1. Section 3.3 Measuring Energy Input and Output 1

2. We use energy in every aspect of our life.
Driving, ipods, heating our homes, cooking food, watching tv.
How can we choose which type of energy to use? (ex: we could use electricity, gasoline, propane, hydrogen)
We need to measure the types and amounts of energy going into and coming out of the devices.

3. Power Power is the rate at which a device converts energy.
Unit of power is the watt (W).
1 watt is equal to 1 joule per second
1Watt = 1Joule/second
Ex: 44 W = 44 J/s
The faster a device converts energy, the greater its power rating.
Is an appliance with high watts or low watts better? Why?

4. Is an appliance with high watts or low watts better? Why?
Depends on the way you look at it.
High watts, is more powerful, stronger, might not have to use it as long
However, low watts is more energy efficient. Might save you money in the long run.

5. Power formula
For an electrical device, the power is the current multiplied by the voltage.
Power = I x V (watts = amps x volts) P I V

6. Power Calculation Example
A hair dryer has a power rating of 1000 W. It is plugged into a 120-V outlet. What is the current flowing through the hair dryer?
Known: Power (P) = 1000 W, Voltage (V) = 120 V
Unknown: Current (I) = ?
Formula: I = P/V
Solve: I = 1000 W/ 120 V
I = 8.33A

7. P t Energy Energy formula E
You can use the power rating of a device to determine the amount of energy the device uses (power is the rate at which a device converts energy).
Energy formula
Energy consumption of an electrical device is its input power multiplied by the time the device is used.
E = P x t (joules = watts x seconds) E P t

8. Energy Calculation Example
A microwave oven has a power rating of 800W. If you cook a roast in the oven for 30 min at high, how many joules of electrical energy are converted into heat by the microwave?
*Remember that W = J/s
Known: P = 800 W (J/s), t = 30 min
30 min = ? s
30min x 60 s/min = 1800s
Unknown: E = ?
Formula: E = P x t
Solve: E = 800 W x 1800s
E = J = 1.4 MJ

9. Kilowatt Hours
in our households, it doesn’t take long to use up a lot of Joules of energy
we use a more practical unit called the kilowatt hour
it is calculated with the same formula
E = Pt
power is measured in kilowatts
(1 kW= 1000 W)
time is measured in hours (h)

10. Summary A. Power power is the rate at which a device converts energy
measured in watts (W)
P = I × V
where: P = power in watts (W)
I = current in Amperes (A)
V = voltage in volts (V)

11. Example: Power
Calculate the power used by an MP3 player if it uses a current of 5.0 A and a voltage of 3.0 volts.
I = 5.0 A
V = 3.0 V
P = ?
  P = IV
= (5.0 A)(3.0 V)
= 15 W

12. Summary
B: Energy
You can use the power rating of a device and the time it runs to calculate the amount of energy used
measured in Joules (J)
E = P × t
where: P = power in watts (W)
t = time in seconds (s)
E = energy in Joules (J)

13. Example: Energy
Calculate the energy usage if a 15 W MP3 player operates for 2 minutes.
P = 15 W E = Pt
T = 2 min x 60 s/min = (15 W)(120 s)
= 120 s = 1800 J
E = ?

14. Assignment Complete Worksheet 10, only #1-5
We’ll finish efficiency on Monday.
Read textbook chapters 1,2 and 3!