This document discusses electric charge, forces, and fields. It begins by defining electric charge and distinguishing between conductors and insulators. It then discusses the properties of electric charge including that like charges repel and unlike charges attract. It introduces Coulomb's law for calculating electric force between charges. It also defines electric field as the field of force around a charged object. Key aspects of electric field include field lines and the four properties of electrostatic equilibrium for conductors. Examples are provided to demonstrate calculating electric force and field.
2. 17.1 Electric Charge
Objectives
• Understand the basic properties of
electric charge
• Differentiate between conductors and
insulators
• Distinguish between charging by
contact, charging by induction, and
charging by polarization
4. Electrostatics - Properties
• Two kinds of charge: positive
negative
• Electric charge is conserved
• Like charges repel, unlike charges attract
• Charges travel between unlike materials
5. Electrostatics – More Properties
• The fundamental unit of electric charge is
denoted by the small letter “e”
• An electron has a charge of –e
• A proton has a charge of +e
• Quantized: Electric charges are always a
multiple of e. (+/- 1e, +/- 2e, +/-3e etc.)
• Value of e = 1.602 x 10-19 Coulombs
and 1.0 C = 6.2x1018electrons
8. Electrostatics - Transfer
• CONDUCTORS – electric charge
moves freely
• INSULATORS – electric charge does
not move freely
• Semiconductors
• Superconductors
9. Electrostatics – Methods of Charging
• By contact
• Two objects touching each other
• Conductors or insulators
• By induction
• No contact required
• Grounding source needed
• Conductors only
• By polarization
• Realignment of charge on surface
• Contact or no contact
• Insulators only
14. Electroscope
• A device that can be used to detect and
demonstrate the presence of a static charge
15. Electric Force
Objectives
• Calculate electric force using Coulomb’s Law
• Compare electric force with gravitational force
• Apply the superposition principle to find the
resultant force on a charge and to find the
position at which the net charge is zero.
16. Coulomb’s Law: Electric Force
Felectric = kC|q1q2|
r2
kC = Coulomb constant = 8.99x109 Nm2
C2
q1 = charge of charge 1 (C)
q2 = charge of charge 2 (C)
r = distance between charge 1 & charge 2 (m)
18. • Felectric is a vector quantity
• Magnitude
• Direction
19.
20. Comparison of forces
Fg = G m1m2
r2
Gravitational
(Fg)
Electric
(Felectric)
Felectric = kC q1q2
r2
G = gravitational constant = 6.67x10-11 Nm2/kg2
kC = Coulomb constant = 8.99x109 Nm2/C2
21. Resultant Force
The sum of all individual vector forces
What is the resultant (net) force C will feel?
A is pushing C away
B is pulling C toward it
FAC = 0.067 N
FBC = 0.022 N
Fnet-C = FAC – FBC = 0.067 – 0.022 = 0.045 N
22. Sample Problem:
Suppose that two point charges, each with a charge
of +1.00 Coulomb are separated by a distance of 1.00 meter.
Determine the magnitude of the electrical force of repulsion
between them.
23. Seatwork:
Two balloons are charged with an identical quantity and type
of charge: -6.25 nC. They are held apart at a separation
distance of 61.7 cm. Determine the magnitude of the electrical
force of repulsion between them.
24. Quiz
1. What is the magnitude of the force a 1.5 x 106 C
charge exerts on a 3.2 x 104C charge located 1.5
m away?
2. Two equal charges of magnitude 1.1 x 107C
experience an electrostatic force of 4.2 x 104 N.
How far apart are the centers of the two charges?
25.
26. Electric Field
Objectives
• Calculate electric field strength
• Draw and interpret electric field lines
• Identify the four properties associated
with a conductor in electrostatic equilibrium
27. Electric Field
• Electric Field is the field that permeates the
space around a charged object and in which
another charged object experiences an electric
force
• Electric Field is a ratio of Force to Charge
E = Felectric / q0
• The direction of E is defined as the direction of
the electric force that would be exerted on a
small positive test charge (q0)
29. Electric Field
A vector quantity with…..
MAGNITUDE determined by: E = kCq N/C
r2
DIRECTION determined by: The direction of the force
that charge (q) would exert
on a small positive test
charge placed in it’s vicinity
30. Electric Field Lines
• Electric Field Lines are lines that
represent both the magnitude and the
direction of the electric field.
• The number of lines shown is proportional
to the electric field strength. As lines get
closer together, the field strength
increases.
31. Electric Field Lines – Rules for Drawing
• Lines begin on + charges (or at infinity)
and terminate on – charges (or at infinity)
• The number of lines drawn leaving a + or
terminating on a – is proportional to the
magnitude of the charge
• No two field lines can cross one another.
32. Electric Field Lines - Monopoles
The electric field from an
isolated positive charge
The electric field from an
isolated negative charge
36. Four Properties of Electrostatic Equilibrium
1. The electric field is zero inside a conductor
2. Any excess charge resides entirely on the
surface of the conductor
3. The electric field just outside a charged
conductor is perpendicular to the conductor’s
surface
4. On an irregularly shaped conductor, charge
tends to accumulate where the radius of
curvature is smallest.
41. Problem Solving: Electric Field
Example 1:
Deepika pulls her wool sweater over her head,
which charges her body as the sweater rubs against her
cotton shirt. a) What is the electric field at a location where
a 1.60x10-19 C-piece of lint experiences a force of 3.2x10-9
N as it floats near Deepika? b) What will happen if Deepika
now touches a conductor such as a door knob?
42. Problem Solving: Electric Field
SEATWORK:
Gordon, the night custodian, dusts off a classroom
globe with a feather duster, causing the globe to acquire a
charge of 8.0x10-9 C. What is the magnitude and direction
of the electric field at a point 0.40 m from the center of the
charged globe?
43. Problem Solving: Electric Field
Example 2:
A fly accumulates 3.0x10-10 C of positive charge as it flies
through the air. What is the magnitude and direction of the electric field
at a location 2.0 cm away from the fly?
44. QUIZ
1.Mr. Patel is photocopying lab sheets for his first period
class. A particle of toner carrying a charge of 4.0x10-9 C
in the copying machine experiences an electric field of
1.2x10-6 N/C as it’s pulled toward the paper. What is the
electric force acting on the toner particle?
2. As Courtney switches on the TV set to watch her favorite
cartoon, the electron beam in the TV tube is steered across
the screen by the field between two charged plates. If the
electron experiences a force of 3.0x10-6 N, how large is the
field between the deflection plates?
