| | Use two spheres each with an electrometer and charge one of them. If you connect the spheres with a glow lamp, the lamp will glow for a brief moment, and the readings of the electrometers become equal (Figure 1). There is a technically relevant equivalent of this phenomenon which allows measurements to be made quite easily. Build an electric circuit of two so-called capacitors (they are the equivalent of the two spheres). Charge one of the capacitors with the help of a battery or a power supply and connect both by a conductor (a so-called resistive element). Use volt-meters to measure the voltage across each of the capacitors as a function of time. The result is seen in the diagram on the right of Figure 1. The voltage of the charged capacitor decreases, and the reading for the uncharged one increases. The readings change until they have become equal.
Interpretation Note the strong similarity of the curves in the graph of Figure 1 and those measured when letting a liquid flow from one tank into another container through a connecting hose (Phenomenon 1.1). The interpretation of the hydraulic phenomenon requires two concepts, those of quantity of liquid and of fluid level or fluid pressure. We say that the liquid flows from the tank with the higher fluid level to the one with the lower level until the levels (or pressures) have equilibrated. This is exactly how we look at electric phenomena. We introduce a quantity of electricity—called electric charge—which can be stored in systems and which can flow. Secondly, we imagine an intensity or level of electricity, called electric potential, whose difference is responsible for flows of electricity. The difference of electric potential is called voltage. That’s what we measure with voltmeters. As long as there is a voltage between the two capacitors in the experiment, charge flows and the voltages across the capacitors change until they are equal. Then the process stops.
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