2. Forms of energy Flashcards
Potential energy equation?
E_pot = mgh
Kinetic energy eq?
E_kin = (1/2)mv^2
Rotational kinetical energy?
E_rot = (1/2)Iw^2
I: moment of inerta
w: rotational frequency (rpm)
Give examples of two energy storage systems that uses mechanical potential energy?
Storage of water in basins and compressed air systems.
Calculate the energy stored of 1 tonne of water brought up 100 meters. How much coal would be needed to achieve this?
E_pot = mgh.
Then convert from J to kWh to kgCE. (see notes)
Derive the energy storage of compressed gases.
See notes
What is the energy density of an infinite plane wave of electromagnetic radiation?
E/V = 1/2 eps eps_0 E(t)^2 + 1/2 µ µ_0 H(t)^2 = 1/2 ( ED + HB).
(see slides)
What is the energy in a capacitor?
E = 1/2 CV^2, with C = eps eps_0 * A/d. (see slides)
What can be done to increase the amount of energy stored in a capacitor?
We have to increase the capacitance. This can be done by increasing the area (through nanostructures, surface area), decreasing the electrode distance (through use of electrolytes which form a Helmholtz layer) or use an isolation medium with a large permittivity.
How can one describe the energy stored in an inductor?
E = 1/2 L I^2 = 1/2 PHI I
Coil with n windings, length L and area A:
L = µµ_0 * n^2 A/l PHI = n B A E = 1/2 B^2 * lA/µµ_0 = 1/2 µ µ_0 H^2 V.
(see slides)
How does the energy density in a supermagnetic storage medium compare to Li-ion batteries?
About one order of magnitude less. (for µ = 1 and B_max = 20T)
How does the energy density of a commerical supercapacitor with C = 30 F compare to a Li-ion battery?
About two orders of magnitude less.
Describe schematically how chemical energy is stored.
When molecules form, the interatomic potential energy is lowered, and thus the excess energy is released (exothermic reaction).
Give examples of the volumetric energy densities for chemical energy.
For hydrogen stored at 700 bar this is about 5 * 10^9 J/m^3.
For gas this is an order of magnitude larger.
Give examples of how chemical energy can be transformed to other forms of energy.
Heat in turbines.
Pressure in explosion engines.
Light in chemo-luminescence.
Electrical energy in batteries and fuel cells.
What are primary, secondary and tertiary elements when talking about transformation of chemical energy into electrical energy?
Primary: non-rechargable.
Secondary: rechargable.
Tertiary: consume external masses.
Explain the basics of a battery.
The basis for such systems is the voltage series of metals:
- Metal ions are dissolved in an electrolyte in contact with the surface of the metal
- Counter charge remains in the electrode
- A voltage, V, between the electrode and electrolyte is established and prevents further dissolution of ions from the electrode
- V is commonly given with respect to the standard hydrogen electrode (SHE), which is defined as a Pt wire surrounded by 1 atm H2
What is the Nernst equation?
The Nernst equation relates the effective concentrations ( activities ) of the components of a cell reaction to the standard cell potential.
Which atom has the highest binding energy per nucleon?
57_Fe
Give an example of a fusion reaction and the associated energy released.
See note
Give an example of a fission reaction and the associated energy released.
See note
What is exergy?
Portion of energy that can be transformed to usable work
What is anergy?
Portion of energy that cannot be transformed to usable work
What happens to the anergy and exergy in a reversible process?
In an ideal reversible process, exergy and anergy are themselves conserved
What happens to the anergy and exergy in an irreversible process?
During an irreversible process (e.g. friction, turbulence, gas mixing, heat conduction, diffusion) exergy is transformed into anergy
How is anergy related to the increase in entropy?
Energy lost in irreversible processes is determined by the increase in entropy
See note
How is the Carnot efficieny defined?
See note
Eff = T_H/(T_H-T_C)
