# Measuring the Charge of Nanoparticles

1. Sep 21, 2003

### eNtRopY

Let's say I wanted to measure the average eletronic charge of an undefined, nano-sized particle. I recently conceived of the following experiment for measuring such charge.

Create an apparatus consisting of a parallel plate capacitor with two serially connected dielectric media. Let one of the media be pure, deionized water H2O(l). Let the other be crystalline silicon dioxide SiO2(c). Place a known number of these nanoparticles in the water. Apply a voltage with a polartiy that causes the particles to press against the SiO2(c) layer. Measure the capacitance of the system by applying a small AC signal. Then, apply a voltage with a polarity that causes the particles to press against the surface of one of the conducting plates. Again, measure the capacitance of the system by applying a small AC signal.

According to Gauss' Law. The total enclosed charge of a system can be found by integrating the exiting electric field lines on the surface of that system. Capacitance is a measure of how many electrons can be placed on the surface of a parallel plate for a given applied voltage. Therefore, we should be able to calculate the charge of our nanoparticles by comparing the values of the two previously mentioned capacitances. In the first case the charged nanoparticles will not effect the measured capacitance of the parallel plates because they will be stuck in the middle of the capacitor. In the second case, the charge will be pressed against one of the conductors which should locally shield charge on that plate thereby reducing the effective capacitance.

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