What it shows:
The use of a parallel plate capacitor to show the dielectric constant of various materials.
How it works:
The capacitor consists of two 1/4" (6mm) aluminum plates 20 cm in diameter. They are mounted on insulated posts to an optics rail (figure 1). The capacitance can be determined with an electrometer, or directly by a capacitance meter. 1 When using an electrometer, one applies a small charge Q to the plates (with a battery), measures the voltage V and then calculates the capacitance C which is given by,
where A is the plate area and l the plate separation. With just air between the plates ε ≈ ε0the permittivity of free space. A set of dielectric sheets (each around 25 × 25cm square) can be inserted between the plates to observe the change in capacitance by affecting the permittivity ε as,
where K is the dielectric constant of the material. Values for K are given in table 1.
figure 1. The capacitor
Setting it up:
A bench-top demonstration. Camera setup needed on the meter display. The water sample is contained in a 25×25×1.5cm Plexiglas box. Supply water in a 500ml beaker and a funnel, maybe a spill tray too.
The electrometer measurement, although a more fundamental measurement, tends to be unreliable. It is hard to avoid static charges on the surfaces of the dielectric, and these static charges transfer to the capacitor plates and make the voltage measurement unreliable. The impedance meter, although more of a "black box", is totally reliable and reads out the capacitance value directly.
|material||thickness (cm)||dielectric constant K|
1 csi impedance meter model 252, Electro Scientific Industries, Portland, Oregon. The impedance meter determines the capacitance with a 1 kHz signal (the capacitor becomes part of an RLC circuit).