energy forms and energy conversion. energy the capacity of vigorous activity the ability to act the...
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Energy Forms and Energy Conversion
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Energy
• The capacity of vigorous activity• The ability to act• The capacity of a body or a system to do work
(and heat).
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Mechanical EnergyKinetic + Potential
• E = U+K
h
€
U +K = mgh +mv 2
2
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Chemical Energy:Microscopic Version of the Potential Energy
Potential energy of a bond between atoms forming a molecule
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Thermal Energy:Microscopic Version of the Mechanical Energy
€
ΔEthermal =CΔT
C – heat capacity
For a gas the thermal energy is just the kinetic energy
In general thermal energy has kinetic and potential components (potential energy stored in the vibrating bonds)
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Electrical Energy: Potential Energy of Electrostatic Interactions
€
Eelectrostatic =q1q2r
q1 q1r
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Electromagnetic Energy
Energy in oscillating electromagnetic waves
These waves can be considered as massless particles called photons traveling with the speed of light
Photon energy hv (h –Planck constant, v frequency)
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Nuclear Energy
Potential energy of bonded protons and neutrons forming nucleus
The energy amount is so high that during nuclear energy release it can be observed that mass is reduced according to
E=mc2
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Energy Conservation
Energy can not be created or destroyed – it can be only converted
Energy of the system can be changed by the flow of energy form or to the outside in the form of work, W, and heat, Q
ΔE = W + Q
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Work
Work = force times displacement
W = Fd
The unit of work is the same as the unit of energy
The unit of force
thus the unit of energy
This unit is called Joule
€
kgm2
s2
€
kgm
s2
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Heat
Heat, as work is also a process – it is essentially work done by molecules thus it can change the energy of the system
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Energy and Power
Power is the energy change rate
W = ΔE/Δt
The unit of power is Watt (W)
The World uses about 15 Terra Watts of power, with about 2.5 TW of electricity
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Energy Units - I
Joule = Watt x second
Kilowatt-hour = 3.6 Megajoules– on your electric bill
Therm = 105 Megajoules = 100,000 British thermal units (BTU) – about 100 cubic feet natural gas burning equivalent
BTU – energy needed to heat one pound of water by one °F
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Energy Units - II
Calorie - heating 1 g of water by 1 °C = 4.2 Joules
Electronovolt – energy gained by moving one electron across potential difference of one Volt = 1.6 x 10-19 Joules (one Volt = 1J/Coulomb)
Energy = Volt x Coulomb = Volt x Ampere x time
Power = Volt x Ampere
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Energy Conversion
Useful output is generally one that can produce work (mechanical, electrical)
Sometimes heat can be useful e.g., to heat a house
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Chemical Energy ConversionMost important is combustion – potential chemical energy is transformed to kinetic energy of the gas (high temp and pressure). For example,
Yields about 240 kJ/mole energy leading to a high pressure gas. This energy shows as the kinetic energy
This energy can be used directly as mechanical energy (car) or converted to other useful energy, such as electrical energy
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Alternator
Converts mechanical energy to AC electricity
Rotating magnet
Stationary wire
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Transformer
Can easily change the voltage of an alternating current
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Why AC won over DC
AC transformers can easily increase and decrease voltage
The input power is P=VI (voltage times current)
The voltage on a transmission wire is V = IR
The power loss on the transmission wire is Ploss = I2R, and since I=P/V, Ploss =R P2/V2
Higher voltage means much lower transmission loss
This was a rare time that Thomas Edison lost big time to Tesla and Westinghouse (War of Currents).
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AC vs. DC in the future
Distributed power generation, e.g., by solar, allows to avoid transmission power losses due to short lines.
Transformation losses are eliminated
Many devices work on DC only
So Edison might be right in the future
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Solar to Electricity
Photon excites negatively charged electrons and separates them from positively charged holes – this generates electrostatic energy and associated voltage (DC)
Limits of efficiency is mostly due to two factors
Not all photons lead to excitation
Electrons and holes recombine and just generate heat
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Efficiency – Carnot Cycle
Efficiency = Work/Input heatHot heat is useful
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Efficiency and Thermal Energy Storage
Carnot conversion efficiency
(Thot-Tcold)/Thot*100%
Increases with increasing Thot
But stored heat radiates at the rate ~ (Thot)4
thus the loss increases with increasing Thot
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Efficiency in General
Efficiency = Useful Energy/Energy Input
Typical fossil fuel power plant up to 40%
Hydro – about 90%
Solar 6-40%, 15% typical
Combustion engine ~ 30%
Electric engine 70-90%
Photosynthesis few percent
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Energy Returned on Energy Invested
EROEI = Usable Energy/Energy Expended
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Sankey Diagram – Electric Plant
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Sankey Diagram – USA