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 EK to EP

EK = ½ mv2

EP =mgh

Solving Problems

1. Solution Find beginning EK

EK = ½ (2 kg) (2 m/s)2 = 4 Joules Assume energy before = energy after

EK = EP

EP =mgh 4 J = mgh

h = (4 Nm)/(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 SurvivalIn 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.