Instructor Outline:
Capacitors
UM Physics Demo Lab 07/2013
Lab length: 70 minutes
Lab objective: Instruct the students about capacitors, capacitance and electrical
energy storage.
Materials
1
1
1
1
Battery Board
Alligator Lead card
Capacitor
Roll Aluminum Foil
3
3
1
1
Transparency sheets
Catalog Sheets
Multimeter
pair of Scissors
1
1
1
1
Heavy Item (Book)
- 3V bulb
Clear Plastic Ruler
Marking Pen
Exploration stage: 30 minutes - Group Lab work
The students observe the behavior of a capacitor in a charging and then a discharging
circuit.
Analysis stage: 10 minutes – Lecture
Capacitance is defined as C 
in a capacitor
Q
and the instructor explains electrical energy storage
V
E = ½ C V 2.
Application stage: 30 minutes – Group Lab Work
The students work in groups again to build capacitors and explore how the geometry
and the permittivity of the dielectric contribute to capacitance. These capacitors cannot store
enough energy to power a bulb or LED but they illustrate clearly how capacitance varies with
area, permittivity and plate separation.
Concepts Developed:
1. Capacitors store electric charge by separating charge of opposite sign on the
plates of the capacitor.
2. The separated charges on a capacitor also store electric potential energy
since a potential difference must exist across the plates of the charged
capacitor on which the charge resides.
3. Capacitance is defined as (charge)/(potential difference): C = Q/V.
4. Capacitance is a measure of efficiency—a large capacitor is efficient because it
stores a large amount of charge at a low potential difference which means
that relatively little work was required to move the charge onto the plates of
the capacitor working against electric forces. By contrast, a small capacitor
requires a large potential difference to store the same amount of charge as
a larger capacitor and much more work must be done against electric forces
to move the charge onto the plates. A small capacitor is therefore less
efficient at storing charge.
5. The energy stored in a capacitor can be calculated in terms of the capacitance
and potential difference across the plates as E = ½ CV 2.
6. The capacitance of a parallel-plate capacitor is proportional to the area of
the plates and the permittivity of the dielectric material between the plates
and inversely proportional to the distance of separation between the
plates: C = ε A/d.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
Suggested Demonstrations:
5C30.20-2 - Killer Capacitor
Shows the magnitude of the electrical energy stored in a capacitor.
5C10.10 - Examples of Capacitors
Includes unraveled miles of aluminum
5C10.20u1 - Potential Rise in a Parallel-Plate Capacitor:
Good for after application stage of lab to test the equation they learned.
Energy Stored in a Capacitor: Introductory Lab apparatus using motor and gears to
lift a 300g mass to demonstrate the energy stored in a 1 F capacitor as mechanical
work. Double the potential and raise the mass four times as high.
Discharge a 1 F intro-lab capacitor charged to 6V through a light bulb, again to
demonstrate stored energy.
Supplemental Tasks:

Go into dielectric materials and permittivity.

Build a functional capacitor: fill a film canister with water. Puncture the lid, insert a
nail. Wrap the canister with aluminum foil. The capacitor can be charged with an
electrophorus plate or a piezo-charger. It can be used to light an LED.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109