Energy How does matter interact with energy?

Definitions Energy is the ability to do work Work means that a mass must be moved by a force against it Mechanical energy Energy of macroscopic systems (large enough to be seen by the naked eye) Nonmechanical energy Energy of microscopic systems (too small to be seen by the naked eye

Mechanical Energy Potential energy Stored energy Wound-up spring Standing at the top of a cliff Kinetic Energy Energy of motion Relaxing spring Falling off a cliff

Nonmechanical Energy Electrical energy Energy of moving electrons Chemical energy Energy stored in chemical compounds Sound energy Magnetic energy Energy in magnetic fields Electromagnetic “radiant” energy Cosmic rays to TV Heat energy Activation Energy The energy required to start a chemical reaction

Conservation of Mass and Energy In any chemical or physical change, matter and energy are neither created nor destroyed. Atoms are rearranged Energy is converted to one form or another Mass and Energy are interchangeable

Energy Conversion- The Car (simplified) Chemical potential energy is stored in the battery. As you turn the key, the potential energy is converted to kinetic electrical energy and flows to the starter motor and coil. The electrical energy is converted to mechanical energy as the starter motor gears turn the flywheel, crankshaft, and camshaft(s). The camshaft lobes transmit this energy to the fuel pump and coil. The coil uses magnetic energy to increase voltage of the electrical energy. Fuel is mixed with air, compressed, and ignited (activated) by an electrical spark from the coil. The potential energy of the fuel is converted to mechanical energy as the gaseous products of the reaction expand to force the pistons. This mechanical energy eventually turns the transmission gears, drive axles, and wheels.

Energy and Chemical Reactions In any chemical reaction, there is a difference in energy between the reactants and products If products have greater energy (absorbed) reaction is ENDOTHERMIC If products have lesser energy (released) reaction is EXOTHERMIC The formation of new substances is always exothermic

Chemical Equations Reactants yield products 2H 2 + O 2 2H 2 O + energy (exothermic) 2H 2 O + energy 2H 2 + O 2 (endothermic) Left of arrow are reactants Right of arrow are products

Endothermic or Exothermic? Combustion (Burning) of fuel like gasoline or natural gas Potential chemical energy of the fuel is liberated EXOTHERMIC Thawing of ice Energy is absorbed by the ice to “unlock” the molecules from crystal lattice ENDOTHERMIC

Heat vs. Temperature Heat One of many forms of energy Measured in Joules (J) or calories (cal) Temperature The average kinetic energy of a system Measured in degrees Celsius (°C), degrees Fahrenheit (°F) or Kelvins (K)

The Fahrenheit Scale Used popularly in some countries, like the USA To convert from °F to °C (°F-32) x 5/9 (be sure to subtract first) To convert from °C to °F °C x 9/5 +32

Conversion Problems What is the normal average human body temperature in °C? ( ) x 5/9 = 37.0 °C What is the boiling point of water in °F? 100 x 9/5 +32 = 212 °F Convert -40 °F to °C ( ) x 5/9 = -40 °C

The Kelvin Scale Starts at absolute zero (0K) Note: we do not use the degree symbol (°) when reporting temperature in Kelvins Each Kelvin is the same size as a Celsius degree. To convert to K, add 273 (K= °C + 273) To convert to °C, subtract 273 (°C= K-273)

Conversion Problems What is the boiling point of water in K? 100°C = 373K Nitrogen boils at –196°C. What is this temperature in K? = 77K Even something extremely “cold” contains heat!!!

More conversion problems In Kelvins, normal human body temperature is 310K. What is this temperature in °C? = 37°C The average surface temperature on Pluto is 44K. What is this temperature in °C? 44 – 273 = -229°C