Zeroth and First Law
Thermal equilibrium, internal energy, heat and work with sign conventions. (Physics › Thermodynamics, NEET UG syllabus.)
What is Zeroth and First Law?
A state where there is no net flow of heat between two systems in contact, implying they are at the same temperature.
Key formula / rule: First Law of Thermodynamics
Key points
- State and explain the Zeroth Law of Thermodynamics.
- Define thermal equilibrium and its relation to temperature.
- State and explain the First Law of Thermodynamics.
- Define internal energy, heat, and work in the context of thermodynamics.
Common exam trap
Confusing the sign conventions for Q and W. Always remember Q is heat 'added to' and W is work 'done by' the system.
Definitions
- Term
Thermal Equilibrium
- Meaning
A state where there is no net flow of heat between two systems in contact, implying they are at the same temperature.
- Term
Internal Energy (U)
- Meaning
The total energy contained within a thermodynamic system, including kinetic and potential energies of its molecules. It is a state function.
- Term
Heat (Q)
- Meaning
Energy transferred between a system and its surroundings due to a temperature difference. It is a path function.
- Term
Work (W)
- Meaning
Energy transferred between a system and its surroundings by means other than temperature difference (e.g., expansion against external pressure). It is a path function.
- Term
State Function
- Meaning
A property of a system whose value depends only on the current state of the system, not on the path taken to reach that state (e.g., internal energy, temperature, pressure, volume).
- Term
Path Function
- Meaning
A property of a system whose value depends on the path or process taken to change the state of the system (e.g., heat, work).
Learning objectives
State and explain the Zeroth Law of Thermodynamics.
Define thermal equilibrium and its relation to temperature.
State and explain the First Law of Thermodynamics.
Define internal energy, heat, and work in the context of thermodynamics.
Apply correct sign conventions for heat and work in problem-solving.
Distinguish between state functions and path functions.
Calculate changes in internal energy, heat, and work for simple thermodynamic processes.
Formulae
- Name
First Law of Thermodynamics
- Note
ΔU is change in internal energy, Q is heat added to the system, W is work done by the system.
- Expression
ΔU = Q - W
- Name
Work done by system (constant pressure)
- Note
P is constant external pressure, ΔV is change in volume (Vfinal - Vinitial).
- Expression
W = PΔV
- Name
Work done by system (general)
- Note
Integral of pressure with respect to volume change.
- Expression
W = ∫PdV
- Name
Change in Internal Energy (for ideal gas)
- Note
n is number of moles, Cv is molar specific heat at constant volume, ΔT is change in temperature.
- Expression
ΔU = nCvΔT
Prerequisites
Basic understanding of heat and temperature.
Concepts of energy and work from mechanics.
Ideal gas equation (PV=nRT).
Common mistakes
Confusing the sign conventions for Q and W. Always remember Q is heat 'added to' and W is work 'done by' the system.
Treating Q or W as state functions; they are path functions.
Incorrectly applying the formula for work done in different processes (e.g., assuming W=PΔV for all processes).
Not understanding that ΔU for a cyclic process is zero.
Misinterpreting thermal equilibrium as simply having the same temperature without considering heat flow.
Keywords
Thermodynamics
Zeroth Law
First Law
Thermal Equilibrium
Temperature
Internal Energy
Heat
Work
Sign Convention
State Function
Path Function
Energy Conservation
Practice preview
The First Law of Thermodynamics is a statement of the conservation of which quantity?…
easy
A system absorbs 100 J of heat and its volume remains constant. What is the change in its internal energy?…
medium
For an ideal gas, the internal energy is solely a function of which thermodynamic variable?…
medium