R

/r/
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Definition
  1. Noun:
    • The 18th letter of the modern English alphabet: "R" is the 18th letter in the English alphabet, a consonant.
    • A symbol for radius: In mathematics, "r" often represents the radius of a circle or sphere, which is the distance from the center to the circumference.
    • A physical constant: In physics and chemistry, "R" (usually capitalized) denotes the universal gas constant in the ideal gas law (PV = nRT).
    • A unit of radiation: In radiology, "r" (often lowercase) is a unit of radiation exposure, specifically the roentgen.
Examples of Usage
  • Noun (Letter):
    • The word "rabbit" begins with the letter R.
    • She has trouble pronouncing the 'r' sound.
  • Noun (Mathematics):
    • In the formula for the area of a circle, A = πr², the variable 'r' stands for the radius.
  • Noun (Science - Constant):
    • The value of the gas constant R is approximately 8.314 J·mol⁻¹·K⁻¹.
  • Noun (Radiation):
    • The patient received an exposure of 50 r.
Advanced Usage
  • "The three R's": A traditional phrase referring to the basic skills taught in elementary education: Reading, wRiting, and aRithmetic.
    • The school's curriculum focuses on mastering the three R's.
  • "The R months": A term, especially in culinary contexts (like oyster consumption), referring to months whose names contain the letter 'r' (September through April).
    • Oysters are said to be best during the R months.
Variants and Related Words
  • R&D: An abbreviation for "Research and Development."
    • The company invests heavily in R&D.
  • R-value: A measure of thermal resistance in insulation.
    • Higher R-value insulation is better for cold climates.
Synonyms
  • Radius: (Specifically for the mathematical meaning) The distance from the center to the edge of a circle.
  • Roentgen: (Specifically for the radiation meaning) The full name of the unit.
Related Phrases (Phrasal Verbs)

(Not typically applicable for a single letter. This section is omitted.)

Related Idioms
  • "Mind your P's and Q's" (Analogous Structure): While not for 'R', this idiom illustrates how letters can be used idiomatically to mean "mind your manners" or "be careful."
  • "The three R's": As detailed above, this is the primary idiom associated with the letter.
Noun
  1. the length of a line segment between the center and circumference of a circle or sphere
  2. the 18th letter of the Roman alphabet
  3. (physics) the universal constant in the gas equation: pressure times volume = R times temperature; equal to 8.3143 joules per kelvin per mole
  4. a unit of radiation exposure; the dose of ionizing radiation that will produce 1 electrostatic unit of electricity in 1 cc of dry air

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