The calorie and the joule

Historically, when heat was still thought of as a fluid, it was measured in calories rather than joules. A calorie is defined as the energy that raises the temperature of $1$ g of water by $1\,°$C. The modern link with the SI unit of energy is

$\ev{1\;\text{cal} = 4{,}186\;\text{J}}$

This equivalence — the mechanical equivalent of heat — is the quantitative proof that heat is energy, measurable in the same units as mechanical work.

Food calorie

The food calorie (Cal with a capital C, or kcal) equals $1000$ cal, i.e. $4186$ J. When a label states ”$800$ Cal”, it means $800$ kilocalories.

To get a sense of the orders of magnitude: a margherita pizza ($\sim 800$ Cal) contains about $3{,}3\;\text{MJ}$. That is enough energy to lift a $70$ kg person almost $4{,}9$ km, if the human body were a machine with efficiency $1$ — which, fortunately or unfortunately, it is not.

Links

Topics: Thermology Concepts: Specific heat and heat capacity

Related exercises: Problem — Copper calorimeter · Worked exercise — Coffee and steam exchange · Problem — Four cups