Erg per Hour per Square Millimeter [erg/(h·mm²)]

The unit erg per hour per square millimeter, written as erg/(h·mm²), measures energy flux density or power per unit area using the cgs system. An erg is defined as the energy required to move 1 dyne over 1 centimeter, or 1 erg = 10⁻⁷ joules. Dividing by hour and mm² provides the rate of energy transfer per small area over time.

This unit is typically used in laboratory-scale physics, radiation experiments, and surface energy studies where small areas and low energy transfers are involved. Applications include:

• 
  

  Measuring radiation incident on tiny surfaces

  
  
• 
  

  Calorimetry experiments on small samples

  
  
• 
  

  Micro-scale energy transfer studies in materials science

  
  

Heat or energy flux can be expressed mathematically as:

where q is energy flux in erg/(h·mm²), E is energy in ergs, A is area in mm², and t is time in hours.

Conversion to SI units:

While largely of historical or specialized use, erg/(h·mm²) provides a convenient cgs-based measure for micro-scale, low-intensity energy transfer in scientific experiments.

Watt per Square Meter [W/m²]

The unit watt per square meter (W/m²) measures heat flux density or radiant flux, representing the amount of energy transferred per unit area per unit time. One watt equals 1 joule per second, so 1 W/m² corresponds to 1 joule of energy passing through 1 square meter every second.

This unit is widely used in physics, engineering, meteorology, and solar energy studies to quantify energy transfer rates across surfaces. Examples of applications include:

• 
  

  Solar radiation: measuring sunlight incident on a surface

  
  
• 
  

  Heat transfer: quantifying conduction or convection per unit area

  
  
• 
  

  Building energy analysis: evaluating heat gain or loss through walls and roofs

  
  

Mathematically, the heat flux is expressed as:

where q is heat flux in W/m², Q is energy in joules, A is area in square meters, and t is time in seconds.

For example, solar constant at Earth’s surface is approximately 1361 W/m², indicating the power received per square meter from the Sun outside Earth’s atmosphere.

W/m² is the standard SI unit for energy flux density, replacing older units like cal/cm²·s or Btu/ft²·h, and is essential for thermal engineering, climatology, and renewable energy calculations.