Energy, Work, and Power Notes

Work

Defined as the product of the applied force and the parallel distance through which the force acts.
Formula: Work = force \times distance
For work to be done, something must move in the same direction as the force.
- Example: Pushing a box across the floor is work; pushing against a stationary wall is not.
Walking a mile and running a mile involve the same amount of work if the mass and distance are the same; the runner does the work faster.

Defined as work per unit of time.
Formula: Power = \frac{work}{time}
Units of power:
- Horsepower (historical comparison to work done by horses).
- Watt: Electric bills charge by the kilowatt-hour (kWh), where 1 kilowatt = 1000 watts.

Potential Energy: Stored energy due to an object's position or chemical composition.
- Example: A rock on a hill or a charged battery.
Kinetic Energy: Energy of motion.
- Example: A rock tumbling down a hill (potential energy converted to kinetic energy).

Mechanical Energy: Associated with objects like cars, baseballs, rocks, machines, etc.
Chemical Energy: Involved in chemical processes (absorbed or given off).
- Example: Photosynthesis (plants absorb light to make glucose and oxygen).
- Example: Burning gasoline in a car (energy released moves the car).
Radiant Energy (Electromagnetic Radiation): Light energy, including visible and invisible light (gamma rays, x-rays, microwaves, etc.).
- Electromagnetic spectrum: Gamma rays (highest energy) to radio waves (lowest energy).
Electrical Energy: Electricity.
Nuclear Energy: Energy obtained from atoms (to be discussed later).

Energy is never created or destroyed; it can only be converted from one form to another.
Total energy remains constant.
Example: Acorn absorbs sunlight and grows into an oak tree; burning the oak tree releases the same amount of energy as it absorbed.
Energy can be converted between potential and kinetic energy, but the total energy remains the same.
Two ways to transfer energy: work and/or heat.
- Example: Add heat to warm a room, remove heat to cool a room.

System: The subject under study (e.g., a flask in a chemistry experiment, the Milky Way Galaxy).
Surroundings: Everything outside the system.

Open System: Matter and heat can be exchanged between the system and the surroundings.
- Example: A dog taking in food and excreting waste, while also exchanging heat with the environment.
Closed System: Heat but not matter can be exchanged between the system and the surroundings.
- Example: A closed bottle of soda that warms up over time.
Isolated System: Neither matter nor heat can be exchanged between the system and the surroundings (no true isolated system exists).
- Example: A thermos bottle (approximates an isolated system but slowly loses heat).

The efficiency of a system is how much of the energy is in the form of work as opposed to heat.
Example: Car engine burning gasoline.
- Gasoline is burned to move mechanical parts, but also produces heat.
- Efficiency of a car: About 25% (25% of the energy from gasoline is converted to useful work, 75% is lost as heat).
Human body: Also about 25% efficient in converting food into energy.

Petroleum (35.43%)
Coal (28.15%)
Natural Gas (23.46%)
Hydroelectric (6.27%)
Nuclear (5.79%)

Fossil fuel from the breakdown of organisms.
Measured in barrels (42 U.S. gallons).
About 50% of a barrel of oil is used to make gasoline, and about 45% for diesel, jet fuel, heating oil, lubricants, etc.
Less than 5% is used for medicines, fabrics, plastics, etc.
- Fabrics and plastics are used in artificial heart valves, prosthetics, protective helmets, and many other very useful devices.

Radioactive materials (uranium, plutonium) used instead of fossil fuels to power plants for electricity production.

Solar cells: Convert sunlight into electricity.
Power tower: Mirrors focus sunlight on a tower with a boiler; heat produces steam to turn a turbine and generate electricity.
Biomass
Geothermal Energy