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 Inch [W/in²]

The unit watt per square inch (W/in²) measures heat flux density or power per unit area, representing the amount of energy transferred through 1 square inch of surface every second. One watt equals 1 joule per second, so 1 W/in² corresponds to 1 joule of energy passing through 1 in² every second.

This unit is commonly used in high-intensity applications with small areas, such as:

• 
  

  Laser beams or focused light sources

  
  
• 
  

  Electronic devices to measure power density on chips

  
  
• 
  

  Industrial heating processes with concentrated energy

  
  

Heat flux can be expressed as:

where q is heat flux in W/in², Q is energy in joules, A is area in in², and t is time in seconds.

Conversion to SI units:

While W/m² is the SI standard, W/in² is convenient in U.S. engineering, electronics, and laboratory experiments involving small surface areas, where reporting flux per square inch provides an intuitive measure of concentrated energy.