Demonstration Highlight: Parallel Plate Capacitor

Today we’re visiting the ever-popular Parallel Plate Capacitor, in its simplest form found in our demonstration index as J4-01, or with a dielectric plate at J4-22. A capacitor stores energy in the electric field between its plates. The capacitance of a capacitor is technically the amount of charge stored per volt – in a sense, how capable it is of storing charge at a given potential. In a parallel plate capacitor, the capacitance goes up with greater surface area, and goes down with greater separation between the plates.

The parallel plate capacitor consists of two large aluminum plates with an air gap. The capacitor is charged with a potential of around 1000 Volts using a low-current DC power supply. The plates may then be separated and the voltage observed, demonstrating that for a fixed amount of charge, the voltage is proportional to the plate separation.

But if you insert a dielectric sheet into a charged capacitor, the voltage goes down, which means the total capacitance of the system has gone up! The capacitance of a system depends on the dielectric constant of the medium – for air, this is very nearly the same as pure vacuum, but some materials have a much greater dielectric constant. This plastic plate has a dielectric constant nearly 5 times that of air.

Now, try out these simulations to see if you observe the same behaviour!

The first, from the PhET collection at the University of Colorado, places a capacitor in a simple DC circuit. In the first simulation tab, you can adjust the input voltage and plate size, and measure the electric field, capacitance, and energy stored. The additional tabs show variations: you can add a dielectric to the capacitor, or place multiple capacitors in series and parallel.

The second simulation, at oPhysics, additionally lets you control many characteristics within an idealized circuit. Compare the results you get for different combinations of capacitor size and input voltage between the two simulations.

Our capacitor has 22cm diameter circular plates, rather than the square plates used in the simulations. In the simulations, try setting the plate area to be the same as ours and see how it responds to other voltages.