Download Energy, work and heat

Energy, work and heat
All processes in nature involve
energy
Learning objectives
• Apply units of energy to simple
calculations of energy changes in
chemical processes
• Describe the main sources of energy
Energy is capacity to do work
•
•
•
•
Work is applying force over distance
Energy comes in different forms
Kinetic energy is energy due to motion
Potential energy is energy due to
position or state
– Height
– Chemical
– Electrical
Energy changes form
• Processes convert
energy from one form
to another
– Falling down stairs
• (potential → kinetic →
pain)
– Chemical reaction
• (potential → heat/light)
– Battery
• (potential → electrical)
– Car 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
Thermodynamic
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?
•
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1 min = 60 s x 100 W = 6 kJ
6 kJ = 1.4 kcal = 1.4 Cal (just a nibble)
Measurements of energy use
Common energy conversions
• Octane is burnt to produce 8500 J. How
much is that in calories?
• 1 cal = 4.18 J
1cal
cals  8500Jx
 2030cal
4.18J
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?
1kW 30days 6h
kWh  1500Wx
x
 270kWh
1000W 1mo 1day
$0.15
cos t  270kWhx
 $40.5
1kWh
Enthalpy and chemical reactions
• Enthalpy of reaction (ΔH) measures heat of
the reaction
CH4 + 2O2 = CO2 + 2H2O ΔH = -11.8 kcal/g
Enthalpy of combustion example
What is energy obtained from burning 10 kg of
coal. Coal has energy density of -28kJ/g
1000g  28kJ
joules  10kgx
x
 280000kJ
1kg
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