Download Static Electricity StudyGuide - Ms. Gamm

12/11
Static Electricity
Physics Fall Semester
Name:
Per.
 This exam will cover all of the topics described below. It is worth approximately 100 points.
Study your homework, reading logs, notes and labs.
 You will be allowed to use a scientific calculator and notecard. No sharing or phones.
 The exam will be on Tuesday, November 13th
A. Visualizing Charges: Conduction, Induction, Induced Polarization & Grounding
 Explain how charges attract and repel
 Describe the properties of an insulator and conductor
 Explain how an electroscope works
 Explain what happens in grounding
 Describe the process of conduction, induction, induced polarization & grounding
B. Coulomb’s Law
 Explain how to predict the change in electrostatic force when distance or charges change
 Be able to use Coulomb’s Law to calculate the electrostatic force given charges and distance
 Know the units of charge
C. Electric Field
 Describe the four rules for drawing electric fields
 Be able to draw electric field lines
 Identify where the electric field is strongest and weakest given a drawing of field lines
 Be able to predict how a charged object will move in an electric field
Vocabulary
 Electron
 Proton
 Neutral
 Induction
 Conduction





Induced Polarization
Grounding
Insulator
Conductor
Electric Field
Electrostatics: The study of stationary charges
 A neutral object has an equal number of positive and negative charges
 When two neutral objects are rubbed together, it is possible for one to physically remove electrons from the other – this
is charging by rubbing.
 If one of the neutral objects becomes negatively charged, the other must become positively charged by the same
amount.
 Charge by conduction is a contact way of charging an object. The charge is equally shared by the two objects.
 Charge by induction is a non-contact way of charging an object (it involves induced polarization and then grounding)
 Polarization is the special arrangement of positive charges on one side, and negative charges on the other. (separation
of charge).
 Grounding neutralizes objects. This occurs whenever electrons are given a path to the ground.
 Charges create an electric field in the space around them
 E-field lines start one positives and end on negatives.
 E-field lines never cross
 The number of lines is proportional to the amount of charge or the strength of the field
 The force which repels like charges and attracts opposite charges is an electric force.
 The closer two charges are together, the greater the force between them.
 The larger the magnitude of the charges the greater the force.


We can calculate the electric force between two charges using Coulomb’s Law: F  k
Q1Q2
d2
12/11
 F: electric force [Newtons /N]
 k: constant [ k  9 109 N  m 2 / C 2 ]
 Q: charge [Coulombs/C]
 d: distance between charges [meters/m]
Practice Problems
1. A negatively charged rod is brought near a pair of touching neutral metal spheres.
Neutral metal spheres
Negatively charged rod
a. Draw the charge distribution of the spheres before the rod is brought near.
b. Draw the charge distribution of the spheres after the rod is brought near.
c. If the metal spheres are separated when the rod is still near, what is the net charge of each
sphere?
d. Is the force between the two metal spheres attractive or repulsive?
2. A charge of 2.0  104 C and another charge of 8.0  10 4 C are separated by 0.3 m. What is the force
between the two charges?
3. Two identical charges (+q and +q) are separated by a distance d. The charges are doubled to +2q and +2q.
a. Is the electric force between the charges attractive or repulsive?
b. Does the electric force between the charges increase, decrease, or stay the same when the charge is
doubled? (Answer this without doing any calculations.)
c. By what factor does the electric force change?
4. Two identical charges (+q and +q) are separated by a distance d. The distance is tripled
a. Does the electric force between the charges increase, decrease, or stay the same when the distance is
tripled? (Answer this without doing any calculations.)
b. By what factor does the electric force change?
5. Two identical charges (+q and +q) are separated by a distance d. The distance is reduced to ½ of the
original. By what factor does the electric force change?
6. Two small spheres hung on threads attract one another as shown. It is known that a negativelycharged rod will attract ball A.
Which of the statements below are correct?
I.
II.
III.
Balls A & B have opposite charges.
Ball A has a positive charge.
Ball B has a negative charge.
Answers: 1c.) Left is + Right - ; 1d) Attractive; 2.) 16,000 N; 3a.) Repulsive 3b.) Increase 3c.) 4; 4b.) 1/9 ; 5.) 4; 6.)
1, II, and III