Capacitorsnj.aaptsections.org/wp-content/uploads/2014/03/Capacitors-NJAAP… · (named after Britain’s Michael Faraday) C=Q/V where Q = total charge in Coulombs (C) V = voltage

Capacitors Have many uses in many types of circuits…

Build a GIANT capacitor:

Like a sandwich:

• Layer of an insulator (plastic garbage bag or square of styrofoam)

• Layer of aluminum foil (metal to hold the charge) – leave a tab for clip

• Layer of dielectric (example, stack of paper, plexiglass, styrofoam)

• Layer of aluminum foil (metal to hold the charge) – leave a tab for clip

• Layer of an insulator (plastic garbage bag or square of styrofoam)

(The top and bottom-most layers are just insulators to isolate the capacitor)

Don’t forget to squish (put a book on top if need be)!

Capacitance (C) in farads(f) – Usually in: mf, nf, pf (named after Britain’s Michael Faraday)

C=Q/V

where Q = total charge in Coulombs (C)

V = voltage in Volts (V)

C=ke0A/d where k = dielectric constant (material dependent)- can google!

e0 = “permittivity of free space (vacuum)” = 8.854 X10-12 f/m

A = Area of plate (same for both) in meters2

d = distance between plates in meters

Units (by thousands)

• mega (M) = 106

• kilo (k) = 103 e.g. km

• ---

• milli (m) = 10-3

• micro (m) = 10-6

• nano (n) = 10-9

• pico (p) = 10-12

Hence: mf, nf, pf

Make your own Capacitor: 1.) Look up dielectric constant K of dielectric material you are using (e.g. plexiglass or stack of paper)

2.) Cut 2 matching pieces of aluminum foil by folding foil over (then cut and separate). Make a square or rectangle shape.

3.) Measure the length (in cm) and width (in cm) of aluminum foil and find area.

4.) Convert area (in cm2) to m2 by dividing by 100 twice (why?). This will be A.

5.) Measure the thickness of the dielectric (preferably with a Vernier caliper for best results). This will be d.

6.) Plug into equation. Do not plug in the 10-12 into calculator--- that we will rename pico!

Some types of capacitors: disk, rolled…

Electrolytic – optimizing charge held because difference in in the dielectric layer-

added oxidizing layer to better hold charge on ONE side only plus liquid soaked “spacer” (often paper, which has dielectric constant too) on other side

Legs marked (one says “+”)

OR

one leg shorter (shorter leg looks more like a negative)

What are some things capacitors do?

1.) To store charge

Temporarily or permanently (at least as long as plugged in)

2.) To create parallel plate situation

Create constant “Electric Field” and control/vary that field

3.) To use the fact that it takes time to fill (and empty) its plates

Variable current in the circuit (even if a DC voltage source such as battery)

A Few Uses of Capacitors:

• to store charge [e.g. allow batteries to have a longer lifetime]

• to use the electric field generated between the plates to divert electrons shot between the plates to have their trajectories controlled [e.g. cathode ray TV screen]

• To use the fact that charges can be moved on and off plates AND that it takes time to fill and discharge capacitor plates, hence there is a short-lived current during fill and during discharge [e.g. electronic timing (BUT NOTE- quartz watches instead use a tiny vibrating crystal for time telling), radio receiver, high and low pass frequency filters]

Another exercise:

a.) Put capacitors in series and predict and then measure capacitance 1/Cseries = 1/ C1 + 1/C2… Remember--- to find the final capacitance, you have to find C (not 1/C) b.) Put capacitors in parallel and predict and then measure capacitance Cparallel = C1 + C2…

SERIES vs. PARALLEL Series = “holding hands”

(2 capacitors touch in 1 place only)

Parallel = “piggybacking”

(2 capacitors touch in 2 places)

Tips: • NOTE--- IMPORTANT: set all up, just leave the alligator clips from the

cables on the capacitance meter lying right next to where they will connect to aluminum foil and then zero because the banana cable wires themselves, lying side by side, have a capacitance (not what we want to test). If you bump drastically then re-zero.

• NOTE: With commercial capacitors (for example: 0.1mf, 1mf and 10 mf), minimize wiring and stick the legs into the slits (for series and parallel, for series and parallel clamp legs together with a free alligator clip and stick in legs---or use technique in previous NOTE above).

• NOTE: Discharging capacitors (recommended in the capacitance meter directions) before testing is done by just touching the legs (these are tiny capacitances so no shock)--- for larger capacitors use a screw driver (hold plastic) touching its metal to capacitor legs and then touch the metal of screwdriver to a ground such as a sink faucet.

Capacitance Meters • Can use auto-ranging capacitance meter - more expensive. However, can

instead use a manual-ranging capacitance meter (just start at the largest setting (mf) and click down to smaller setttings (mf, nf, pf) until get full screen reading (always try one lower setting to see if more digits). The lowest setting on a good capacitance meter should be 200pf (so it reads down to 0.1pf). Highest just needs to be in mf range for this experiment series.

Examples of manufacturers: NOTE: capacitance meter =“component testers”

• BK Precision Compact Capacitance Meter to 20 mF (this is manual-ranging) Model# 810C, MSRP $85.00

• BK Precision Dual Display Capacitance Meter to 50 mF (this is auto-ranging) Model# 890C, MSRP $149.00 At: http://www.bkprecision.com/products/component-testers.html#!/capacitance-meters

Other manufacturers out there too – I am not advertising, this is just an example.

Another is Bulb America (http://www.bulbamerica.com/digital-capacitance-meter.html?mr:referralID=caf16621-acb5-11e3-beb7-001b2166c2c0 )