| 释义 |
▪ I. pi, n.1|paɪ|
The name of the Greek letter π (in Gr. πῖ, |piː|).
1. Used in Math. to express the ratio of the circumference or periphery (περιϕέρεια) of a circle to its diameter: see P (the letter) II.
[1748Euler Introd. in Anal. Infinit. i. viii. (1797) I. 93 Satis liquet Peripheriam hujus Circuli in numeris rationalibus exacte exprimi non posse, per approximationes autem inventa est..esse = 3,14159 [etc., to 128 places], pro quo numero, brevitatis ergo, scribam π, ita ut sit π = Semicircumferentiæ Circuli, cujus Radius = 1, seu π erit longitudo Arcus 180 graduum. 1841Penny Cycl. XIX. 186/1 This number π must be the same for all circles. Ibid. 186/2 This measure of Archimedes gives 3·14286 for the approximate value of π, the ratio of the circumference to the diameter. ]
2. Electr. Applied to a four-terminal set of three circuit elements in which one element is in series between two in parallel. Usu. written as π or Π.
1924K. S. Johnson Transmission Circuits for Teleph. Communication xi. 124 If the structure shown in Fig. 1 is considered to be made up of Π sections.., each section may be regarded as terminating in a mid-shunt iterative impedance. 1930Dannatt & Dalgleish Electr. Power Transmission v. 118 This network is frequently used as an equivalent circuit for a transmission line, and is referred to as a π-circuit. 1930[see ladder network s.v. ladder n. 6]. 1950[see lattice n. 2 d]. 1961Amat. Radio Handbk. (ed. 3) vi. 175/1 The pi-network coupler is often used for delivering power to an aerial feeder. 1972R. H. Warring Ham Radio v. 68 The adoption of a pi-network tank circuit does not automatically ensure that no harmonics are radiated which may show up..on near-by television receivers, when a transmitter is being worked on the various amateur bands.
3. Physics and Chem. [After P III. 2.] Used to designate electrons, orbitals, molecular states, etc., possessing one unit of angular momentum about an internuclear axis; pi- (or π-) bond, a bond formed by a π-orbital.
Usu. written π when it refers to one electron or orbital and Π when it refers to a molecule as a whole.
1929R. S. Mulliken in Chem. Rev. VI. 532 The molecule contains two 1sσ electrons (1sσ2)..and two 3sσ electrons, and (in NO and O2) one or two 3ρπ electrons... The second (Greek) letter gives the value of a quantum number λ which does not exist for the atom, (σ, π, δ,{ddd}mean λ = 0, 1, 2,{ddd}). Ibid. 534 Two σ electrons of any one kind.., or four π, or δ, electrons of any one kind..constitute a closed shell for a diatomic molecule. 1930― in Physical Rev. XXXVI. 616 The use of the symbols σ, Π, Δ, ϕ, Γ, Η,..to indicate λ = 0, 1, 2, 3, 4, 5,..is recommended. 1939J. W. T. Spinks tr. Herzberg's Molecular Spectra I. v. 291 Examples of 2σ - 2Π transitions are the ultraviolet OH bands. 1947Q. Rev. Chem. Soc. I. 157 A double bond is normally a σ-bond and a π-bond together. 1952L. N. Ferguson Electron Struct. Org. Molecules ii. 19 The π electrons are bound less firmly and can be more easily polarized (that is, attracted to either end of the molecular orbital) than σ electrons, so the former are commonly referred to as mobile electrons, whereas the σ electrons are said to be localized. 1954G. I. Brown Introd. Electronic Theories Org. Chem. iv. 49 Two atoms linked by a σ-bond can rotate freely about the bond, unless there is some steric interference.., but free rotation is prevented when two atoms are linked by a π-bond. 1964D. F. Eggers et al. Physical Chem. xvi. 630 Since many molecular properties seem to be governed largely by the pi electrons, most theoretical calculations for such molecules are carried out for the pi electrons only. 1966New Scientist 29 Dec. 735/2 So-called pi-bonded systems containing alternating sequences of single and double bonds. 1972J. C. Schug Introd. Quantum Chem. xi. 263 The remaining six valence electrons [in the benzene molecule] occupy the unhybridized p orbitals of the carbon atoms, which are perpendicular to the plane of the molecule... Each of these so-called pi electrons can be paired with a pi electron on a neighboring atom to form three additional pi-type bonds, as in ethylene. 1973A. W. Adamson Textbk. Physical Chem. xvii. 838 Pi bonding is considered not so much as providing the primary bonding holding a molecule together as supplementing an already present sigma bond. 1975H. W. Kroto Molecular Rotation Spectra x. 224 The 2Π ground state of NO..has both orbital and spin angular momentum.
▪ II. pi, a. (n.2) Public School and Univ. slang.|paɪ|
[abbrev. of pious.]
Pious, religious, sanctimonious. Also absol. = a pious person; and as n. = pious exhortation, etc. See also pie a.1
c1870[at Eton], ‘What did your tutor say to you?’ ‘Oh, he gave me a pi; asked me how I could reconcile my behaviour with my duty to God and my parents’. 1891Wrench Winchester Word-bk., Pi, virtuous, sanctimonious. He's very pi now, he mugs all day. 1897Westm. Gaz. 1 Sept. 8/1 The man who regularly affects the ‘pi’ and who ‘plays up’, with ready catholicity of spirit, the ‘special missions’ of every religious denomination in turn. 1916[see fit a. 5 b]. 1968J. R. Ackerley My Father & Myself viii. 79 He invited the two of us into the billiard-room..for a ‘jaw’, which could hardly be called ‘pi’ and which he himself described as ‘man to man’. 1972South China Morning Post (Hong Kong) 20 Nov. 16/4 The subject emerges, I believe falsely, as a kind of overgrown school-prefect, bossy, frustrated and a bit pi. 1975J. Hitchman Such Strange Lady ii. 27 ‘That were only sparrers... They aren't good for nothing.’ ‘God made them,’ retorted the clergyman's daughter, ‘so they must be good for something.’ All very pi' of course. 1978Broadcast 3 Apr. 42/2 ‘Blue Peter’, though never pi or holier than thou, is always on the side of the..decencies.
▪ III. pi
variant of pie n.4 and v.2 (disordered type, etc.); also of pie n.5 (Indian copper coin). |