First Law of thermodynamics Flashcards
What is thermodynamics?
Thermodynamics studies relationships involving heat, mechanical work, energy, and energy transfer.
How does the First Law of Thermodynamics extend conservation of energy?
It includes internal energy (of particles) alongside kinetic and potential energy from mechanics.
What is an example of thermodynamics in action?
A steam engine: heat flows into water, raising its temperature, boiling it, and expanding steam does work.
What is a thermodynamic system?
A collection of objects that can exchange energy with its surroundings.
What variables describe the state of a thermodynamic system?
Pressure (p), volume (V), temperature (T), and mass (m).
What happens in a thermodynamic process?
Changes occur in the state of the system, with energy exchanged as heat (Q) or work (W).
When is work positive in thermodynamics?
When it leaves the system
When is work negative in thermodynamics?
When it enters the system
What type of work is focused on in thermodynamics?
Work that causes the state of the system to change (e.g., altering temperature or pressure).
How is mechanical work calculated for constant force?
W=F⋅x (force times displacement).
What does positive work mean in mechanics?
Force and displacement are in the same direction.
What does negative work mean in mechanics?
Force and displacement are in opposite directions.
How is work calculated for a variable force?
W=∫ F(x)dx, the area under the force-displacement graph.
How is force related to pressure in a gas-piston system?
F=pA (pressure times piston area).
What is the small amount of work done in a gas-piston system?
dW=Fdx=pAdx=pdV
How is total work calculated for a volume change?
W=∫ ^_(V initialV final)
pdV, the area under the pV graph
Why can’t work be simply
p(Vf−Vi)p(Vf−Vi) for a gas?
Pressure changes as volume changes, requiring an integral.
What does the area under the pV graph represent?
The work done during a volume change.
When is work positive on a pV diagram?
When volume increases (V_initial < V_final), work is done by the system.
When is work negative on a pV diagram?
When volume decreases (V_initial < V_final), work is done on the system.
What is the work for a constant pressure process?
W=p(Vf−Vi)
What is an isothermal process?
A process at constant temperature.
Quiz: For a pV diagram from a to b (path 1) and back to a (path 2), which path has W>0?
Path 1 only (expansion), assuming path 1 increases volume and path 2 decreases it.
How much work does an ideal gas do in isothermal expansion from V_1 to V_2
W = nRT ln(V2/V1)
Why is work positive in isothermal expansion?
V2 > V1, so ln(V2/V1) >0
What is the ideal gas law used in isothermal work calculation?
p = (nRT)/V
What is the First Law of Thermodynamics?
ΔU=Q−W, where
ΔU is internal energy change,
Q is heat added,
W is work done by the system.
What does internal energy depend on?
Only the system’s state, not the path taken.
Do heat and work depend on the path?
Yes, they depend on the path between initial and final states.
What is an adiabatic process?
No heat transfer
Q=0,
ΔU = −W
What is an isochoric process?
Constant volume
W=0,
ΔU=Q
What is an isobaric process?
Constant pressure
W=p(V2−V1)
What is an isothermal process for an ideal gas?
Constant temperature
ΔU=0,
Q=W
What does internal energy depend on for an ideal gas?
Only temperature, not pressure or volume.
What is the relationship between molar heat capacities for an ideal gas?
Cp = C_V + R, where
R is the gas constant.
What is
γ in thermodynamics?
γ = Cp/Cv
What is constant in an adiabatic process for an ideal gas?
pV ^γ
and
TV ^(−γ-1)
.
What is the Second Law of Thermodynamics?
When all systems in a process are included, entropy (disorder) either remains constant or increases.
What does a sandcastle example illustrate?
Spontaneous processes increase disorder (e.g., destruction), not order (e.g., building).
Why doesn’t the First Law explain process direction?
It holds for both spontaneous and non-spontaneous processes; the Second Law (entropy) determines direction.
Can mechanical energy be fully converted to heat?
Yes, with 100% efficiency (e.g., friction in brakes).
Can heat be fully converted to mechanical energy?
No, it’s impossible to build a 100% efficient engine.
What is needed to transfer heat from cold to hot?
Work (e.g., in refrigerators or heat pumps).
How does heat flow spontaneously?
From a hot body to a cold body, without work.
What is the Zeroth Law of Thermodynamics?
Two systems are in thermal equilibrium if they have the same temperature.
What is the Third Law of Thermodynamics?
It’s impossible to reach 0 K in a finite number of steps
What can a thermodynamic system exchange with its surroundings?
Energy via heat transfer or mechanical work.
What is the work formula for constant pressure?
W=p(V2−V1).
What happens to internal energy in an isolated system?
It remains constant.
The First Law of Thermodynamics, in words
The change in the internal energy of a system is equal to the heat added to the system minus the work done by the system
When is work done
if and only if volume changes
