kW•h)
Cost = Rate x E

21. We know the rate, we need to find energy
Example 4
The power company charges 6.67 cents per kW•h. How much does it cost (rounded to the nearest cent) to watch 2 hours of TV on a big screen 40” LCD (400W)?
Cost = Rate x E
= $/kW•h x 0.8kW•h
E = Pt
E = 400W x 2h
We know the rate, we need to find energy
Cost = $0.05
E = 0.4kW x 2h
E = 0.8kW•h

22. Try This
An air conditioner has a power rating of 1000W per room and the energy company charges 6.67 cents per kW•h. How much does it cost a family to use the air conditioner in their living room during July and August for 10 hours a day?
Cost = Rate x E
= $/kW•h x 620kW•h
E = Pt
E = 1000W x t
We know the rate, we need to find energy
62 days in July and August multiplied by 10 hours a day
Cost = $41.35
E = 1.0kW x 620h
E = 620kW•h

23. 1 year has 52 weeks and the dishwasher is used for 1 hour each week
Try This Challenge
A family uses a dishwasher that draws 10A from a 120V source. The dishwasher is run 1 hour a week. What rate is the power company charging the family if the family if at the end of 1 year the family is charged $4.99.
P = IV
P = 10A x 120V
E = Pt
Rate = Cost E
P = 1200W
Cost = Rate x E
Rate = $4.99
62.4kW•h
E = 1.2kW x 52h
Cost = Rate x E E
P = 1.2kW
E = 62.4kW•h
1 year has 52 weeks and the dishwasher is used for 1 hour each week
Rate = 0.08$/kW•h

24. Summary of Formulae V = IR P = IV E = Pt Cost = Rate x E
Remember, you don’t have to memorize formulas. However, you must know how to use them!
V = IR
P = IV
E = Pt
Cost = Rate x E
V = Voltage (V)
I = Current (A)
R = Resistance (Ω)
P = Power (W)
E = Energy (J or kW•h)
t = time (s)

25. ~ fin~

26. Do BLM 3-40 The Price of Energy