NMT-Physics Intranet

Electrostatic Pendulum

This demonstration, allows for a nice visualization of the electric field inside a parallel plate capacitor. The idea is that an electric charge will be push by the electric field towards one of the capacitor's plates, according to their polarity. The twist is that the charged object between the plates is a graphite-covered sphere, which makes it a good conductor. Once the sphere touches the plate, it deposits its charge and acquires charge of the opposite sign from that plate, and is therefore immediately repelled. The electric field inside the capacitor will now pushed the sphere towards the other plate where the story repeats. The end result is an oscillatory motion of the sphere inside the plate capacitor. Note that there is no equilibrium point for the sphere and the force on the sphere due to the electric field is constant inside the capacitor - the oscillation is not harmonic. The capacitor for this demo is a PASCO variable capacitor, which allows us to vary the distance between the plates and therefore the magnitude of the electric field. Hence, by changing the distance between the plates you can vary the frequency of the oscillator. The graphite-covered sphere (also from PASCO) is hanging from a fishing line right at the center of the capacitor. The potential difference is supplied by a DC high-voltage-low-current source. Changing the potential difference, is another way to control the electric field and therefore the frequency of the oscillation. The voltage range is typically between 1kV and 3kV. The currents are very low, since the sphere can only carry a few micro Coulombs per oscillation, and the frequencies are less that 100 Hz.

ID: d0013

Set up time:

2 minutes in class.

Requires:

Electrical outlet. Components.

Notes:

You may need assistance the first time you use this demo.The demo works even on rainy days. A sphere hanging from a string forms a pendulum whose equilibrium point should be at the middle point of the distance between the plates of the capacitor. Otherwise, the motion will be asymmetric. When varying the distance between the capacitors' plates, make sure to keep the equilibrium point at the mid point. I have not yet explore voltage differences above 3kV.

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