Electric Forces and
Fields
CHA PT E R 17
17-1 Electric Charge Essential Concepts:
• Understand the basic properties of electric charge.
• Differentiate between conductors and insulators.
• Distinguish between charging by contact, charging by induction, and
charging by polarization.
17-1 Electric Charge
• There are two kinds of charge – positive and negative
• Like charges repel, while opposite charges attract.
• In all circumstances, charge is conserved
Interaction between charges…
• Rub your hair with a balloon
and electrons get transferred.
• The balloon and your hair are
attracted to each other.
• Two balloons, in this context,
tend to repel each other.
Interaction between charges…
• Rub your hair with a balloon
and electrons get transferred.
• The balloon and your hair are
attracted to each other.
• Two balloons, in this context,
tend to repel each other.
Electric charge is quantized
Millikan found that charge is always a multiple
of a fundamental unit of charge, e.
Transfer of electric charge
• Material in which electric charges move easily are conductors.
• Materials in which electric charges do not move freely are insulators.
• Semiconductors have properties between the two.
Charging
• Insulators and conductors can be charged by contact.
• Conductors can be charged by induction.
• A surface charge can be induced on insulators by polarization.
Charging by induction
Ground object
Bring charge
near surface
Remove
ground
Charging by induction
Charging by induction
• A charged object induces a
charge on the surface of an
insulator.
• A charged comb induces a
charge on the surface of small
pieces of paper that have no
net charge.
Summary
• Charge is a fundamental property of matter.
• Charge is conserved.
• Similar charges repel, different charge attract.
• There are three ways to charge objects:
• By contact
• By induction
• By polarization
17-2 Electric Force Essential Concepts:
• 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 force on a charge is
zero.
Coulomb’s Law
Coulomb’s Law
Practice 17A Problems, Page 636
Number 1
Number 2
Electric force is a field force
Notice the similarity between the
mathematical form of Coulomb’s Law and
that of Newton’s Law of Universal
Gravitation.
Principle of Superposition
The resultant force on any single charge is
equals the vector sum of all the individual
forces exerted on that charge by all other
charges present.
Coulomb quantified
electric force
17-3 The Electric Field Essential Concepts
• Calculate electric field strength.
• Draw and interpret electric field lines.
• Identify the four properties associated with a conductor in
electrostatic equilibrium.
By Convention:
• A charged object sets up an
electric field in the space around
it.
• The direction of E is defined as the
direction of the electric force on a
small positive charge.
By Convention:
• A charged object sets up an
electric field in the space around
it.
• The direction of E is defined as the
direction of the electric force on a
small positive charge.
• We assume a SMALL test charge.
Electric field strength depends
on charge and distance.
A charge, q 1 = 5.00 μC, is at the origin, and a second charge, q 2
= -3.00 μC, is on the x-axis 0.800 m from the origin. Find the
electric field at a point on the y-axis 0.500 m from the origin.
Electric Field Lines – lines that represent both the
strength and direction of the electric field.
Electric Field Lines – lines that represent both the
strength and direction of the electric field.
• The number of field lines is proportional to the electric field
strength.
• In this case, only half the lines
originating from the positive
charge terminate on the negative
charge because the positive
charge is twice as great as the
negative charge.
Conductors in Electrostatic Equilibrium
• Electrostatic equilibrium implies no net movement of charge.
When a field becomes great enough, the air around
the sharp edges becomes ionized and a bluish glow
called a corona is seen.
A Van de Graaff Generator collects charge