Usando um voltímetro para medir tempo

Abstract

The most direct way to measure time intervals is through the use of a chronometer. But sometimes we need to use some non-conventional method. In this work we explore an indirect method to measure time intervals based on the discharge curve of a capacitor through a resistor. The capacitor previously charged is connected in series with the resistor and a proper switch initially open. When the switch is closed for a certain time, the capacitor discharges partially and the potential difference across its terminals decays from a initial value V 0 to V. Using the discharge curve or the discharge equation of a capacitor in RC circuit, and measuring both the voltage V 0 and V, we can determine the time t during which the switch remained closed. The method consists in associating the duration of an event to the time t: To avoid the capacitor to discharge through the voltmeter resistance when measuring V and V 0, we use a operational amplifier(CA 3140) set up as a unity gain voltage follower. The method was successfully tested in a series of experiments we perform. In one of them, for instance, we measured the duration of a collision between two steel balls. In another, we measured the speed of a pellet fired by an air rifle. © Sociedade Brasileira de Física.