1. 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:

2. Determination of the ohm (Kelvin) B0B0 ω  B i  θ R

3. Determination of the ohm (Rayleigh, Sidgwick) DC B ω F v I F = q v  B

4. Determination of the ohm (Rayleigh, Sidgwick) DC ω I -- + + + + --

5. 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

6. Determination of the ampere F F F F I O

7. F F F F I O

8. F F F F I O

9.    O G

10. O   z max zz

11. 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