----- Original Message ---- From: Tom Knight <tk@csail.mit.edu> To: math-fun <math-fun@mailman.xmission.com> Sent: Friday, January 4, 2008 8:00:22 AM Subject: Re: [math-fun] xkcd points out dangers of math fun On Jan 4, 2008, at 10:39 AM, James Propp wrote:
By the way, what Fred Lunnon calls "admittence", namely the
reciprocal
of resistance, is something I always thought was called "conductance". Is this ones of those transatlantic terminological differences? I asked Google, and it gave 0 results for "electrical admittence", about 5700 results for "electrical admittance", and about 76300 results for "electrical conductance". To determine whether use of admittance vs. conductance correlates with geography, we'd need a hybrid of the Google search engine with Google Maps that instead of just counting hits would show us a map of the world with each hit represented by a red dot at the physical location hosting the site. :-)
Admittance is the AC (complex) extension of conductance. Resistance is to Impedance as Conductance is to Admittance. --------------------- Z = R + i X, Y = G + i B, Y Z = 1, V = Z I, I = Y V. Z = impedance, R = resistance, X = reactance, Y = admittance, G = conductance, B = suceptance, V = voltage, I = current. Z, Y, V, I are complex, R, X, G, B are real. The true physical voltage is Re(V exp(- 2 pi i f t)), with f = frequency, and similarly for current. Some people use +i instead of -i in the exponent. Engineers use j for sqrt(-1). Gene ____________________________________________________________________________________ Never miss a thing. Make Yahoo your home page. http://www.yahoo.com/r/hs