Name __________________________________ Period ______ Date ___________
Practice Problems: Energy part 2 – The Power Problem
In previous lessons, we learned that a Joule it a unit of work equivalent to 1 Newton-meter as
well as 1 Watt-second. These two equations are given below:
1 J = 1 Nm
1 J = 1 Ws
These equations can be used to solve a variety of problems involving the relationships between
work, energy and force. In this activity, we are going to focus on the relationship between
Joules and Watts, and as it applies to the cost of energy.
Directions – use the above equations to solve the following problems. You may need to rearrange the variables in the equations to find the answers. Remember to include the correct
unit in your answers. You should work groups of 3 or 4.
1. What Watt rating would you give to an appliance that consumes 50 J of power each
second?
2. The human body gives off 60 J of radiant energy per second. What is the Wattage of a
human being?
3. A standard residential rate (from PEPCO) is about $0.09 per kilowatt hour (this is
$0.09/kWh). How much does this cost in terms of Watt seconds? (hint: this is cost
divided by total number of kilowatt hours, so figure out the conversion of kilowatt hours
to Watt-seconds first, then divide that into the cost – it’s a very small number!).
4. Based on your answer to number 3, how much would it cost to operate each of the
following devices (assume continuous usage – which is of course not entirely realistic)
for the times indicated:
Device/Wattage Estimated Watt
time on
seconds
per day (in per day
seconds)
Cost per
day
(Wattseconds X
cost per
Joule)
Cost per Cost per
month
year
(assume
all months
have ~ 30
days)
Cell phone
charger/ 4
Watts
CFL bulb/ 20 W
Incandescent
bulb/ 60 W
LCD TV/ 213 W
Washing
Machine/
425 W
Toaster Over/
1200 W
Hair Dryer/
1538 W
Central Air
Conditioner/
5000 W
5. Which device(s) had a cost that surprised you? Explain why (briefly! But give more than
one)`