Please review the example problems before answering the assigned questions/problems.

Review Problems

E1. Energy cannot be created or destroyed, it can only change form. Certain devices or processes help converting energy from one type to another. List four devices which convert electrical energy to

a) thermal energy,
b) mechanical energy,
c) invisible electromagnetic radiation, and
d) visible electromagnetic radiation.

a) A toaster converts electrical energy to thermal energy (or heat).
b) A loudspeaker converts electrical energy to mechanical energy (sound is air in motion).
c) A remote control device converts electrical energy to electromagnetic radiation (usually in infrared region).
d) A laser pointer converts electrical energy to visible light (electromagnetic radiation).
 

E2. A typical adult in U.S. converts 2000 Calories of food energy per day. Calculate the average power of an adult, assuming that the energy is converted evenly throughout the day (24 hours). (This is not strictly true. One converts much less energy by sleeping than by walking or running).

Power is defined as the amount of energy converted divided by the time period it takes to convert that amount of energy (for example per second, per hour, per day). In other words, power is the rate (speed) of converting energy.

Average Power of an Adult = (Energy/Time) = 2000 Calories/24 hours = ~83 Calories/hour.

Note that Calories/hour is not a commonly used power unit, although it is a physically legitimate unit. A typical power unit is Watt, 1 Watt = 1 Joule/Second. One should convert Calories into Joules, and day into seconds to calculate the power in Watts.

1 hour is 60 minutes, and 1 minute is 60 seconds; then 1 hour = (60)(60) = 3600 seconds (Probably you knew this!)

Now, convert Calories into Joules: 1 Calorie is equal to 4184 Joules; 83 Calories = (83)(4,184) = 347,272 Joules

Power = (347,272 Joules/ 3600 seconds = ~96 Joules/second or 96 Watts (Remember the power of a typical household incandescent light bulb is 60 Watts.)

E3. An electric utility company measures electricity consumption (conversion) in energy units of kilowatt-hours to bill its customers. Calculate how much it would cost to keep a 60- Watt bulb on for 24 hours if the cost of electricity is 10 cents for one kilowatt-hour.

Energy = Power x Time

Power of the light bulb = 60 Watts, or (60 Watts/1000) = 0.06 kilowatt

Electrical energy needed to keep the bulb on for 24 hours =

(0.06kilowatt) x 24 hours = 1.44 kilowatt-hours

Cost = (1.44 kilowatt-hours ) x (10 cents/kilowatt-hour) = 14.4 cents
 

Assigned Questions/Problems
 

1) Write down all the energy conversion steps that enable you to listen to a live news broadcast on the radio - in the path from the newscaster's mouth to your ear.  Start with mechanical energy (sound from the newscaster) ---> electrical energy (in the microphone) and continue.
(20 pts)

2) The power of a typical blow dryer is 1.5 kilowatt. (1 kilowatt is 1000 Watts). Calculate how many Calories of electrical energy one converts using a blow dryer for 4 minutes. (You can solve this problem; just remember 1 minute is 60 seconds and Energy = Power x Time. Watch your units.)
(20 pts)

3) Solve Problem 3 in Chapter 1 of your textbook (page 28).
(20 pts)

4)There are approximately 30,000 Calories of energy in one gallon of gasoline. Imagine that you can convert all this energy to food energy. How many days would an adult feed on this amount of energy? What is the power of an automobile (in kilowatts), if it burns one gallon of gasoline in 30 minutes? Compare the power of an automobile and an adult human being.
(20 pts)

5) Solve Problem 8 on page 28.

For questions e-mail Abraham J. Brandt  (ajb220@psu.edu).