Energy: the Ability to Do Work Or Cause Change

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Energy

Unit Notes

Energy: the ability to do work or cause change

When an object or organism does work on another object, some of its energy is transferred to that object, so work is the transfer of energy.

EX/ a moving bowling ball has energy; when it hits the pins, they move because energy was transferred to them from the bowling ball, therefore, the bowling ball did work on the pins.

 There are two basic kinds of energy, dependent on whether an object is moving or not:

Kinetic energy (KE) is the energy of motion.

 KE increases with more mass – a bowling ball moving at 5 mph has more KE than a golf ball moving

at 5 mph

 KE increases with more velocity – a golf ball moving at 10 mph has more KE than a golf ball moving

at 5 mph

Potential energy (PE) is stored energy (there is the “potential” to do work).

 PE increases with greater height – a boulder on the top of a cliff has more PE than one with the

same mass at the bottom of the cliff

 PE increases with more tension – a wind-up toy has more PE when it is wound than after it is

released

 PE increases with more mass – a boulder sitting on a cliff has more PE than a pebble sitting on

the same cliff

 PE increases with stronger chemical bonds – explosives have more PE than cinder blocks

 Energy can also take on six main forms, each of which can be classified as either kinetic or

potential:

Mechanical/Physical energy -- the KE or PE associated with the motion or position of an object

EX/ a frog leaping, a moving school bus, sound waves (because molecules are vibrating), motors

Thermal energy (Heat) -- the KE of moving particles of matter (the faster they move, the more heat energy they have)

Temperature is a measure of how fast the molecules are moving. Absolute zero is a theoretical temperature that exists when all molecular motion has stopped, which is around 273 C or 0 Kelvin.

Chemical energy -- a type of PE stored in the bonds that hold chemical compounds together

EX/ chemical energy is stored in food (released when digested) and also stored in a match (released when struck)

Electrical energy -- the KE of moving electrons

EX/ anything that runs on a battery or electricity

Electromagnetic energy (EM) -- KE with both electrical and magnetic properties (electromagnetic waves). This includes light, radio and TV waves, microwaves, gamma rays, X-rays, ultraviolet, etc.

Nuclear energy -- a type of PE stored in the nucleus of an atom and released during nuclear reactions (either when the nucleus is split or joins with another)

EX/ the sun, nuclear power plants

Don’t confuse this with “conserving” energy

instead of wasting it -- that’s different!

 Law of Conservation of Energy

The Law of Conservation of Energy states that energy cannot be created or destroyed, it can only change form. When one type of energy changes into another type of energy, an energy transformation (conversion) has taken place.

Examples of some energy transformations:

- A toaster converts electrical energy into thermal energy

- A gas-powered motor converts chemical energy into mechanical energy

- Your body digests food to run, thereby transforming the chemical energy in food into thermal and

mechanical energy in your body

- Lighting a match: mechanical (striking)  thermal (friction) AND

chemical (sulfur, etc.)  thermal (heat) and EM (light)

A common energy transformation is the exchange between KE and PE, as in stretching a rubber band or throwing a ball into the air.

 Energy Interactions

The different types of electromagnetic energy (EM) -- gamma rays, X-rays, visible light, ultraviolet light, radio and TV waves, infrared, microwaves, etc. -- all differ from each other because of their wavelengths. This difference in wavelength causes energy to do different things when it comes into contact with various substances.

When energy meets a new material, it can be:

Reflected -- bounced back off the surface

- Materials that are light in color and have smooth, shiny surfaces are good reflectors of energy

Refracted -- bent as it passed through the material

Absorbed -- taken into the material

- Materials that are dark in color and have rough textures are good absorbers of energy

- A material that is a good absorber of energy is also a good radiator of energy (an object that

absorbs a lot of heat will feel hot because it’s giving off that heat)

Scattered -- randomly reflected and refracted all over

Transmitted -- passed through the material completely unchanged

Exactly which thing happens to the energy depends on the type of energy and the type of material it hits.

EX/ Visible light can easily get through glass, but heat cannot; X-rays can go through your skin but not lead

 Energy Transfer

Energy can be moved (transferred) from one place to another in three ways, depending on the material:

Conduction is the transfer of energy through molecular vibrations. This occurs in solids because the molecules are close enough together so that when they vibrate, they bump into each other, essentially passing the energy along. An object that transfers heat easily is said to be a good conductor (all metals) but an object that does NOT transfer heat easily is said to be a good insulator (cloth, rubber, Styrofoam, glass, wood, etc.)

EX/ If a metal fork is placed into a fire, the end that is in the fire will become hot because the fire is causing those molecules in the fork to vibrate quickly. They bump into each other, bumping all along the fork, causing the molecules in the end of the fork that is NOT in the fire to vibrate too (like dominoes) so even that end gets hot. [This is why the handles of pots and pans often have wood or plastic to insulate them from the heat.]

Convection is the transfer of energy due to temperature and density differences in fluids (a fluid is either a gas or liquid). Hot material rises because it is less dense, it spreads out, cools off, becomes more dense, and sinks. This circular motion creates a pattern called a convection current. Convection occurs in gases and liquids because the molecules are free to move from place to place.

EX/ When a pot of water is placed over a fire, the water directly over the fire gets hot because the molecules are vibrating. The molecules spread out, become less dense, and the hot water rises. When it reaches the top, it spreads out, cools off, becomes more dense, and sinks. It reaches the bottom where the heat is, the molecules vibrate, and the process repeats itself until the heat is uniformly spread out through the pot.

Radiation is the transfer of energy by electromagnetic waves. No molecules are used; therefore, radiation can occur in solids, liquids, gases, or vacuums (where no molecules exist, such as in outer space).

EX/ Starlight/sunlight reaches Earth due to radiation; the light in the ceiling reaches your desk by radiation; microwaves move through your food by radiation; anything in the EM spectrum moves by radiation.

 Phase Changes

There are essentially four phases (states) of matter: solid, liquid, gas, and plasma.

Plasma is a kind of superdense gas that only exists in stars.

Imagine starting with a solid substance and continuously adding heat to it (like heating ice on the stove):

The less heat a substance has, the slower its molecules are moving. In a solid, the molecules are stuck in place and cannot move around each other. If heat is added to the substance, the molecules will vibrate back and forth but remain in place. However, as the vibrations increase, the KE of the molecules increases so the temperature of the substance increases. As more heat is added, the molecules will continue to vibrate faster and faster until they begin to actually break the bonds between them and spread out. Now the substance is becoming a liquid, and while this happens, the heat energy being added is used to separate the molecules, not make them move faster, so the temperature doesn’t change. Once the substance is a liquid, the added heat will make the molecules move faster and cause the temperature to rise again. When a substance is in the liquid phase, the molecules are very close to each other…..they can slide between each other but that’s the limit of their motion. As more heat is added, the molecules will move around each other faster, increasing their KE and making the temperature go up. Eventually, the heat will cause the molecules in the liquid to completely separate, allowing them to move freely and very rapidly all over the place. It is becoming a gas, and while this happens, the temperature of the substance stays the same. Once turned into a gas, the heat will again make the molecules move faster, raising the temperature.

Energy is absorbed Energy is released

Melting: solid  liquidFreezing: liquid  solid

Boiling/Vaporizing: liquid  gasCondensing: gas  liquid

Sublimation: solid  gas

[Evaporation is changing from liquid to gas without reaching the boiling point.]

Evaporation is a cooling process. Since energy needs to be absorbed by the liquid molecules to turn them into a gas, that energy comes from the surface that the liquid is on, thus cooling the surface down. (This is how sweat helps you to control your body temperature.)

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