Electric Forces and FieldsCHAPTER 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

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 quantifiedelectric 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 dependson charge and distance.

A charge, q1 = 5.00 μC, is at the origin, and a second charge, q2 = -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