Glossary

11.4 First law of thermodynamics

4 min read•august 21, 2024

The first law of thermodynamics is a cornerstone of energy conservation in mechanical systems. It links changes in to transfer and work done, providing a framework for understanding energy transformations.

This law states that energy can't be created or destroyed, only converted between forms. It's expressed as , where U is internal energy, Q is heat added, and W is work done by the system.

Definition and concept

  • First law of thermodynamics establishes the fundamental principle of energy conservation in thermal systems
  • Relates changes in internal energy to heat transfer and work done, forming the basis for understanding energy transformations in mechanical systems

Statement of first law

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  • Energy cannot be created or destroyed, only converted from one form to another
  • Total energy of an remains constant over time
  • Mathematically expressed as ΔU=Q−W\Delta U = Q - W, where U is internal energy, Q is heat added, and W is work done by the system

Conservation of energy principle

  • Fundamental concept underlying the first law of thermodynamics
  • Energy transfers between system and surroundings must balance
  • Applies to all physical processes, including mechanical, thermal, and chemical interactions

Closed vs open systems

  • Closed systems exchange energy but not matter with surroundings
  • Open systems allow both energy and matter exchange
  • First law applies differently to each system type, accounting for mass flow in open systems

Internal energy

Microscopic kinetic energy

  • Contributes to internal energy through molecular motion
  • Includes translational, rotational, and vibrational energy of particles
  • Directly related to temperature, increasing as temperature rises

Microscopic potential energy

  • Arises from intermolecular forces and atomic bonds
  • Includes electrostatic interactions and chemical bond energies
  • Changes during phase transitions and chemical reactions

Relation to temperature

  • Temperature measures average kinetic energy of particles
  • Higher temperature indicates greater internal energy
  • Absolute zero represents minimum possible internal energy state

Work in thermodynamics

Pressure-volume work

  • Most common form of work in thermodynamic systems
  • Calculated as W=∫PdVW = \int P dV, where P is pressure and V is volume
  • Positive work done by system when expanding against external pressure

Other forms of work

  • Electrical work in batteries and fuel cells
  • Magnetic work in superconductors
  • Surface tension work in liquid droplets
  • Gravitational work in hydrostatic systems

Sign conventions

  • Positive work indicates energy leaving the system
  • Negative work represents energy entering the system
  • Consistent sign convention crucial for accurate energy balance calculations

Heat transfer

Conduction vs convection vs radiation

  • Conduction transfers heat through direct molecular collisions
  • involves heat transfer by fluid motion
  • transmits energy via electromagnetic waves
  • Each mechanism dominates in different scenarios (solids, fluids, vacuum)

Heat capacity and specific heat

  • measures energy required to raise temperature by one degree
  • normalizes heat capacity per unit mass
  • Values vary widely among materials (water has high specific heat)

Mathematical formulation

Equation for first law

  • General form: ΔU=Q−W\Delta U = Q - W
  • For open systems: ΔU=Q−W+∑mihi−∑mehe\Delta U = Q - W + \sum m_i h_i - \sum m_e h_e
  • Accounts for energy flows across system boundaries

Change in internal energy

  • Calculated as ΔU=mcvΔT\Delta U = m c_v \Delta T for ideal gases
  • Depends on specific heat capacity at constant volume (cv)
  • Independent of path taken between initial and final states

Work done on system

  • Negative of work done by system: Won=−WbyW_{on} = -W_{by}
  • Includes all forms of work (pressure-volume, electrical, etc.)
  • Path-dependent quantity, unlike internal energy change

Applications

Isochoric process

  • Volume remains constant (ΔV=0\Delta V = 0)
  • No done
  • All heat added goes into increasing internal energy

Isobaric process

  • Pressure remains constant
  • Work done equals W=PΔVW = P \Delta V
  • Commonly encountered in constant pressure heating/cooling

Isothermal process

  • Temperature remains constant (ΔT=0\Delta T = 0)
  • Internal energy unchanged for ideal gases
  • All heat added converted to work (or vice versa)

Adiabatic process

  • No heat transfer with surroundings (Q=0Q = 0)
  • Work done entirely at expense of internal energy
  • Occurs in rapid compressions/expansions (diesel engines)

Limitations and assumptions

Ideal gas approximation

  • Assumes negligible intermolecular forces
  • Valid for low densities and high temperatures
  • Breaks down for real gases under extreme conditions

Quasi-static processes

  • Assumes system remains infinitesimally close to equilibrium
  • Allows use of equilibrium thermodynamic relations
  • Real processes often deviate from this idealization

Relation to other laws

Zeroth law of thermodynamics

  • Establishes concept of thermal equilibrium
  • Provides foundation for temperature measurement
  • Complements first law in describing heat flow direction

Second law of thermodynamics

  • Introduces concept of entropy and irreversibility
  • Limits efficiency of and
  • Complements first law by addressing quality of energy

Experimental verification

Historical experiments

  • Joule's paddle wheel experiment demonstrated mechanical equivalent of heat
  • Rumford's cannon boring observations linked heat to motion
  • Mayer's work on in biological systems

Modern laboratory techniques

  • Calorimetry measures heat transfer in chemical reactions
  • Bomb calorimeters determine energy content of fuels
  • Differential scanning calorimetry analyzes phase transitions

Practical implications

Engineering applications

  • Design of heat engines and power plants
  • Optimization of refrigeration cycles
  • Thermal management in electronic devices
  • Energy recovery systems in industrial processes

Energy efficiency considerations

  • First law efficiency measures useful output vs input energy
  • Guides development of energy-saving technologies
  • Informs policy decisions on energy conservation measures
  • Highlights importance of waste heat recovery in industrial processes

Key Terms to Review (25)

Adiabatic process: An adiabatic process is a thermodynamic process in which there is no heat exchange between a system and its surroundings. This means that any change in the internal energy of the system results solely from work done on or by the system, rather than heat transfer. In an adiabatic process, the temperature of the gas can change as a result of this work, which is crucial in understanding how systems behave under various thermodynamic conditions.
Calories: Calories are units of energy that measure the amount of energy food provides when consumed. They play a crucial role in the first law of thermodynamics, which states that energy cannot be created or destroyed, only transformed from one form to another. Understanding calories helps to explain how energy is conserved and transferred in physical systems, especially in terms of heat and work.
Closed System: A closed system is a physical system that does not exchange matter with its surroundings but can exchange energy. In a closed system, the total mass remains constant, and any changes in energy can occur in the form of work or heat. This concept is critical in understanding various principles, particularly those related to conservation laws and thermodynamic processes.
Conservation of energy: Conservation of energy is a fundamental principle in physics stating that the total energy in a closed system remains constant over time, meaning energy cannot be created or destroyed, only transformed from one form to another. This principle connects various forms of energy, including kinetic and potential energy, and applies to many physical scenarios, illustrating how systems behave under different forces and conditions.
Convection: Convection is the process of heat transfer through the movement of fluids, such as liquids and gases, where warmer parts of the fluid rise and cooler parts sink. This movement creates a circulation pattern that effectively transfers thermal energy throughout the fluid. Convection is crucial in understanding various phenomena, including the heating of fluids, energy transfer in the atmosphere, and even cooking processes.
Energy transfer: Energy transfer refers to the process by which energy moves from one system or object to another, changing its form or state in the process. This concept is fundamental in understanding how energy is conserved, how it can be transformed between potential and kinetic forms, and how it plays a role in various physical phenomena such as oscillations and thermodynamic processes.
Heat: Heat is a form of energy that is transferred between systems or objects with different temperatures, moving from the hotter object to the cooler one. This transfer of energy can lead to changes in temperature, phase, or even the physical state of a substance. Heat is crucial in understanding thermodynamic processes and is a key component in the first law of thermodynamics, which deals with energy conservation and transformation.
Heat capacity: Heat capacity is a physical property of a substance that quantifies the amount of heat energy required to change its temperature by a given amount. It plays a crucial role in understanding how materials respond to heat transfer, linking directly to the principles of energy conservation and transformation in thermodynamic processes, as well as the mechanisms by which heat moves through materials.
Heat engines: Heat engines are devices that convert thermal energy into mechanical work by utilizing the flow of heat from a high-temperature source to a low-temperature sink. They operate based on the principles of thermodynamics, particularly focusing on how energy transfers and transforms, making them essential in many practical applications like power generation and transportation.
Ideal gas approximation: The ideal gas approximation is a model that simplifies the behavior of gases by assuming that they consist of a large number of particles that do not interact with one another except during elastic collisions. This concept allows for the use of equations like the ideal gas law, which connects pressure, volume, temperature, and the number of moles of a gas in a straightforward manner. It is particularly useful in thermodynamics to analyze energy changes, work done, and heat transfer within systems involving gases.
Internal energy: Internal energy is the total energy contained within a thermodynamic system, arising from the kinetic and potential energies of the molecules in that system. It includes contributions from various forms of energy such as thermal energy, chemical energy, and any other forms that might be present. Understanding internal energy is crucial as it helps to explain how energy is transferred and transformed in processes involving heat and work.
Isobaric process: An isobaric process is a thermodynamic process in which the pressure of a system remains constant while its volume changes. This type of process is significant in understanding how energy is transferred within a system, particularly when heat is added or removed, affecting both temperature and volume without altering the pressure. In the context of energy conservation, it connects to the first law of thermodynamics, which states that energy cannot be created or destroyed, only transformed from one form to another.
Isochoric Process: An isochoric process is a thermodynamic process in which the volume of a system remains constant. During this type of process, no work is done on or by the system because the volume does not change, and all energy transfer is in the form of heat. This concept is crucial for understanding energy conservation and transformations in closed systems, especially when applying the first law of thermodynamics.
Isolated System: An isolated system is a physical system that does not exchange matter or energy with its surroundings, meaning that the total energy and mass remain constant over time. This concept is essential in understanding various physical phenomena, as it simplifies the analysis of complex interactions by allowing us to focus solely on the system itself without external influences.
Isothermal Process: An isothermal process is a thermodynamic process in which the temperature of a system remains constant while heat is transferred in or out of the system. This concept is crucial as it links the principles of energy conservation and transfer, providing insights into how systems can maintain thermal equilibrium even when energy is exchanged. Understanding this process helps clarify the roles of temperature and heat in energy transformations and supports the foundation of thermodynamic laws.
Joules: Joules are the standard unit of energy in the International System of Units (SI). This measurement is critical in understanding various forms of energy transfer and conversion, such as gravitational potential energy, rotational energy, thermodynamic processes, and elastic potential energy. Essentially, joules help quantify how much work is done or how much energy is stored in a system when forces act on objects.
Pressure-volume work: Pressure-volume work refers to the work done by or on a system when it expands or compresses against an external pressure. This type of work is essential in thermodynamics, particularly in understanding how energy transfers occur within a system when its volume changes under constant or varying pressure conditions. It highlights the relationship between pressure and volume, revealing how they influence the energy states of the system.
Quasi-static processes: Quasi-static processes are idealized thermodynamic processes that occur infinitely slowly, allowing a system to remain in thermal equilibrium at all times. In these processes, any change happens so gradually that the system is always nearly at equilibrium, enabling accurate descriptions of states and transitions according to thermodynamic laws.
Radiation: Radiation is the process by which energy is emitted as particles or waves. It plays a crucial role in heat transfer, affecting how energy moves through different mediums, and is essential in understanding temperature variations and thermal dynamics in systems. Radiation can occur in various forms, including electromagnetic waves like light and infrared radiation, which influence thermal energy exchanges.
Refrigerators: Refrigerators are mechanical devices that use the principles of thermodynamics to remove heat from a designated area, thereby preserving food and other perishable items by keeping them cool. By utilizing the first law of thermodynamics, refrigerators transfer heat from inside the appliance to the outside environment, effectively lowering the temperature within to prevent spoilage and maintain freshness.
Second Law of Thermodynamics: The Second Law of Thermodynamics states that in any energy transfer or transformation, the total entropy of an isolated system can never decrease over time. This principle highlights the directional nature of energy processes and is crucial for understanding heat engines and their efficiencies, as well as the interplay between heat and work in thermodynamic systems.
Specific heat: Specific heat is the amount of heat energy required to raise the temperature of one unit mass of a substance by one degree Celsius (or one Kelvin). This property varies between different materials and is crucial in understanding how substances absorb or release heat energy during temperature changes, playing a key role in energy transfer processes and the first law of thermodynamics.
Thermal conduction: Thermal conduction is the process by which heat energy is transferred through a material without any movement of the material itself. This transfer occurs due to collisions between neighboring particles, where faster-moving particles pass on their energy to slower-moving ones. This phenomenon is crucial in understanding how heat interacts with different materials and directly ties into the principles of energy conservation and transformation.
Zeroth Law of Thermodynamics: The Zeroth Law of Thermodynamics establishes the concept of thermal equilibrium, stating that if two systems are each in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other. This law is fundamental in defining temperature and enables the comparison of temperatures between different systems.
δu = q - w: The equation δu = q - w represents the first law of thermodynamics, which states that the change in internal energy (δu) of a system is equal to the heat added to the system (q) minus the work done by the system (w). This relationship highlights the conservation of energy principle, indicating that energy can be transformed from one form to another but cannot be created or destroyed. It emphasizes the interconnection between heat, work, and internal energy in thermodynamic processes.
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