Name:
PHY 227 F16 Pre-REC 4
Section: ________________
Due: Week of October 3, 2016
ELECTROSTATIC POTENTIAL ENERGY AND POTENTIAL
A. Inventing a Charge Storage Index
You are designing a new generation of phone chargers, and you need some way of temporarily storing
charge as part of the circuit design. You try the four devices below, attaching a couple of different batteries
to each one (one at a time). For each device you measure the battery voltage and the amount of charge
stored. Your results are shown in the table below.
Device name
Battery Voltage
Charge Stored
Pack-a-Proton
3 Volts
1 micro-Coulomb
Pack-a-Proton
9 Volts
3 micro-Coulombs
Charge Master
25 Volts
10 micro-Coulombs
Charge Master
125 Volts
50 micro-Coulombs
Volt Miser
8 Volts
4 micro-Coulombs
Volt Miser
12 Volts
6 micro-Coulombs
Stor-a-Shock
60 Volts
27 micro-Coulombs
Stor-a-Shock
20 Volts
9 micro-Coulombs
Charge storage index
Create an index that allows you to compare the devices based on their charge storage ability. The charge
storage index should allow you to predict the amount of charge stored by the device for any battery voltage.
Follow these rules:
•
•
A single device gets a single index value.
A big index value should mean that the device is better at storing charge.
Enter your values for the charge storage index in the table.
Follow up questions:
1. How, if at all, does the value of the charge storage index depend on the voltage applied?
2. Suppose that a 15 V battery is connected to the Pack-a-Proton device. How much charge would be
stored?
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Name:
PHY 227 F16 Pre-REC 2
Section: ________________
Due: Week of Oct 3, 2016
-8
3. Another device, ACME, has a charge storage index of 2.5 x 10 (using the same units). Using everyday
language, describe the specific information that the number 2.5 x 10-8 tells about devices from ACME.
As you have probably guessed by now, the charge storage devices are nothing other than capacitors. A
capacitor (for an example, a parallel plate capacitor) is generally made up of two conductors separated by
a dielectric like vacuum or air.
4. Is the charge storage index the same as capacitance, or is it different?
5. What is the unit of capacitance?
6. What kind of energy is stored inside a capacitor: electrical, magnetic or gravitational?
7. If a capacitor has capacitance C and the two conductors of the capacitor are kept at a potential difference
V, how much energy is stored inside the capacitor?
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