The SI prefix "zepto" represents a factor of
10-21, or in exponential notation, 1E-21.
So 1 zeptojoule = 10-21 joules.
The definition of a joule is as follows:
The joule (symbol J, also called newton meter, watt second, or coulomb volt) is the SI unit of energy and work. The unit is pronounced to rhyme with "tool", and is named in honor of the physicist James Prescott Joule (1818-1889).
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