Work and Energy

Energy is defined as the ability to do work, therefore W = ΔE, or work is equal to the change in energy. When work is done an object gains energy. In the absence of any outside forces, such as friction, the total work input is equal to the total work output/energy output.

Forms of Energy

1. Chemical Energy: Is the potential energy stored in the chemical bonds of compounds such as fossil fuels.
2. Electrical energy is the work done by moving charges.
3. Nuclear energy is the potential energy stored in the nucleus of an atom.
4. Solar energy results from a hydrogen-hydrogen nuclear fusion reaction with the release of radiant energy.
5. Kinetic energy is the energy an object has due to its motion.
6. Gravitational potential energy is the energy an object has due to its position relative to the Earth’s surface.
7. Mechanical energy is the sum of an object’s kinetic energy and gravitational potential energy.
8. The amount of thermal energy in a substance is determined by the average kinetic energy of the individual atoms.
9. Potential Energy
   • Elastic Potential Energy: The energy stored in an object that is stretched or compressed and will return to its original form if released.
   • Chemical Potential Energy: The energy stored in the bonds of chemical compounds. Any substance that can be used to do work through a chemical reaction has chemical potential energy.

Kinetic energy

Kinetic energy is the energy associated with the motion of an object. Kinetic energy is calculated as:

E_k = 0.5*mv²

As shown in the formula, an object with a mass and a speed will have a certain amount of kinetic energy. Another object with twice the mass and the same speed will have twice the kinetic energy. A third object with the same mass but twice the speed will have four times as much kinetic energy.

Mechanical Energy

Mechanical energy is defined as the energy due to the motion and position of an object. Mechanical energy can be calculated as:

E_m = E_p + E_k

Energy Conversions

• Photosynthesis converts light energy, carbon dioxide, and water into chemical energy in the form of glucose.
• Cellular respiration converts chemical energy in the form of glucose into chemical energy in ATP and heat energy.
• Combustion reactions convert chemical energy in fossil fuels into thermal energy.
• Hydroelectric dams convert the potential energy of water stored behind the dam into electrical energy - Hydroelectric power.
• A coal-burning power station converts the chemical energy of fossil fuels into electrical energy - Thermal power station.
• Solar cells convert solar energy directly into electricity
• In a hydrogen fuel cell, hydrogen reacts with oxygen to form water and release energy.

Energy Flow in Systems

A system is a set of interconnected parts, everything else is considered the environment.

An open system is one that exchanges both matter and energy with its surroundings (such as a tree).

A closed system is one that cannot exchange matter but can exchange energy with its surroundings (such as the Earth).

An isolated system is one that cannot exchange matter or energy with the environment (such as the Universe).

Laws of Thermodynamics

The first law of thermodynamics states that energy cannot be created or destroyed. It can only be transformed from one form to another, and the total amount of energy never changes.

The second law of thermodynamics states that thermal energy always flows naturally from a hot object to a cold object.

Machines

Machines make work easier by converting an initial energy input to the type of desired (useful) energy output. All energy output that is not being used to do useful work is considered waste energy (such as energy lost due to friction).

Efficiency (of a machine) is a measurement of how effectively a machine converts energy input into useful energy output. It is calculated as:

Efficiency = (Useful work output) / (Total work input).

Percent efficiency is Efficiency *100.

Renewable and Non-Renewable Energy

Renewable energy sources are continually and infinitely available (such as solar, wind, water, geothermal, tidal, and biomass).

Non-renewable energy sources are limited and irreplaceable (such as nuclear and fossil fuels).

The Effects of Energy Use

The extraction and combustion of fossil fuels may negatively affect environment in ways such as:

• Ecosystem disruptions.
• Oil spills
• Production of greenhouse gases (GHGs)
• Release of chemicals and other toxins, such as those that contribute to acid rain