ENERGY 7.3

Chapter Seven: Energy

 7.1 Energy and Systems  7.2 Conservation of Energy  7.3 Energy Transformations

Chapter 7.3 Learning Goals  Explain what it means when energy is conserved.

 Use energy conservation to solve problems.

 Discuss applications of energy conservation in daily living.

Investigation 7B

Conservation of Energy



Key Question: What limits how much a system may change?



7.3 Conservation of Energy

The idea that energy tranforms from one form into another without a change in the total amount is called the

law of conservation of energy

.

 The law of energy conservation says the total energy before the change equals the total energy after it.

7.3 Conservation of Energy

 When you throw a ball in the air, the energy transforms from kinetic to potential and then back to kinetic.

Solving Problems  A 2 kg car moving with a speed of 2 m/sec starts up a hill.  How high does the car roll before it stops?

Solving Problems 1.

 Looking for: …height of hill 2.

 Given … mass = 2 kg, v = 2 m/s 3.

   Relationships:

Energy transformed from E K E K

= ½ mv

2 E P

=mgh

to E P

Solving Problems 1.

     Solution Find beginning E

K E K

= ½ (2 kg) (2 m/s)

2 = 4 Joules

Assume energy before = energy after 

E K = E P E P

=mgh

4 J = mgh

h = (4 N  m)/(2 kg)(9.8 N/kg) = .2 m

 

7.3 Conservation of Energy

Many people are concerned about “running out” of energy. What they worry about is running out of certain

forms

of energy that are easy to use, such as fossil fuels like oil and gas.

 

7.3 Conservation of Energy

It took millions of years to accumulate these fuels because they are derived from decaying, ancient plants that obtained their energy from the Sun when they were alive. Because it took a long time for these plants to grow, decay, and become oil and gas, fossil fuels are a limited resource.



7.3 Conservation of Energy

Regular (incandescent) light bulbs convert only 10% of electrical energy to light.

 That means 90% of the energy is released as wasted heat.

7.3 Conservation of Energy

 Other forms of energy, such as thermal energy, flowing water, wind, and solar energy are not as limited.

Investigation 7C

Energy and Efficiency



Key Question: How well is energy changed from one form to another?

A Matter of Survival  In 2005, the U.S. Defense Advanced Research Projects Agency (DARPA) launched its VHESC. program.

 The goal of the program is to develop solar cells that would operate at or above 50 percent efficiency.