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Heat flux or thermal flux is the rate of heat energy transfer through a given surface. The measurement of heat flux is most often done by measuring a temperature difference over a piece of material with known thermal conductivity.

Heat rate is a scalar quantity, while heat flux is a vectorial quantity. To define the heat flux at a certain point in space, one takes the limiting case where the size of the surface becomes infinitesimally small.

The flux of electric and magnetic field lines is frequently discussed in electrostatics. This is because Maxwell's equations in integral form involve integrals like above for electric and magnetic fields. Heat, a form of kinetic energy, is transferred in three ways: conduction, convection, and radiation. Heat transfer (also called thermal transfer) can occur only if a temperature difference exists, and then only in the direction of decreasing temperature.

**Example:**

Convert 5 calorie/second square centimeter to various units.

**Solution:**

BTU/hour square foot = 66360.42984099999

BTU/minute square foot = 1106.007164836955

calorie/second square centimeter = 5

dyne/hour centimeter = 753624000001.6965

erg/hour square millimeter = 7536240000.016965

foot pound/minute square foot = 860660.7879943552

gram calorie/hour square centimeter = 17999.99999879024

horsepower (metric)/square foot = 26.442359541914602

horsepower (UK)/square foot = 26.08062992958372

CHU/hour square foot = 36866.90548278824

joule/second square meter = 209340.00000078825

Kilocalorie/hour square foot = 16722.547192856655

kilocalorie/hour square meter = 180000.00000059672

kilowatt/square meter = 209.34000000078822

watt/square centimeter = 20.93400000007882

watt/square inch = 135.05779430146677

watt/square meter = 209340.00000078825