Joule per Second per Square Meter [J/(s·m²)]

The unit joule per second per square meter, written as J/(s·m²), is a measure of heat flux density or radiant flux. Since 1 joule per second equals 1 watt, this unit is equivalent to watt per square meter (W/m²). It represents the amount of energy transferred or radiated through 1 square meter of surface every second.

This unit is widely used in physics, thermodynamics, meteorology, and solar energy studies. Examples include:

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  Measuring solar radiation incident on the Earth’s surface

  
  
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  Evaluating heat transfer through walls, roofs, and other surfaces

  
  
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  Calculating power density in radiant heating or cooling systems

  
  

Mathematically, the heat flux is expressed as:

where q is the flux in J/(s·m²), Q is energy in joules, A is area in m², and t is time in seconds.

For example, solar constant outside Earth’s atmosphere is approximately 1361 J/(s·m²), indicating the power received per square meter from the Sun.

Using J/(s·m²) or W/m² provides a standardized, SI-compliant measure for energy transfer per unit area, replacing older units like cal/cm²·s or Btu/ft²·h in modern engineering and scientific applications.

Calorie (th) per Minute per Square Centimeter [cal(th)/(min·cm²)]

The unit calorie (th) per minute per square centimeter, written as cal(th)/(min·cm²), measures heat flux density or thermal power per unit area using the thermochemical calorie (1 cal(th) = 4.184 J). It represents the amount of heat energy transferred through 1 cm² of surface every minute.

This unit is often used in laboratory experiments, solar energy studies, and material testing, particularly when small surfaces and longer exposure times are involved. Applications include:

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  Solar radiation measurements on small panels or surfaces

  
  
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  Thermal testing of materials over extended periods

  
  
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  Calorimetry experiments on small samples

  
  

Mathematically, the heat flux is expressed as:

where q is heat flux in cal(th)/(min·cm²), Q is energy in calories, A is area in cm², and t is time in minutes.

Conversion to SI units:

Although SI units such as W/m² are standard, cal(th)/(min·cm²) remains useful in historical calorimetry, laboratory experiments, and solar radiation studies, providing an intuitive measure of energy transfer per minute per small surface area.