CHAPTER 10 ENERGY

10.1 The Nature of Energy The usual definition of energy: the ability to do work Work is moving an object against an opposing force Work = distance × opposing force [ w = d x f ] SI unit of work or energy: the joule (J)

A joule A joule is the mechanical equivalent of heat meaning the number of units of work which the unit of heat can perform. Its value was found by James Prescott Joule in experiments that showed the mechanical energy which must be expended to raise the temperature of a unit weight of water by 1 degree F. was equivalent to 772 foot-pounds upon the Fahrenheit, thermometric scale, by reason of which it is often called Joule's equivalent, and represented by the symbol J.

Two basic types of energy Potential Energy: energy of position Examples Boulder on a ledge Cations and anions Chemical bonds

Two basic types of energy Kinetic Energy: energy of motion. It depends on the mass and velocity of the object. KE = ½ mv2 Examples Pool balls Molecules

There are several forms of energy Chemical Energy - stored energy that depends on the composition of the substance. Food and gasoline Nuclear Energy - energy involved in nuclear reactions. Electrical Energy - the chemical reaction that occurs in the battery of a car produces electrical energy used to start the car and operate the headlights, radio, etc.

There are several forms of energy Light Energy - plants use the radiant energy of sunlight to do the work of making carbon dioxide and water into sugar. This process of creating stored chemical energy is photosynthesis. Mechanical energy – used to do mechanical work Sound Energy Thermal Energy – heat Two things energy is NOT Some sort of invisible fluid Something which can be measured directly

Energy can be converted from one form to another form, which is more useful. This occurs all around you on a daily basis. Consider when electrical energy is converted into heat energy in a hair dryer. A fundamental law governs these energy conversions:

Law of Conservation of Energy in any chemical or physical change, energy is neither created nor destroyed. Energy may be converted from one form to another but it is always conserved. All the energy involved can be accounted for as work, stored energy, or heat.

10.2 Temperature and Heat Thermal energy Definition: energy due to chaotic molecular motions Three factors affecting thermal energy Temperature – Definition: a measure of the random motion of the components (atoms/molecules) of a substance Higher temperature leads to higher thermal energy Sample size A cup of hot coffee has more energy than a teaspoon of coffee, all other things being equal. Composition E (solid) < E (liquid) < E (gas), all other things being equal

Heat Anything that changes temperature, sample size and/or composition of an object can change its thermal energy Heat Definition: transfer of thermal energy due to a temperature difference Thermal energy isn't measurable, but heat is

10.3 Exothermic and Endothermic Processes According to chemistry/physics, when studying reactions etc… the universe is divided into 2 parts: The System: the part of the universe which you focus your attention. The Surroundings: everything else in the universe.

The energy changes accompanying the burning of a match