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 energyStored energyWound-up springStanding at the top of a cliff

Kinetic EnergyEnergy of motionRelaxing springFalling off a cliff

Nonmechanical Energy

Electrical energy Energy of moving electrons

Chemical energy Energy stored in chemical compounds

Sound energyMagnetic energy Energy in magnetic fields

Electromagnetic “radiant” energy Cosmic rays to TV

Heat energyActivation 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

2H2 + O2 2H2O + energy (exothermic)

2H2O + energy 2H2 + O2 (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 energyMeasured in Joules (J) or calories (cal)

TemperatureThe average kinetic energy of a systemMeasured 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?

(98.6 -32) 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

(-40 -32) 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 + 273 = 373KNitrogen boils at –196°C. What is this temperature in K?-196 + 273 = 77KEven something extremely “cold” contains heat!!!

More conversion problems

In Kelvins, normal human body temperature is 310K. What is this temperature in °C?

310 - 273

= 37°C

The average surface temperature on Pluto is 44K. What is this temperature in °C?

44 – 273

= -229°C