Question 1

How do kilojoules differ from the kilocalories on a food label?

Accepted Answer

1 kcal = 4.184 kJ, so 100 kcal = 418.4 kJ. Nutrition labels often show both. The "calories" in dieting are actually kilocalories (large calories, Cal); the small calorie (cal = 4.184 J) is only used in the lab.

Question 2

Why is electricity billed in kWh?

Accepted Answer

The kilowatt-hour is the natural household unit: a 1 kW appliance running for one hour uses 1 kWh. In joules that is 3,600,000 J — awkward to work with. So 1 kWh = 3.6 MJ, and your meter counts in kWh.

Question 3

What is a BTU?

Accepted Answer

A British Thermal Unit is the energy needed to heat one pound of water by 1°F: 1 BTU = 1,055 J. Air conditioners in the US and Japan are rated in BTU/h — 9,000 BTU/h ≈ 2.64 kW.

Question 4

How much energy is in food?

Accepted Answer

A daily intake of about 2,000 kcal equals 8,368 kJ ≈ 2.3 kWh. A chocolate bar (~250 kcal) is about 1,047 kJ. A kilogram of pure fat stores roughly 9,000 kcal = 37.7 MJ. An hour of running burns 500–700 kcal.

Question 5

What is a gigacalorie (Gcal) used for?

Accepted Answer

1 Gcal = 10⁶ kcal = 4.187 GJ = 1,163 kWh. It is the standard unit for district heating: an apartment might use 1–2 Gcal per month in winter. Boiler plants are rated in thousands or millions of Gcal per year.

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