A lesson on electric power and its application to energy consumption in household appliances (50 min).

1. Topic: Electric Power
Before we start to talk about electric power we need to review some terms.
We know that moving charge can deliver energy to multiple appliances or areas
where this energy will be converted into work or other types of energy.
Review questions:
1. How do we call moving charge? - Electric Current - amount of
charge going through a crossestion of a wire in a unit of time.
1. What moves charge in a wire? - Difference in electric potential
(AKA: voltage). Electrons move all the time. But chaotic motion is not
an electric current. Only directed motion of electrons will produce the
electric current. Difference in electric potential (voltage) makes
electrons to move in a specific direction.

2. Review questions:
3. What is voltage? - Difference in the amount of electric potential
energy per charge in different locations of a magnetic field. Electrons
would move from area of high electric potential to area of low electric
potential.
3. What opposes the electric current? - All the atoms and ions that
make up the material through which electric current is traveling. They
create resistance.
3. What is the relationship between electric current, voltage and
resistance? - Voltage makes electrons move in a specific direction
while resistance opposes the motion of electric current.

3. Work-Energy Theorem:
Whenever work is done - energy changes.

4. Electric Power (Pe)
- rate at which electric energy is supplied to an element.

5. Electric power is the same thing as mechanical power, a rate
at which work is done; both measured in Watts.
Watt is equivalent to

7. What do we buy from power companies?
a) Electrons
b) Electric Current
c) Voltage
d) Power
e) Energy

8. Power companies usually give us the price for 1 kWatt・hour
of energy and then send us a bill based on how much energy
we’ve consumed.

9. How can I use 1 kWatt・hour of energy?

10. How can I use 1 kWatt・hour of energy?
With 1 kW・h I can burn 100W light bulb for 10 hours.
With 1 kW・h I can burn 60W light bulb for 16.7 hours.
With 1 kW・h I can boil water in 1500W tea kettle for 0.67 hours.

11. 1. How many amperes flow through a 60-watt bulb when 120
volts are impressed across it?
How many amperes flow through a 100-watt bulb when 120
volts are impressed across it?
Higher power consumption → Bigger current is drawn

12. 2. How much current in drawn by a 1200-watt hair dryer when
it operates on 120 volts? What is the resistance of the dryer?
Can we answer the second question in this problem without
solving for electric current first?

13. 3. Calculate the power dissipated in a toaster that has a
resistance of 14 ohms plugged in a 120-V outlet.
Another way to solve it:

14. 4. How much power is used by a calculator that operates on 8
volts and 0.1 ampere? If it is used for one hour, how much
energy does it use?

15. 5. Calculate the yearly cost of running a 5W
electric clock continuously in a location where
electricity costs 30 cents per 1 kW•h.

16. 6. Power consumption of my hair drier is 1200W. The cost of
electric energy in my area is 30 cents per kW•h. How much
does it cost me to dry my hair (~15 min)?

17. 7. Power consumption of my tea kettle is 1500W. If electricity
company charges me 30¢ /kW•h that I’m using, how much does
it cost me to boil water? It takes about 5 min for water to boil.

18. Homework:
At home, find and electric device. Look up its power
consumption and calculate the cost of one time use of that
device. You would need to estimate or measure time of the use
and ask your parents for the cost of electric power in your
household.
Estimate how frequently this device is used in a year and
calculate the cost of energy for this device per year.