## Chapter Objectives and Summaries

### CHAPTER 21 Electrostatics

LEARNING OBJECTIVES
Knowledge of subject matter
• Define and give examples of the process of electric charging by friction (Electrification).
• List the properties of electrostatic force and charge.
• Explain the theory of atomic structure as it applies to electrostatic charging.
• Describe the differences between conductors, insulators and semiconductor materials.
• Identify common instruments used to produce and detect electrically charged objects.
• Recognise the shape of various electric field patterns.
• State the mathematical definitions of Coulomb force, electric field, electrostatic potential and voltage.
• Recognise an application of electrostatic effects.

Scientific Processes

• Construct physical representations of various types of electric field patterns given the nature of charges involved.
• Use diagrams and written descriptions to describe the process of charging objects by conduction and by induction.
• Record observations of electrostatic interactions using detecting and generating instruments.
• Identify variables in the electric fields and forces surrounding point charges or charged parallel plates.
• Use mathematical and vectorial operations to quantify and analyse the nature of electric forces and fields.
• Determine the work involved in moving electric charge across potential differences within a given field.
• Use electrostatic units of measurement correctly in calculations, reports and tables.
• Relate modern ideas to the historical development of the nature of electricity.

Complex Reasoning Processes

• Solve problems involving combinations of electric force, electric field and electric potential quantities.
• Gain an understanding of the interactions possible between charged objects and develop the electric field concept as a means of describing the strength and direction of these interactions using mathematical models to analyse the interactions.
• Select relevant properties from electric charge and fields to predict the outcome of a given electrostatic interaction.
• Propose reasons for observable effects in our common experience which may be due to electrostatic effects.

### CHAPTER 21 SUMMARY

• All matter is made up of atoms which contain electrically charged protons (positive) and electrons (negative) as well as neutrons (neutral).
• Materials can become electrically charged by gaining or losing electrons, which may be caused by friction or rubbing processes (triboelectric generation) or by charge sharing (induction).
• Two states of charge exist for any material, positive if it has lost electrons and negative if it has gained electrons. Opposite charges attract and like charges repel.
• Charge is a quantity of electricity measured in coulombs. The charge on one electron is 1.6 x 10-19 C.
• The Earth itself acts as either an infinite sink or an infinite source of electric charge.
• Charging devices using frictional processes available in the laboratory are the Van de Graaf generator and the electrophorus.
• Insulators do not allow electricity to flow through them, whereas conductors do. Semiconductors are modern materials whose conductivity can be controlled.
• Charge will remain where placed onto an insulator material such as a perspex rod, but will distribute itself across the surface of a conductor material such as metallic solid. Charge accumulates along edges or points at a higher density than on smooth surfaces.
• An electroscope is a charge measuring device which can be used to detect the quantity and type of charge present on insulators or conductors.
• Coulomb's inverse square law describes the nature of the force between electrically charged objects. The force is proportional to the charges Q1, Q2 and inversely proportional to the square of the separation distance (d). The constant of proportionality (k) in Coulomb's law has a value of 9.00 x 109 N m2 C-2. Mathematically this is written as F=k.Q1.Q2/(r2)
• An electric field is a region in 3D space surrounding an electrically charged object. Electric field lines define the direction and strength of the electric field in terms of the force on a test positive charge.
• Electric field strength (E) is a vector quantity defined as the force per unit charge at a point in the electric field. Measured in newtons per coulomb (N C-1) or volts per metre (V m-1)
• The electric field surrounding point charges or systems of point charges is not uniform, whereas the electric field between a set of parallel metallic plates is uniform (Millikan's plates).
• The potential difference between any two points in an electric field is defined as the work done per unit charge as the charge is moved from one point to the other. Potential difference V = W/q and is measured in volts (V). One volt (V) equals one joule per coulomb (J C-1).
• An electrostatic gun is a device using an electric field to linearly accelerate electric charges to very high speeds.
• Many practical applications of electrostatic phenomena are found in nature or have been invented by human beings.