Physics 102-current balance-HK

Physics 102
The Current Balance
March 21, 2006
Wai N Man, HoKoon Yau
The purpose of this experiment is to find out the permeability of free space and
combine the result from the previous lab to find the speed of light.
1. Current balance
2. Ammeter
3. Power supply
4. Rheostat
5. Laser
6. Lab jack
7. Analytic weight set
1.) Set up all the equipments as shown in the lab manual.
2.) Put a coin on the pan to make sure the beams are probably aligned without
3.) Adjust the counterweight of beams to make the gap not more than .5cm
4.) Turn on the power supply and the laser to reject the laser by the mirror to
the backboard.
5.) Marked down the rejected laser beam on the backboard.
6.) Adjust the rheostat until the laser beam on the backboard move up. Then,
read the ammeter.
7.) Place a small weight on the pan to counter balance until the laser beam on
the backboard go back to the original mark.
8.) Repeat the process from 6-7 until the end of the rheostat.
Questions/ Calculations:
1. The distance r between the conductors adds the radius of each beam to the equilibrium
distance is 0.00519m. The radius of each beam is 0.0016m, and the distance between the
conductors is 0.002m.
2. See the Excel Spreadsheet.
3. Positive to Negative: slope = 1.231x10-05, intercept = -5.11197x10-05
4. Negative to Positive: slope = 1.31x10-05, intercept = -1.6x10-05
5. The y-intercepts of both directions are negative.
6. F = (µoI2L) / 2πr
F/I2 = µoL /2πr
Slope ave = µoL /2πr
2πr Slope ave /L = µo
µo = -5.334x10-07
permeability of free space = -5.334x10-07
7. Speed of light is c = 1/√(µoε0)
µo = -5.334x10-07 ε0=8.987x1016
c = -4.567x10-06
Error analysis:
1. The counter weight given in the experiment may not be able to bring down the
laser beam to the original mark point on the board.
2. Moving the rheostat by hand may not be accurate.
3. Movement from the surrounding like the experiment table may affect the result.
This experiment has proven that the current is proportional to the permeability of
the free space. By increasing the current, the space between the bars will be increased.
After combining the result from the previous experiment, we concluded that the
permeability and the permittivity of free space are inversed proportional to the speed of
grade = 100