Ley Del Coulomb

Electricity
Electrostatics Coulomb’s law

LEYBOLD Physics Leaflets

P3.1.2.2

Confirming Coulomb’s law

Measuring with the force sensor and newton meter
Objects of the experiments
Measuring the force F between two charged balls as a function of the distance d between the balls. Measuring the force F between two charged balls as a function of their charges Q1 und Q2. Estimating thepermittivity of free space ε0.

Principles
According to Coulomb’s law, the force between two pointlike charges Q1 and Q2 at a distance d is F= Q1 ⋅ Q2 1 ⋅ 4 ⋅ ε0 d2 As : permittivity of free space Vm (I)

with ε0 = 8.85 ⋅ 10 − 12

The force F is positive, that is repulsive, if both charges have the same sign. If the signs of the charges are different, the force is negative, that is attractive.The force between two charged spheres is approximately the same if the distance d between the centres is considerably larger than the radii r of the spheres so that the uniform charge distribution on the spheres remains undistorted. At smaller distances d, measuring results are changed by an “image force” caused by mutual electrostatic induction. The force between two charged balls will bemeasured in the experiment by means of a force sensor. You will study the proportionalities F 1 ,F d2 Q1 and F Q2 (II).

The device for measuring the force contains two parallel flection elements and four strain gauges connected in bridge circuit. The electric resistance of the strain gauges changes under mechanical stress. This change in resistance is proportional to the acting force, which isdirectly displayed by a newtonmeter.

0909-Wie

An electrometer operated as a coulombmeter enables the charges on the balls to be measured almost without current. Any voltmeter may be used to display the output voltage UA. From the reference capacitance C Q = C ⋅ UA (III)

is obtained. For example, at C = 10 nF, UA = 1 V corresponds to the charge Q = 10 nAs. If other capacitances are used, othermeasuring ranges are accessible.

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P3.1.2.2

LEYBOLD Physics Leaflets
Preliminary remark

Apparatus
1 set of bodies for electric charge . . . . . 1 trolley 1.85 g . . . . . . . . . . . . . . . 1 precision metal rail, 0.5 m . . . . . . . . 1 force sensor . . . . . . . . . . . . . . . . 1 newtonmeter . . . . . . . . . . . . . . . 1 multicore cable, 6-pole, 1.5 m . . . . . . 1 high voltagepower supply 25kV 1 high voltage cable, 1 m . . . . . 1 insulated stand rod, 25 cm . . . 1 saddle base . . . . . . . . . . . 1 electrometer amplifier . . . . . . 1 plug-in unit 230 V/12 V AC/20 W 1 STE capacitor 1 nF, 630 V . . . 1 STE capacitor 10 nF, 100 V . . . 1 voltmeter, up to U = ±8 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 263 337 00 460 82 314261 314 251 501 16 521 721 501 05 590 13 300 11 532 14 562 791 578 25 578 10 531 100 546 12 590 011 532 16 300 02 300 41 301 01

Carrying out this experiment requires particular care because “leakage currents” through the insulators may cause charge losses and thus considerable measuring errors. Moreover, undesirable effects of electrostatic induction may influence the results. The experiment mustbe carried out in a closed, dry room so as to prevent charge losses due to high humidity. Cleaning the insulated rods, which hold the balls, with distilled water is recommended because distilled water is the best solvent of conductive salts on the insulators. In addition, the insulated rods should be discharged after every experiment by quickly passing them through a non-blackening flame severaltimes; for example, that of a butane gas burner. The high voltage power supply and the point of the high voltage cable must be at a sufficient distance from the rest of the experimental setup so as to avoid interference by electrostatic induction. For the same reason, the experimenter – particularly while measuring charges – must keep the connection rod of the electrometer amplifier in his hand...