Chapter 11: Using Energy – Energy Transformations, Efficiency, and Thermodynamics

Chapter 11: Using Energy

Introduction

This chapter explores practical energy transformations and transfers, focusing on the efficiency limits of energy use. It covers how energy is used in biological systems, the nature of heat and temperature, and the operation and limitations of heat engines and pumps.

Energy Use in the Body

Chemical Energy and Biological Work

• Chemical energy from food powers bodily functions such as movement, maintaining body temperature, and cellular processes.

• Energy requirements can be calculated for various physical activities.

Temperature and Heat

Definitions and Concepts

• Heat is energy transferred due to a temperature difference between objects.

• Temperature measures the average kinetic energy of the particles in a substance.

Heat Engines

Definition and Function

• A heat engine is a device that transforms thermal energy into useful work, such as a geothermal power plant.

• The efficiency of a heat engine is limited by fundamental physical laws.

Energy Models and Conservation

Basic Energy Model

• Energy can be transferred as work or heat, changing the system's total energy.

• If the system is isolated, total energy is conserved.

Sample Problem: Energy Transformation

Javelin Example

• Work done on an object increases its total energy, which can be partitioned into kinetic and potential energy.

• Example: If 270 J of work is done and potential energy increases by 70 J, the kinetic energy at the highest point is 200 J.

Efficiency

Definition and Calculation

• Efficiency ($e$) is defined as:

$e = \frac{\text{what you get}}{\text{what you had to pay}}$

• Lower efficiency results from greater energy losses, which can be due to process limitations (practical details) or fundamental limitations (physical laws).

• Typical human body efficiency is about 25% for converting food energy into mechanical work.

Problem-Solving Approach: Energy Efficiency

1. Identify the useful energy output ("what you get").

2. Identify the total energy input ("what you had to pay").

3. Ensure all energy values are in the same units.

4. Calculate efficiency using the formula above.

5. Assess the reasonableness of your answer based on typical efficiencies.

Example: Lightbulb Efficiency

• Given: 75 W incandescent and 15 W fluorescent bulbs both produce 3.0 W of visible light.

• Efficiency (incandescent): $e = \frac{3.0}{75} = 0.04 = 4\%$

• Efficiency (fluorescent): $e = \frac{3.0}{15} = 0.20 = 20\%$

• Conclusion: Fluorescent bulbs are more efficient at converting electrical energy into light.

Key Concepts Table: Efficiency and Energy Transformations

Process	Typical Efficiency	Limitation Type
Human body (mechanical work)	~25%	Process
Power plant	~35%	Fundamental
Incandescent bulb	4%	Process
Fluorescent bulb	20%	Process

Summary

• Energy transformations are governed by the laws of thermodynamics.

• Efficiency quantifies how much useful energy is obtained from a process.

• Understanding energy transfer and efficiency is crucial for analyzing biological systems, machines, and power generation.