Resumen de Using Capacitors to Measure Charge in Electrostatic Experiments
Francisco Vera, Manuel Ortiz, P Diego Romero Maltrana, Francisco Antonio Horta Rangel
In this work we describe a simple setup for measuring electrostatic charge and polarity in electrostatic experiments. This low-cost detector only requires a graphite pencil, a capacitor, two switches, and a voltmeter, and holds the promise of transforming qualitative activities that are commonly used to teach electrostatics into quantitative activities. In order to measure the charge and polarity of charged objects, a pencil is used as an antenna to collect charge from the object under study. The collected charge is transferred to a capacitor, and the resulting voltage at the capacitor is measured using a voltmeter. In contrast to other devices, our detector can be used to measure the amount of charge stored in objects at several thousand volts, as for example Van de Graaff domes.
When touching the object with the antenna, almost all the charge is transferred to the capacitor, the high voltage of the object drops at the capacitor, and it can be measured safely with regular voltmeters. The capacitor’s voltage and its capacitance can be used to estimate the initial charge at the originally charged object. Note that the capacitance of objects such as Van de Graaff (VdG) domes are usually orders of magnitude lower than commercial capacitors, which explains two important issues of this process: 1) the large difference in voltage related to the same amount of charge, and 2) why most of the charge is transferred into the capacitor after the antenna touches the object.
It should be stressed that there are other simple or low-cost polarity detectors, as for example neon lamps obtained from screwdriver pole testers.1 In such devices a bright red color lights up at the negative terminal as one end of the screwdriver pole tester is grounded while the other end gets close to objects charged at high voltages. However, current neon test lamps have reduced the size of inner electrodes, making it hard to see which electrode lights up. To the best of our knowledge, this problem was not noticed in the literature previous to the first draft of our work. However, during the time the draft was reviewed and edited, Ref. 2 highlighted this pitfall too. A member of our group also conceived a simple way of detecting polarity using LEDs by approaching one of the LED terminals to a charged object while the other terminal is grounded. The LED will light up if the object’s charge and polarity are capable of producing a current through the LED. Although this method is simple, low cost, and fairly direct, a possible problem is that if the LED does not light up, it might imply different things: the object under study may be uncharged, the polarity may be wrong, or the voltage applied may have burned out the LED. Finally, it is also possible to use some simple electronics to build an electrometer to measure the sign of a charged object.3–7 Despite some minor limitations, these methods are all simple and pedagogical; however, none of them is able to deliver quantitative measurements, and this is a notorious advantage of our device. The standard alternative to measure the electric field and the polarity of objects charged with high DC voltages is to buy a commercial electronic electrometer, whose cost is considerably higher in comparison to our simple device.
