Chapter 9: Heat and Thermodynamics

Chapter 9: Heat and Thermodynamics

• Thermodynamics is the study of the effects of work, heat and energy on a system.
• A thermodynamic system is a definite quantity of matter contained within defined boundaries and everything outside of these boundaries are called the surroundings.
• Systems can exchange energy with surroundings via heat and/or work.
• Heat is the energy in transfer from a hot to a cold body.
• Objects are in thermal contact if heat can flow between them.
• Eventually, thermal energy will stop flowing between systems in contact when they reach thermal equilibrium.
• This means the objects will be at the same temperature.
• Temperature is a scalar quantity that has the same value for objects in thermal equilibrium.
• It is an operational definition that tells us how warm or cold an object is.
• A thermometer is an instrument used to quantitatively measure the temperature of a body.
• The zeroth law of thermodynamics states that if objects A and B are each in thermal equilibrium with a third object C, then A and B will be in thermal equilibrium with each other if placed in contact.

The Concept of Temperature and Heat

• Temperature is a physical quantity that tells us how warm or cold an object is.
• Qualitatively, temperature is based on our senses e.g. hot, warm, cold.
• Our senses can be misleading and inaccurate.
• An operational definition of temperature is required
• A thermally isolated system will not exchange energy with the surroundings.
• Using two isolated systems that are different temperatures gives us a way to define temperature.

Thermal Energy (Heat) versus Temperature

• Thermal energy (heat) is kinetic energy in transit due to a temperature difference.
• Thermal energy is measured in calories or joules.
• Any object above absolute zero has thermal energy.
• Temperature is the average kinetic energy of particles, not the total amount of kinetic energy.
• Temperature is measured in degrees Celsius (°C), Fahrenheit (°F), or Kelvin (K)
• Absolute zero is 0 Kelvin.

Thermodynamic Systems

• Open system: Matter and heat can cross the boundary.
• Closed system: Matter cannot cross the boundary, but heat can.
• Isolated system: Neither matter nor heat can cross the boundary.

Work - Mechanical Energy Transfer

• Work done by an expanding gas (change in volume):
• W = -FAs = PA∆s = PAV
• Work done depends on the thermodynamic process.
• Work done is path dependent and equals the area bounded by a P-V curve (Pressure-Volume curve).

Internal Energy of a Thermodynamic System

• Internal energy of a system is the sum of all kinetic and potential energies of the atoms or molecules in the system.

The First Law of Thermodynamics

• The first law of thermodynamics is a statement of conservation of energy.
• ΔU = Q - W (change in internal energy = heat - work).
• If the volume is constant, and heat is added, the internal energy increases.
• If work is done on the external world, and the system doesn't gain heat, the internal energy of the system will decrease.

Tables - Signs and Q or W.

• Summarising what 'Q positive' means and other key points.

Processes in Thermodynamics

• Isothermal process:
• Takes place at constant temperature.
• Adiabatic process:
• No heat transfer.
• Isobaric process:
• Constant pressure.
• Isochoric process:
• Constant volume.
• Any heat or work entering or leaving a system can be calculated based on thermodynamic processes.

Other Processes in Thermodynamics

• Reversible process: A series of equilibrium states.
• Cyclic process: The initial and final states are the same.
• The internal energy for closed cycles is always zero.
• Work and heat in these processes depend on the process itself.

Specific Heats for an Ideal Gas

• Defined as the heat required to raise the temperature of 1 mole of a substance by 1°C, at constant pressure or constant volume.
• Specific heats at constant pressure (Cp) and constant volume (Cv) must be measured at these conditions.

Calculating Energy Transfer

• Energy transferred is calculated using the equation Q= mCpΔT.
• Work is a key term involved in energy transfer and depends on the process being used to transfer the energy.
• Heat flow between objects can be determined.

Modes of Heat Transfer

• Conduction: Heat transfer through a solid or between solids in direct contact due to molecular collisions. It requires a temperature gradient.
• Convection: Heat transfer due to the bulk movement of a fluid (liquid or gas). it is driven by buoyancy or external forces like fans.
• Radiation: Heat transfer via electromagnetic waves, which doesn't require a medium. It depends on temperature and surface properties.