Dynamics Track
Inclined Plane
Momentum

Capacitor
Plate Sep
Plate Sep/Volt
Dielectrics

Circuits
Ohms Law
Series/Parallel

Wave Tank
Freq/Wavelength
Two Pt Interf.

Optical Bench
Refraction
Focal Length

 

 

Variable Capacitor

Dielectrics

A dielectric is a material that, when placed between the plates of a capacitor, increase the capacitance.   Air itself is a dielectric, but it does not change the capacitance much from that of a vacuum so we will consider air and vacuum to be the same for this experiment.   The ability of a dielectric to raise capacitance is stated in a number called the dielectric coefficient which is usually symbolized by the Greek letter kappa ( k).   This dielectric coefficient is simply the ratio between the capacitance with the dielectric in place, and that with no dielectric in place, all other parameters being kept the same.

Equipment:

  • Variable capacitor
  • Digital multimeter
  • Capacitance Tester (short leads that plug into multimeter)
  • Masonite dielectric
  • Plastic dielectric
  • Paper dielectric

Cautions:

This equipment is delicate.   Everything should go together with the lightest of touches.   Do not force anything!

Procedure to set up the variable capacitor

  1. Place the variable capacitor in the middle of the lab table, with the 0cm mark to your left.   Don’t put the capacitor too close to the edge of the table!
  2. Place the multimeter close to the capacitor plates.   You will find it most convenient to put the meter behind the capacitor.
  3. Plug the capacitance tester into the Cx socket on the multimeter.   One pin of the tester goes into each of the slots of the Cx socket
  4. Clip the leads of the capacitance tester to the plates of the capacitor.   The best place to clip them is onto the radial flanges on the back side of the plates.   There are binding posts present, but the flanges work better!


 The meter and leads themselves have some capacitance (about 5pF), so the leads have to be kept short.   Try to keep the leads as far away from each other as you can.
You will now test several dielectrics.   The dielectrics are all the same thickness, so the only variable in this experiment will be the dielectric material.

  1. Turn the big dial on the multimeter to “2000p”.   This turns the meter on and adjusts it to read in picofarads (pF).
  2. Place the plastic dielectric between the plates and gently slide the plates together until both are touching the dielectric.
  3. Back away from the apparatus, let the meter settle to a constant reading and record the reading here:

  4. Gently slide the dielectric out from between the plates.   Do not disturb the position of the plates!
  5. Again, back away from the apparatus, let the reading settle, and then record the reading here:
  1. Use your data in the following table.   Fill in the cells under “Plastic”:

Plastic

Masonite

Paper

Capacitance with Dielectric

Capacitance with Air

Dielectric Coefficient ( k )


Since each dielectric is a slightly different thickness, you will record a reading for “Air” along with each of the dielectric materials to make sure that you can calculate an accurate dielectric coefficient..   Since the thicknesses of the materials are close to each other, the “Air” readings should be within a few pF of each other.   If not, figure out what’s not right!

  1. Repeat steps 7-10 for the masonite and paper materials.   Record their data in the table.
  2. Turn the meter dial to “0” (straight up).   This turns the meter off.
  3. Disconnect the meter and return the lab equipment to the condition in which you found it.
  4. The dielectric coefficient ( k ) is simply the ratio between the capacitance with the dielectric in place and that with a vacuum between the plates.   Since we’re fresh out of vacuum, we’ll make do with air, which is within .05% of vacuum.   For each of the materials in your table, divide the capacitance with the dielectric in place by the capacitance with the dielectric removed and write your answer in the appropriate place.

  5. Answer the following questions:

Which material was the best dielectric?













What advantages are there in using dielectrics in capacitors?
















Why were the leads on the capacitance tester so short?