Energy and Work

Physical Science

Energy/Work
Unit 3

Conservation of Energy and the Increase in Disorder
National Standards The total energy of the universe is constant. Energy can be transferred by collisions in chemical and nuclear reactions, by light waves and other radiations, and in many other radiations, and in many other ways. However, it can never be destroyed. As these transfers occur, the matter involved becomes steadily less ordered.

National Standards All energy can be considered to be either kinetic energy, which is the energy of motion; potential energy, which depends on relative position; or energy contained by a field, such as electromagnetic waves.

Georgia Standards - distinguish between kinetic and potential energy
describe how energy can be changed from one form to another
apply the Law of Conservation of Energy
define and calculate work

(Text - pp. 124 - 129)
Potential vs. Kinetic Energy
** Scientists have difficulty defining energy because it exists in so many different forms.
*** Kinetic Energy - Energy in the form of motion.
** The greater the mass, the greater the kinetic energy.
*** Potential Energy - Stored energy
** The amount of potential energy a sample of matter has depends on its position or its condition.

Kinetic Energy
* Forces can set objects in motion.
** Objects in motion can cause changes.
** Kinetic energy - energy of an object in motion.
** Kinetic means moving.
** The greater the speed of a moving object, the greater the kinetic energy.
*** The kinetic energy of moving objects is equal to one- half the mass times the square of the speed.
formula
K.E. = 1/2 mv2
** All moving objects have kinetic energy.

Potential Energy
*** The ability of an object to cause change due to its position is called potential energy.
** Potential energy is stored energy.
** In general, the potential energy increases when an object is pushed farther away from the object pulling on it.
** Gravitational Potential Energy - Potential energy due to position above ground.
** G.P.E. = mass x acceleration due to gravity x height
P.E. = mgh

Energy Conversion
** The change of any energy from one form to another is called energy conversion.
** Potential energy may be converted to kinetic energy and kinetic energy may be converted to potential energy.
* When a pendulum swings back and forth, energy is continually being converted.
** At the end of the swing, the pendulum has no kinetic energy, but has potential energy.
** When you stretch a rubber band or spring and let go, energy conversion takes place.
* The potential energy of the rubber ban is converted into kinetic energy.
** An archer has to stretch a bow to put the arrow in. When the archer lets go, the potential energy of the bow is converted into kinetic energy for the arrow.

Friction and Mechanical Energy
** Kinetic energy and potential energy are two forms of mechanical energy.
** Mechanical energy is energy due to motion or position.
** If a pendulum swings long enough, it will come to a halt, because it has lost both kinetic and potential energy.
*** Friction brings most moving objects to a halt.
* When friction is reduced, motion continues longer.
** Lubricants cut down on friction.
** Friction cannot be totally eliminated.
*** Temperature increases are found wherever friction is present.
** Large amounts of mechanical energy appear to be used up whenever large temperature increases occur.

Einstein's Theory of Relativity
** There is no way to tell if something is at rest or is moving at constant velocity.
*** You can say you are moving relative to some reference object.
** Motion is completely relative.
** No object may move faster than the speed of light.
** When objects are accelerated to high speeds they gain mass.
** The relationship between mass and energy is the formula:
E = mc2
** Einstein thought of mass and energy as being equivalent.
** For everyday objects with everyday energy changes, the change in mass with increase in speed is undetectable.

Defining Work
** Two conditions have to be met for work to be done.
a. object must move
b. a force must be acting on the object part or entirely in the direction of motion
*** If you push on a wall and it does not move, you have not done work.
** Pushing a lawn mower is work.
* You exert a force and the lawn mower moves.
** The greater the force in the direction of motion, the more work is done.
** The farther you push the lawn mower, the more work is done.
*** Work = force x distance
W = f x d
*** Force is measured in Newtons (N).
*** Distance is measured in meters (m).
*** Work, like energy, is measured in Joules.
*** One JOULE equals the work done by a force of 1 N that moves an object a distance of 1 m.
** Large units of work are the kJ - 1000 J, and the megajoule - 1,000,000 J.
** The Joule is named after James Joule, a 19th century English physicist.
** newtons - Unit of FORCE - Required to accelerate 1 kg 1 meter/sec/sec
** Joules - Unit of WORK - also equal to 1 newton-meter

w = f x d
Joules (newton-meters) = newtons x meters
watt - unit of POWER (rate at which work is done)
(watts are measured in Joules / second)
power = work done / time
1 Joule/sec = 1 watt
1 kilowatt = 1000 watts
1 horsepower = 746 watts

Law of Conservation of Energy
** A substance can lose internal energy either by giving off heat or by doing work.
*** Law of Conservation of Energy - Energy cannot be created nor destroyed, but may change forms.

National Standards Heat consists of random motion and the vibrations of atoms, molecules, and ions. The higher the temperature, the greater the atomic or molecular motion.

Georgia Standards Temperature and Heat
recognize the difference between the motion of the particles of an object and the motion of the object itself
relate changes in particle motion to thermal energy and temperature
compare and contrast heat and temperature
(Text - pp. 134 - 141)

Temperature Changes and Heat
** Temperature is a measure of how hot or cold something is.
** In everyday metric units, temperatures are measured in degrees Celsius.
** When something is warmer than its surroundings, it tends to cool down.
** Something that cools down is said to lose heat.
** Cold is a lack of heat.
*** Thermal energy is the total energy of the particles in a material.
** Heat tends to move from a hotter substance to a colder substance.
** The colder substance takes in heat lost by a hotter substance.
*** When the two substances are at the same temperature, no further movement of heat is observed.
** When hot and cold water are mixed, the temperature of the mixture will be somewhere in between.
** When equal amounts of hot and cold water are mixed, the temperature of the mixture will be halfway in between.
** Heat is measured in joules and involves transfer of energy.

A Model for Specific Heat
** It takes different amounts of heat to raise the temperature of equal masses of different substances by the same number of degrees.
*** Water has a very high specific heat.
** A water molecule has little mass and absorbs much more heat than an equal mass of copper, or lead.
(Text - pp. 141 - 144)

Specific Heat
** The temperature of the change of a substance that gains or loses heat depends on the substance.
*** The amount of heat gained or lost by a substance is determined by three factors.
a. the kind of substance
b. the amount of the substance
c. the temperature change that takes place
** The amount of heat gained or lost by a substance is measured in Joules.
** The amount of heat needed to raise the temperature of 1 kg of water by 1 degree C is 4180 Joules.
** Only 450 Joules of heat is needed to raise the temperature of 1 kg of iron by 1 degree C.
*** The quantity of heat required to raise the temperature of 1 g of a substance by 1 degree C is referred to as the specific heat of a substance.
**Many uses of water depend on water's high specific heat.
* A lot of excess heat from the engine can be absorbed by water.
* Electric generating plants often use water to absorb excess heat.

Phase Changes and Heat
** When heat is added to ice at 0 degrees C, the ice melts and the temperature does not rise until all the ice is melted.
** When heat is added to to water at its boiling temperature, the water boils and the temperature does not increase until all the liquid has boiled off.
** Heat of Fusion - the amount of heat necessary to melt a substance.
* Ice at 0 degrees C must take in 334 kJ of heat for each kilogram of ice that melts to liquid water at 0 degrees C.
** Ice absorbs 334 J from a drink for each gram of ice that melts.
** Water at 0 degrees C absorbs only 84 J for each gram of water that warms 1 degree C.
** Heat of Vaporization - amount of heat required for a liquid to become a gas.
** Water requires 2260 kJ/kg to change boiling water to steam.
** An equal amount of heat is released when a gram of water vapor condenses, or becomes a liquid.
** The condensation of water vapor to form small droplets in the air (cloud) releases much heat into the air.

Explaining Heat
* Early models described heat as caloric or an invisible fluid.
** Benjamin Thompson (Count Rumford) (1753-1814) became convinced that heat was not a fluid.
* James Joule (1818 - 1889) showed that a certain amount of work always produces the same temperature change as adding a certain amount of heat.
* Joule measured temperature changes in different substances and the unit of energy is named after him.
** Because of Joule's work, scientists became convinced that heat, like work, is a form of energy.
*** The modern definition of heat is that heat is the form of energy transferred because of a temperature difference.
*** Heat always moves from the warmer substance to the cooler substance.
** A substance may give off heat or take in heat. However, it does not contain heat.
** Internal energy - Energy when it is inside a substance.
** Internal energy does not produce visible motions nor reflect a change in position of a substance.
*** A rise in temperature is a sign of increased internal energy.
*** A change in phase is also a sign of increased internal energy.

National Standards Everything tends to become less organized and less orderly over time. Thus, in all energy transfers, the overall effect is that the energy is spread out uniformly. Examples are the transfer of energy from hotter to cooler objects by conduction, radiation, or convection and the warming of our surroundings when we burn fuel.

Georgia Standards Measuring and Using Thermal Energy
relate specific heat to the rate of temperature change in an object
apply concept of specific heat to lab situation/setting
identify the 3 methods of thermal energy transfer and cite examples of each
compare and contrast effectiveness of various heat insulators and conductors
describe and/or construct models of heating and cooling systems
identify problems associated with thermal pollution and possible solutions to these problems
explain how heat is used to do work using examples such as engines: 4- stroke, steam, etc.

(Text - pp. 153 - 152 - 154)
A Model for Heat Transfer
*** When particles with a lot of kinetic energy collide with particles with little energy, some of the kinetic energy is transfered and those particles are caused to move faster.

Types of Heat Transfer
** Any material that flows is a fluid.
** Heat moves from matter at a higher temperature to matter at a lower temperature.
** Heat moves in three basic ways: 1) conduction 2) convection and 3) radiation.
** Conduction - heat moving through a substance without it moving. ex.- metal
* Insulator - substance that does not transfer heat energy well. ex.- glass, wood, air
** Convection - The movement of heat energy through a substance by movement of currents.
** Radiation - energy that travels through space as pure energy.
** Radiation may travel through some kinds of energy. ex. air.
** Infrared - heat radiation that is invisible to the human eye, but may be detected by special film.

(Text - pp. 155 - 158)
Cooling Systems
* In order to cool anything, it is necessary that heat be removed from it.
** One of the simplest ways to cool a space is to radiate heat into the air.
** In cooling a room by using a fan, the warm air inside the room is pushed out of the room through a door or window and cool air from outside is circulated inside the room.
** Evaporation is the principle that is used to operate refrigerators, freezers, and air conditioners.
** These devices take advantage of the heat transfer that takes place as a result of the evaporation and condensation of a substance called a coolant.
** As freon is pumped through a refrigerator by a motor, it repeats the cycle of evaporation and condensation over and over again.
** Each time the cycle takes place, the freon withdraws heat from the inside of the refrigerator and releases heat on the outside.

Insulation
** Heat is transferec through the walls, windows, and roofs of buildings.
** There are several ways to cut down on heat loss, such as attics and walls can be lined with insulating materials.
** Storm windows can be placed over windows.
* Cracks around doors can be sealed.
** When it is very hot outside, heat moves readily through the walls, windows, and roof to the inside of the building.
** Insulating materials, roof overhangs, and shade trees can cut down on heat from direct sunlight.
** Insulating materials prevent heat transfer through fibers not conducting heat well and heat cannot be transferred by convection currents through the air.

Georgia Standards identify problems associated with thermal pollution and possible solutions to these problems
explain how heat is used to do work using examples such as engines: 4-stroke, steam, etc.

(Text - pp. 166 - 168)
Heat Engines
** Heat engines are devices that convert thermal energy into mechanical energy.
** Heat engines burn fuel in a process called combustion.

Internal Combustion Engines
** Fuel burns inside cylinders. ex - gasoline and diesel engines
** As fuel in burned inside the engine, the up and down motion of the cylinder is transferred to the wheels of the automobile through a series of moving parts.
** Each up and down movement of the piston is called a stroke.
*** An automobile engine is called a four stroke engine because the piston makes four strokes in each cycle.
** In an internal combustion engine, only part of the thermal energy produced by burning fuel is converted to mechanical energy.
** The rest is left over as waste thermal energy.