340 likes | 598 Views
Energy, work and heat. All processes in nature involve energy. Learning objectives. Define heat and energy and differentiate among different types of energy Describe main uses of energy nationally and globally
E N D
Energy, work and heat All processes in nature involve energy
Learning objectives • Define heat and energy and differentiate among different types of energy • Describe main uses of energy nationally and globally • Apply units of energy to simple calculations of energy changes in chemical processes • Define heat capacity and use in calculations of energy consumption • Describe exothermic and endothermic reactions • Describe principle of entropy and the “heat tax” and the limitations of heat engines • Describe the main sources of energy • Describe basic principles of the Greenhouse effect and global warming
Heat and chemical reactions • The burning candle is a chemical reaction • It releases energy which heats up the air molecules • Heat is the flow of energy due to a temperature difference
Energy from chemical reactions performs work • In what way is a human like a car? • It’s unreliable • Both burn fuel to run • Humans burn glucose • C6H12O6 + O2 = CO2 + H2O + energy • Cars burn petrol • C8H18 + O2 = CO2 + H2O + energy
Energy is capacity to do work • Work is done in different ways • Kinetic energy is energy due to motion • Potential energy is energy due to position or state • Height • Chemical • Electrical
Energy is mercurial • Processes convert energy from one form to another • Falling down stairs • (potential → kinetic → pain) • Chemical reaction • (potential → heat/light) • Battery • (potential → electrical) • Engine • (potential →heat→ kinetic)
But it never goes away • Energy is conserved in any process • None is lost • None is gained • But it goes from one place to another • Law of Conservation of Energy: Energy is neither created nor destroyed in a chemical reaction • Also known as the First Law of Thermodynamics • System and Surroundings • The process under study is the system • Everything else is the surroundings
The First Law says that “perpetual motion” can’t be • If a friend asks you to invest in his new free energy machine – don’t • Visit the museum of unworkable machines for a history of conmen and futile energy ideas
Heat and work • The industrial revolution was founded on conversion of heat into mechanical motion • Joule (1843) was first to recognize connection • Heat: energy in transit – molecular motion • Calorie is energy required to raise temperature of 1 g water by 1ºC • Work: force applied over a distance • Joule is a force of 1 Newton applied for 1 meter 1 cal = 4.18 J
Heat engines • Newcomen steam engine invented 1712 • Watt improves design 1760 • Carnot (1820) described the operation of heat engines in abstract terms – the Carnot cycle: the foundation of modern thermodynamics • All engines based on burning fuels are heat engines
Rules of the road • First law: energy is conserved • Is that all there is to it? • Heat engines convert heat into mechanical work • Question: Can all heat be converted into work with 100 % efficiency? • First Law says yes • Second Law says No!
Limitations on heat engines: the Carnot cycle and entropy • Limit on efficiency of a heat engine is ruled by temperature difference • Entropy and energy dispersal • The second law of thermodynamics Entropy of the universe is always increasing • Processes only occur spontaneously when energy becomes more dispersed – spread out
Various spontaneities: dispersal • Matter disperses – gas fills a container, two liquids mix • Heat disperses – hot object cools on cold surface • Motion disperses – a ball stops bouncing • These processes never reverse spontaneously
Socks and spontaneity • Would you be stunned if the tumble dryer matched the socks? • Okay, you never match the socks anyway • Chaos in the sock drawer is natural • The same principles apply to chemical change (sort of)
Consequences for efficiency • All processes use some of the energy in dispersal • More energy is lost due to inefficiency – friction, wind resistance etc.
Measuring energy: calories are case sensitive • calorie is the energy required to raise temperature of 1 g of water 1 degree C • Calorie is the food version = 1,000 cal • Raises temperature of 1 pint of water 3.8ºF • Joule is SI unit derived from mechanical work: the work done when a force of 1 newton is applied for 1 meter 1 cal = 4.18 J
What’s watt? • Watts measure the rate of delivery of energy or power 1 W = 1 J/s • How many Mars Bars to power a 100 W bulb for one minute? • 1 min = 60 s x 100 W = 6 kJ • 6 kJ = 1.4 kcal = 1.4 Cal (just a nibble)
Common energy conversions • 10 g of octane is burnt to produce 8500 J. How much is that in calories? • 1 cal = 4.18 J
Power consumption • An air conditioner is rated at 1,500 W. How many kWh are used per month if it operates 6 h per day? • What is cost at $0.15 per kWh?
Energy: in or out? • Do chemical processes always create heat? • If you think yes then how does a cold pack work? • Answer: some processes absorb heat from the surroundings
Exo-thermic and endo-thermic • H2 + O2 gives out heat – exothermic • N2 + O2 absorbs heat - endothermic
Enthalpy and chemical reactions • Enthalpy of reaction (ΔH) measures heat of the reaction CH4 + 2O2 = CO2 + 2H2O ΔH = -11.8 kcal/g
The sun as our energy source: directly and indirectly • Indirect: • Solar radiation provided energy for fossil fuels • Heats the air (wind power) • Evaporation of water (hydro power) • Direct: • Solar panels • Photovoltaic cells • Nonsolar: • Nuclear • Geothermal
Energy sources • 85 % comes from fossil fuels • Fossil fuels are hydrocarbons • Petroleum • Coal • Gas • 15 % is everything else • Nuclear (8) • Hydro (3) • Renewables (3)
Electrical generation • More than 70 % comes from fossil fuels • Heat of combustion boils water • Steam turns a turbine • Turbine generates electricity
Finitude • Gambling on the “Other”: how to turn 3 % into 85 % without hurting anybody