thermodynamics Flashcards
what is the first law of thermodynamics?
states that energy cannot be created or destroyed. it can only be converted from one form to another
total energy of a closed system remains constant
what is the second law of thermodynamics?
systems will tend towards disorder
some energy is always lost or wasted - impossible for a system to be 100% efficient
Not all heat can be converted into useful work/ energy
work?
work done is the energy transferred from one place to another
work equation
W=F x distance
work in joules
describe arrangement of particles in solid
tightly packed
vibrating in fixed positions
strong IM bonds
slow moving kinetic energy
describe arrangement in liquid
IM forces are weaker
not as slightly packed
particles can flow or slide over another
less negative potential energy
describe arrangement in gas
no potential energy
no IM bonding
fills entire container
rapid and random movement
higher kinetic energy
define an ideal gas
modelled as a series of point particles randomly moving around the space colliding with each other and the walls of space
what are the three variables that describe a gas in a box
pressure Nm-2 - collisions between particles and container walls
volume m3 - size of container
temperature K - average speed of molecules (kinetic energy)
describe what would happen to pressure if a closed vessel eg glass bottle was placed over a flame?
due to heat increasing, particles would gain kinetic energy and collide more faster with other particles. pressure increases in closed vessel so the frequency and magnitude of collisions between container walls and particles increases
adiabatic
there is no transfer of heat
temperature changes due to variations in internal energy (which changes when work is done on or by system).
expansion and compression occurs so quickly there is little time for heat to be exchanged with surroundings
1st law of thermodynamics equation
AU = Q-W
heat engine
allows heat to flow from a hot reservoir to a cold reservoir and converts heat into mechanical work energy to do work
give equation for efficiency of general heat engine
efficiency = 1 - Q out / Q in
define carnot cycle
ideal cycle of changing pressures and fluids
internal energy
potential and kinetic energy combined - the sum of all energy of molecules
isothermal
exchange of heat occurs to ensure temperature remains constant
to ensure system is in thermal equilibrium
TEMP DOES NOT CHANGE
compression
reduced volume of gas
work is done on gas by piston
expansion
increase in volume of gas
gas does work on piston
ideal gas law equation
PV = NkT
what is the value for Boltzmann constant
1.38 x 10 -23m2 kgs-2 K-1
explain the stages of the carnot cycle
- isothermal compression - piston does work on gas to reduce volume which increases pressure. slight increase in heat energy is given out to cold reservoir to keep temperature constant and ensure it does not change
- adiabatic compression - working material is compressed adiabatically. piston does work on gas which decreases volume and increases pressure. Temp rises as work is done but there is no heat exchange with surroundings
- isothermal expansion
working substance expands isothermally and gas does work on piston as it expands and pushes out piston
temp of gas remains constant and pressure drops slightly
4.adiabatic expansion -
working substance expands adiabatically
pressure decreases and temp increases due to internal energy variations
no heat is exchanged with surroundings and does work on surroundings
what does the carnot cycle consists
4 reversible processes:
2 isothermal processes - constant temp
2 adiabatic processes - no heat exchange
thermal equilibrium
No net transfer of thermal energy between them
Thermal capacity
Specific heat capacity - energy required to change the temperature of 1kg of material by 1C
