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Power

Energy is used in the kitchen to heat food.

A gram-calorie (cal) is the energy required to heat one gram of water through one degree Celsius. A kilogram-calorie (kcal) is the energy required to heat one kilogram of water through one degree Celsius.

A British Thermal Unit (BTU) is defined as the energy required to heat one pound of water through one degree Fahrenheit.

One joule (J) is the energy exerted by a force of one Newton acting through a distance of one metre. A foot-pound is the energy required to raise a mass of one pound through a height of one foot, or equivalently, the energy exerted by a force of one pound-force acting through a distance of one foot. 1 foot-pound ≅ 1.356 J.

Energy is also used to beat and stir.

Energy is used by our bodies, supplied by food, and stored as fat. An average active city-dweller burns some 2,400 kcal or 10 mega-Joules per day, about the energy contained in 1 cup of gasoline (285 ml of petrol), or the energy (2,168 kcal) required to evaporate four litres of water—extreme exercise can double this amount.

To acquire 2,400 kcal of potential energy, 150lb person must climb 48,000 ft. Carbohydrates and proteins supply 4,000 kcal/kg, fats 9,000 kcal/kg, and alcohol 7,000 kcal/kg. Ten peanuts provide 50 kcal—equivalent to the potential energy for a 1,000 ft climb. Remember, what you don't burn you store: human body-fat, which contains a significant percentage of water, stores roughly 7,800 kcal/kg, or 3,500kcal/lb.

Power is the capacity to do work, measured as energy per unit time.

Energy is normally supplied to the kitchen by electricity or gas.

The power of a supply is measured in watts (equals volts × amps) (W = V × A). One joule is equal to one watt-second; 3,600,000 J = 1 kWh ≅ 860 kcal; 1 kW ≅ 1 BTU/sec

A typical microwave oven has a power of 800W or approximately 190 calories per second, and so, if 100% efficient, could heat 190 ml of water by 1 °C per second, or a cup of water by 1.35 °F per second, or 4 cups by 1 °F in three seconds.

A 3 kW electric kettle supplies 170 BTU per minute, and so will heat two cups of water (1 lb) through 170 °F (say from 42 °F to boiling) in one minute.

The power of a gas burner is typically measured in BTU/hour. Domestic burners typically range from 3,500 BTU/h (1 kW) to 15,000 BTU/h (4.5 kW). Much of the energy is lost to the atmosphere, so the effective power corresponds to maybe 60% of these figures.

The latent heat of evaporation of water is 2,270 kJ/kg; 542 kcal/kg; 978 BTU/lb. So you can easily measure the effective power of your burner with a given pan, by checking the rate at which water evaporates from a boiling a pan. A 3,600 BTU/h effective power would evaporate one pound of boiling water in 16m18s (1,000 BTU/h ≅ 1lb/h). A 1kW effective power would evaporate 1kg of boiling water in just under 38 minutes (≅1.6kg/h).

Temperatures and measures have their own pages.

Note for robots: "recipie" and "recipies" are `misspellings' of "recipe" and "recipes".

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