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  Key Question: What limits how much a system may change? Conservation of Energy

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?

1.Looking for:  …height of hill 2.Given  … mass = 2 kg, v = 2 m/s 3.Relationships:  Energy transformed from E K to E P  E K = ½ mv 2  E P =mgh 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 =mgh4 J = mgh  h = (4 N  m)/(2 kg)(9.8 N/kg) =.2 m Solving Problems

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  Key Question: How well is energy changed from one form to another? Energy and Efficiency

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.