36th International Physics OlympiadsSalamanca, Spain2 -12 July 2005 Theoretical Question 2: Absolute measurements of electrical quantities u Objective: To find resistance and current in terms of mechanical quantities of mass, length and time u Three case studies:
Determination of the ohm (Kelvin) B0B0 ω B i θ R
Determination of the ohm (Rayleigh, Sidgwick) DC B ω F v I F = q v B
Determination of the ohm (Rayleigh, Sidgwick) DC ω I
G D’D’ 4 C’C’ C'’ S' S F-2 3 D' 1 2 R I G D C C' S ' R I D' G IGIG I R D ' D C ' C S Determination of the ohm (Rayleigh, Sidgwick) I G = 0 R
Determination of the ampere F F F F I O
F F F F I O
F F F F I O
O G
O z max zz
Physical concepts required u Current and Magnetic Field: Current, resistance, magnetic field Ohm´s law, Kirchhoff’s laws Law of electromagnetic induction u Mechanics of Rigid Bodies: Centre of mass, torque Conditions of equilibrium