Kilojoule to Erg

Australia, New Zealand, and the EU mandate SI-based labeling, so food packages list energy in kilojoules. The US and Canada stuck with kilocalories (branded as "Calories"). To convert, divide kJ by 4.184 — a 500 kJ snack bar is about 120 kcal. Most Australian shoppers learn the kJ scale by familiarity rather than converting every time.

A 70 kg person walking briskly at 5.5 km/h burns roughly 600–700 kJ in 30 minutes (about 150–170 kcal). That is roughly one banana or a small flat white. Running the same distance roughly triples the kilojoule burn because the body must lift itself off the ground with every stride.

They measure the same thing — food energy — in different units. One kilocalorie (kcal) equals 4.184 kilojoules (kJ). European and Australian labels show both; US labels show only kcal (labelled "Calories"). A 2,000 kcal/day diet is 8,368 kJ/day. Nutritionists consider the two interchangeable for dietary guidance.

A typical smartphone battery rated at 15 Wh holds about 54 kJ. That is roughly the food energy in a single sugar cube (17 kJ per cube times three). A laptop battery at 60 Wh stores about 216 kJ, and a Tesla Model 3 battery pack at 60 kWh stores 216,000 kJ — enough dietary energy to feed a person for about 25 days.

It is a middle-ground unit — too large for electronics (which use millijoules) and too small for household energy bills (which use megajoules or kWh). One kilojoule is the kinetic energy of a tennis ball served at about 160 km/h, the energy in a small sip of juice, or the heat generated by a 100 W bulb in ten seconds. It sits at the human snack-and-exercise scale.

Astrophysics literature built decades of reference data in CGS units before SI became dominant. Key constants like solar luminosity (3.828 × 10³³ erg/s) and supernova energy (10⁵¹ erg, called a "foe") are baked into textbooks and databases. Switching to SI would require rewriting thousands of reference values, so the field maintains CGS by convention.

A core-collapse supernova releases roughly 3 × 10⁵³ ergs total, of which about 99% escapes as neutrinos. The visible light and kinetic energy of the ejected shell account for about 10⁵¹ ergs — a unit so common in astrophysics it has its own name: one "foe" (ten to the Fifty-One Ergs). In joules, that is 10⁴⁴ J, or the Sun's total output over 10 billion years.

An erg per second is the CGS unit of power, equivalent to 10⁻⁷ watts. Astronomers quote stellar luminosities in ergs per second because the numbers align well with astrophysical scales: the Sun emits 3.846 × 10³³ erg/s, and supernovae peak at ~10⁴³ erg/s. Using watts would give the same exponents minus seven — less tidy for a field that already juggles 40-digit numbers daily.

CGS (centimeter-gram-second) is a metric system that predates SI, formalised in the 1870s. It derives mechanical units from cm, g, and s: force in dynes (g·cm/s²) and energy in ergs (dyne·cm). CGS was standard in physics until the mid-20th century, and its Gaussian variant remains preferred in electromagnetism and astrophysics because Maxwell's equations take a simpler form.

One erg is 10⁻⁷ joules — roughly the kinetic energy of a mosquito in flight or the energy of a single grain of sand falling one centimeter. You would need about 10 million ergs to equal one joule, or 42 billion ergs to match the energy in a single dietary Calorie. The erg is useful precisely because atomic and astronomical quantities span so many orders of magnitude.