1. Practice #6—Electric Charge, Fields, and Capacitors
Circuit Lab
Practice #6—Electric Charge, Fields, and Capacitors
Mr. Burleson

2. Agenda 15 minutes—Grading homework.
30 minutes—Learning Lesson of the Day
15 minutes—In Practice quick test on Lesson of the Day
25 minutes—Practical testing
5 minutes—Sending out homework

3. What is an electric charge?
Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field.
There are two types of electric charges; positive and negative (commonly carried by protons and electrons respectively). Like charges repel and unlike attract. An object with an absence of net charge is referred to as neutral
Charge is in unit Columbs (C) , it is always conserved
An electron has a charge of x C
Electric field lines are drawn as radiating from a positive (+) charge and towards a negative (-) charge. Electric fields can be drawn as below.

4. Coulomb’s Law
Coulomb’s Law determines the force of attraction of repulsion between two charges.
F is the force in Newtons
ke is Coulomb’s constant 8.99 x 109 N-m2/C2
q1 and q2 are the charges in Coulombs
r is the distance in Meters between the centers of charges
Groups of charges can be combined into an equivalent charge (i.e. like for an ion)

5. Capacitors (Division C Only)
A capacitor is a passive two-terminal electrical component that stores potential energy in an electric field
Most capacitors contain at least two electrical conductors often in the form of metallic plates or surfaces separated by a dielectric medium (glass, ceramic, plastic, paper, mica, etc.). A conductor may be a foil, thin film, sintered bead of metal, or an electrolyte.
Unit of capacitance is a Farad (F). Most capacitors have small values like μF, pF, etc.
Charge equals Capacitance times Potential
Q = C V

6. Capacitors (Division C Only)
A capacitor consists of two conductors separated by a non-conductive region where charge builds up on both sides
When charging the current flows freely at first so it appears to be a short (with zero voltage)
When fully charged the current is completely stopped so it appears to be an open (with maximum voltage).
A capacitance of one Farad (F) means that one Coulomb of charge on each conductor causes a voltage of one Volt (V) across the device
Q = C V

7. Capacitors (Parallel Plate Model)
The simplest model of a capacitor consists of two parallel plates with a thin dielectric in between.
The dielectric is much thinner than the dimensions of the plates
The dielectric permittivity (ε) of the material determines how much capacitance can be held
C = ε A / d
Energy stored in a capacitor is E = ½ CV2
Free space/vacuum has a permittivity of εo = x F/m, all other materials are higher

8. In Practice Quiz - - + - - - + - + - - - + - + - - - + - - - + - - - +
What are conducting materials? Two examples?
What are insulating materials? Two examples?
What are semi-conducting materials? Two examples?
Draw Electric Field lines for the following charges, show attraction or repulsion - - + - - - + - + - - - + - + - - - + - - - + - - - + -

9. In Practice Quiz V = 100 V R1 = 100Ω R2 = 220Ω R3 = 680Ω
What is the voltage drop across each resistor?

10. In Practice Quiz
If two electrons are 1 mm apart, answer the following:
Are they attracted or repulsed?
What is the force between them?
Draw an electric field diagram not to scale
If two protons are 1 mm apart, answer the following:
If an electron and a proton are 1mm apart, answer the following:

11. Homework
Know what common materials are used to charge materials (silk cloth, glass, etc.) and which provide positive and which provide negative charges?
Explain how a balloon that is rubbed on someone’s head sticks to the ceiling?
What are common ways to prevent static charge or discharge?
Draw the electric field lines for positive, negative, and mixed charges
Determine the force between a 10C and a -100C set of charges that are 1 m apart (repulsed or attracted?)