Week 11 Flashcards
(23 cards)
What is thermodynamics?
Thermodynamics is the study of the conversion between heat and mechanical work, which are two forms of energy.
What are the laws of thermodynamics?
-Zeroth Law: Relates to law of temperature
-First Law: Deals with conservation of energy
-Second Law: Law of ‘disroder’ and it thus deals with the efficiency of energy conversion
-The Third Law concerns matter related to the absolute zero of temperature
What is the first law of Thermodynamics and it’s formula?
The First Law is a statement of the law of conservation of energy in a system which converts energy between heat and
work.
dQ = dU + dW
Q: Heat energy
U: Internal energy stored
W: Mechanical work
d: Very small change
What does the first law say when we add a small amount of heat energy?
-Increase its internal energy or
-Some work will be done or
-A combination of the two
-The sum of the increase in internal energy and the workdone will always be exactly equal to the amount of heat
supplied
What is the formula of work done by gas?
dW=pdV
W= Work
p= Pressure
V= Volume
d = Small change
What happens when volume increases or decrease?
If volume increases, we can say that gas does work against the restraining force (Positive), it expands.
If volumes decrease, we can say that work is done on a gas by its surroundings (Negative), it compresses.
What is the formula for the first law of thermodynamics when pressure is constant?
dQ = dU + pdV
Q= Amount of heat
U: Internal energy
pdV: Work done by the gas
What is heat capacity?
The amount of heat required to increase
the temperature of the body by 1 K (or 1°C)
What is the formula for heat capacity?
Change in Q = C x change in T
Q: Heat energy
C: Heat capacity
T: Time
What is specific heat capacity?
The amount of heat required
to increase the temperature of 1 kg of the substance by 1 K (or 1°C)
What is the formula of specific heat capacity?
Change in Q = mc x Change in T
Q: Heat energy
mc: Specific heat capacity
T: Time
What are the two principal specific heat capacities
The specific heat capacity at constant volume, cv is the heat required to raise the temperature of 1 kg of gas by 1 K at constant volume. (J kg-1 K-1).
- The specific heat capacity at constant pressure, cp is the heat required to raise the temperature of 1 kg of gas by 1 K at constant pressure. (J kg-1 K-1)
What are molar heat capacities?
Molar heat capacities’ refer to the heat required to raise the temperature of 1 mole of gas by 1 K (or ˚C)
What are molar heat capacities in:
-Constant volume
-Constant pressure
-cv ( J mol^ K^-1)
-CP ( J mol^ ^01 K^-1)
What is the formula of Heat energy when the volume is constant
dQ=dU
therefore
mcv x dT=dU
m: Mass
cv: Specific heat capacity
dT: Change in temperature
dU: Interal energy
If gas is heated at constant pressure, what would the heat capacity be?
dQ = mcp x dT
dQ: Change in heat energy
m: Mass
cp: Specific heat capacity at constant pressure
dT: Change in temperature
How to get work done when the gas expands to maintain constant pressure?
dW = pdV
W: Work done
p: Pressure
V: Volume
d: Small change
mcpdT = mcvdT + pdV
m: Mass
cp: Specific heat at constant pressure
T: temperature
cv: Specific heat at constant volume
p: Pressure
d: Small change
Why is cp > cv
cp: Specific heat capacity at constant pressure
cv: Specific heat capacity at constant volume
-At constant volume all heat increases temperature
-At constant pressure some heat is used for expansion
-Since more heat is needed at constant pressure cp>cv
What is adiabatic expansion?
The expansion of gas in an
aerosol spray can is adiabatic,
and this leads to the can cooling
as it sprays (though often there
is also evaporation which adds
to the cooling).
Examples of adiabatic expansion?
-Cloud formation on adiabatic expansion
-Cloud after the pressure reduced in a warm water bottle
-Bicycle pump gets warm due to rapid pumping (compression)
-A fog or smoke forms when champagne bottle is opened
What is the difference between adiabatic and isothermic expansion?
Constant: Isothermal has now change in temperature but it must have heat flow dU=0. Adiabatic has no heat flow but with a change in temperature dQ=0
Characteristics: Iso is a slow process meanwhile adia is a very rapid process
Rule: Iso is pV = constant, while adia is pV=constant
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