Topic 2 Ch 4 Energy
Topic 2: Energy and Radiation Balances
Chapter 4: Energy
Introduction
Energy is the capacity to do work.
Forms of Energy:
Seen as light,
Felt as heat,
Experienced as movement.
Law of Conservation of Energy: Energy cannot be created or destroyed but can be transformed and transferred.
Energy Transfer Mechanisms
As Heat: Flow of energy from warmer to colder objects.
Conduction: Transfer between molecules in contact.
Convection: Transfer by vertical motions in fluids.
Radiation: Transfer via electromagnetic waves.
As Work:
Adiabatic heating and cooling occur when the atmosphere expands or contracts.
Heat
Definition and Characteristics
Heat is energy transferred due to temperature differences.
Addition of heat increases molecular movement.
Kinetic Energy: Energy of moving molecules increases with temperature.
Internal Energy
Total Energy: Consists of thermal and potential energy:
Thermal Energy: Increases kinetic energy, resulting in temperature changes (sensible heat transfers).
Potential Energy: Alters molecular attractive forces, causing phase changes (latent heat transfers).
Sensible Heat
Refers to heat transfers affecting temperature changes.
Temperature changes affect sensations of warmth or coolness.
Heat flow equation:
Q (heat flow in joules) = m (mass in kg) × c (specific heat) × ΔT (temperature change in K or °C).
Latent Heat
Components
Definition: Heat absorbed or released during a phase change with no temperature change.
Processes that Absorb Latent Heat:
Melting,
Evaporation,
Sublimation.
Processes that Release Latent Heat:
Freezing,
Condensation,
Deposition.
Gases and Work
Work and Energy Transfer
Work: Energy transfer through mechanical means (e.g., pushing a cart).
Gas Expansion: Expanding gas does work on its surroundings, decreasing its internal energy.
Gas Compression: Requires work input, increasing internal energy.
Work equations: W = F Δx, W = PA Δx, W = P ΔV.
The First Law of Thermodynamics
Temperature Changes in Gases
Add Heat: Increases temperature.
Add Work: Also increases temperature.
Internal Energy Equation:
Q = U + W,
Includes heat transfer (Q), change in internal energy (U), and work (W).
Heat and Work Components
m c ΔT: Amount of heat used for internal energy change.
P ΔV: Heat involved in work.
Constant Volume vs. Constant Pressure
Constant Volume:
All added heat raises temperature.
Specific heat, cv = 717 J ∙ kg–1 ∙ K–1.
Constant Pressure:
Added heat causes temperature and volume increase.
Specific heat, cp = 1004 J ∙ kg–1 ∙ K–1.
Adiabatic Processes
Characteristics
Adiabatic Process: Temperature change without heat transfer; results from work.
Driven by pressure changes in rising or falling air parcels:
Expansion causes cooling (work done by parcel).
Compression causes heating (work done on parcel).
Atmosphere and Adiabatic Processes
Pressure decreases with height, leading to adiabatic heating or cooling as the air rises or descends.
Distinction between adiabatic and diabatic (heat transfer-based) processes.
Heat Transfers
Conduction
Definition: Transfers heat molecule to molecule.
Conductivity: Highest in solids.
Laminar Boundary Layer: Thin air layer in contact with the ground;
Warms through conduction, transferring heat upwards via convection.
Convection
Definition: Transfers heat through fluid movement.
Types:
Thermal Convection: Driven by density differences (warm air rises).
Mechanical Convection: Driven by external mechanical forces (winds and turbulence).
Radiation
Energy travels as electromagnetic waves (e.g., light, microwaves).
All objects emit and absorb radiation:
Emission: Transfers energy out,
Absorption: Transfers energy in.
Hotter substances emit more radiation.
Heat Transfers at Earth’s Surface
Daytime and Night-time
Daytime: Solar radiation warms the surface; some heat transfers upward to the atmosphere and downward to the ground.
Night-time: Radiation emission cools the surface; some heat transfers downward from the atmosphere to the surface and upward from the ground.