Hi all! This is my first post... so have mercy on me We had an exam from computer technologies today and there were only 8 of us loosers who still didn't pass it and noone is sure about the correct solutions: 1.) What is wave frequency of 0,5m long whistle sound? What is the frequency of the next higher tone? 2.) A 20A current runs through a Cu wire with intersection 5mm^2. What is the travelling speed of charge carrying particles in this wire? 3.) We have some light diode which has "forbidden" belt width 3,5eV. What is the wave length of emitted light, energy and momentum of a single photone? 4.) Diode has a saturation current 200microA. Temperature is 140°C and the voltage on diode is 0,5V... what current goes through diode? 5.) Hall's coeficient of a doped silicium at room temperature (25°C) is 0,05Vm/AK. With what kind of dopand is this silicium doped and what is the concentration of dopands? Extra info: k_B = 1,38*10^-23J/K h=6,63*10^-34Js e_0=1,6*10^-19As rho_Si=2,33g/cm^3 relative atomic mass of Si = 28 N_A = 6*10^26/kmol M_Cu = 64kg/mol rho_Cu = 9 (relative density ) - WTF is relative density supposed to mean? speed of sound = 340m/s I'm aware that this must be a horrible translation so I won't wonder if you die :rofl: laughing like crazy. 1: I didn't have problems with 1. exercise... you just have to know that this is wave motion is "fixed" on one side and "free" on the other. (v_0=170/s, v_1=340/s I hope) 2: It bothered me cuz I got a very small speed for electrons in my opinon. 3: Didn't have a clue what to do with this E_g (gap energy)... really need help here. 4: No current at all :uhh: (but I guess it's possible and I'm just used to getting non zero results on exam) 5: I calculated this concentration (n): n = 1/(R_h*e_0) but I don't know if this is overall concentration of all charge carrying particles or just dopands I'm sorry if I'm asking simple and stupid things... I have serious problems finding any books on these topics in libraries and we didn't do much exercise at school.