The First Law of Thermodynamics Flashcards
State the first law of thermodynamics.
The change in internal energy of a system is equal to the difference between the heat input to the system and the work done by the system.
What is a reversible process?
- Always in equilibrium
- the work done by a system during a reversible process is the maximum possible
What is an irreversible process?
Real process which can’t be reversed without changing surroundings.
What is another way of writing/finding work done?
dW = p dV, where V is the volume. Work done is the area under the p-V curve.
Describe the graph for an isothermal process of a system going from state 1 to state 2.
- pressure on y-axis, volume on x-axis
- negative curve with gradient of 1/V
- Goes from p1 and v1 to p2 and v2
What is the algebraic version of this isothermal curve?
- pV = nRT
- Work done from 1 state to another is integral from v1 to v2 of p dV
- nRT is constant so just integral of 1/V dV
What is a different version of this graph?
Isochore and isobar (constant volume and constant pressure). 2 straight lines.
What is the equation for molar heat capacity?
C = 1/n * dQ/dT
What is the equation for Cv?
dQ/dT at constant volume. If heat if added isochorically and reversibly, dW = 0, dU = dQ and Cv = dU/dT
What is the equation for Cp?
dQ/dT at constant pressure. If heat is added isobarically and reversibly dU = dQ - dW
How can you link Cv and Cp?
- For isobaric process, dW = pdV, so dU = dQ - dW
- CvdT = dQ - pdV
- Cv = dQ/dT - pdV/dT = Cp - R
Summarise an isothermal process.
- ΔT = 0
- ΔU = 0 - work done by gas equal to heat put into it
- Work done = nRT * integral between V1 and V2 of 1/V dV
Summarise an isochoric process.
- ΔV = 0, so ΔW = 0
- Q = ΔU = nCv ΔT
- For decreasing pressure, ΔT will be negative
Summarise an isobaric process.
- Δp = 0
- W = pΔV, Q = nCp ΔT
- ΔU = Q - W
Summarise an adiabatic process.
- Q = 0
- pV^gamma = const
- ΔU = -W = nCv ΔT
