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CHAPTER 15: AN INTRODUCTION TO ENERGY (Energy efficiency (The ratio of…
CHAPTER 15: AN INTRODUCTION TO ENERGY
What is Energy?
The capacity to perform work
Objects have energy either because of their motion or because of their position
Example of energy being used:
Pile driver that uses the impact of a heavy mass to drive a pile (pole) into the ground. Energy is required to lift the mass to a height - a motor may perform the work to life the heavy object, which then hains potential energy equal to the work done on it. When released, the object accelerated until it strikes the pile. As it falls, the obkect's potential energy is converted to kinetic energy. Upon striking the pile and drriving it a relatiely small distance into the ground, the kinetic energy is tranformed into sound and heat.
Forms of Energy
Classified as:
Kinetic Energy
Energy associated with an object's speed (motion)
Potential Energy
Refers to 'stored' energy that can be released to perform work
Mechanical Energy
The movement of objects and the conversion of potential energy into other forms of energy involving movement.
Example: The potential energy stored in water can be converted into mechanical energy either in a waterwheel, which can be used to grind grain, or in a turbine, which can be used to produce electrical energy.
Heat and Temperature
Heat or thermal energy are important in nearly all energy transformations. Define strictly, heat is this transfer or flow of energy.
Temperature: Measure of the average kinetic energy of the molecule within a material.
At lower temperatures, the speed of vibrations is lower.
At higher termperatures, the speed of vibration is higher and has a higher average molecular kinetic energy
In solids, the molecules vibrate around basically fixed positions.
To increase molecular vibration (kinetic energy), and therefore temperature, energy must be transferred into the material. The energy transferred is thermal energy (heat). Thermal energy is also required to cause a change in state
For example: Melting ice - the molecules need to gain sufficient energy to slip past one another. Thus, heat energy must be supplied to break the bonds between molecules and cause the solid to melt.
Whenever the total thermal energy increases, temperature will increase or a change of state occurs.
When two objects of different termperature are in contact with one another, thermal energy will transfer from the hotter one to the cooler one until they are at the same temperature.
Chemical Energy
Form of potential energy that is due to the relative positions of the atoms and their electrons in the molecules that make up a substance.
Example: Methane has chemical energy due to the particular arrangement of the carbon and hydrogen atoms within the methane molecules. This stored energy can be converted into other forms of energy(CO2) by combustion.
Combustion:
The reaction of a substance with oxygen resulting in the release of the chemical energy as chemical bonds are broken.
Energy Resources
Naturally occurring fuels or energy sources that can be converted into forms of energy useful to society.
Fuels: Substances that contain chemical (potential) energy that is released when oxidised; usually by combustion.
The combustion of methane, the main component of natural gas
Exothermic Reaction: Reactions that release heat.
Endothermic Reactions: Reactions that take up heat.
Physical changes can fall under both (e.g freezing water is exothermic and forming steam from water is endothermic)
The Units of Energy
Basic unit of energy is the joule (J), which is a very small amount of energy.
Multiple joules = Megajoules
Power
It is the amount of energy available or used over a given time.
Unit for power is watt (W)
Formula: Power (watts) = energy (joules) / time (seconds)
The energy rating of electrical appliances is usually given in watts.
Energy Transformations
Laws that govern energy conversion:
Law of Conservation of energy OR the first law of thermodynamics
Energy is neither created nor destroyed in normal physical and chemical processes; it is merely changed from one form to another. Consequently, the total quantity of energy remains thee same.
Second Law of Thermodynamics
Energy is converted from one form to another, some of it becomes degraded to a lower quality of energy. This degraded energy is less able to do work and in most conversions is released as heat.
Energy efficiency
The ratio of energy input to a system to the useful energy output.
Formula: Energy efficiency (%) = useful energy output / total energy input X 100
Energy conversion is never 100% efficient.
Implications of the laws of thermodynamics
Formula: Useful energy = Total energy in the resource - Energy needed to obtain and process the energy (first law) - Energy lost in obtaining and processing (second law)
High-grade energy resources are continually being run down. Some renewable energy source (e.g solar energy) are high-grade sources of energy and will be able to replace fossil fuels, but they still need an input of energy to collect and concentrate them.
The laws of thermodynamics and the fact that fossil fuels are a limited resource highlight the need for the development of renewable energy and energy conservation strategies.
STUDY DESIGN:
Comparison of Different Energy Sources
the conservation of energy (First Law of Thermodynamics) in energy conversions
efficiency of energy conversions between different forms of energy including mechanical, kinetic, heat, and potential
Fossil Fuels
the process of fossil fuel combustion (exothermic and endothermic reactions not required), and consequences
for the carbon cycle
