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Revista Brasileira de Ensino de Física
Print version ISSN 1806-1117
HESSEL, Roberto and PERINOTTO, Ângelo C.. Using a voltmeter to measure time. Rev. Bras. Ensino Fís. [online]. 2011, vol.33, n.4, pp.4316-4316. ISSN 1806-1117. http://dx.doi.org/10.1590/S1806-11172011000400017.
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 V0 to V. Using the discharge curve or the discharge equation of a capacitor in RC circuit, and measuring both the voltage V0 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 V0, 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.
Keywords : measurement of time interval; operational amplifier; voltage follower; collision time; unity gain buffer.