How is BTU defined?

BTU, or British Thermal Unit, is defined as the amount of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit. This unit of measurement is widely used in heating and cooling systems to quantify the energy content or energy transfer related to heat.

The foundational principle behind BTU is related to temperature change in water, which is a standard reference, given its common use in thermal calculations. Option B accurately captures this definition by specifying both the weight of the water (1 pound) and the temperature increase (1 degree Fahrenheit).

While raising the temperature of water by 1 degree Celsius, as mentioned in option A, does have a correlation to BTUs, it represents a different measurement since the conversion factors between Celsius and Fahrenheit are involved. The statement about boiling 1 liter of water in option C focuses on a different thermal process and does not define BTU accurately. Lastly, while electrical energy is measured in joules or kilowatt-hours, stating BTU as a measure of energy in electrical appliances as in option D strays from its definition pertinent to heating and thermal energy. Thus, option B is the most precise definition of BTU in the context of thermal energy.