Electrostatics

10.1 Properties of Electric Charges

Static electricity – not moving Two types of charge positive (+) when electrons are lost negative (-) when electrons are gained Objects can gain charges by rubbing

10.1 Properties of Electric Charges Like charges repel Unlike charges attract Law of Conservation of electric charge – the net amount of electric charge produced in a process is zero

10.1 Properties of Electric Charges Robert Millikan – charge is always a multiple of a fundamental unit Quantized – occurs in discrete bundles The discrete bundle is an electron The charge on a single electron is

10.2 Insulators and Conductors

Conductors – outer electrons of atoms are free to move through the material Insulator – electrons tightly held, do not move

10.2 Insulators and Conductors Semiconductors – conduct electricity under some circumstances, don’t under other conditions Charges can be transferred by contact Called Charging by Conduction

10.2 Insulators and Conductors Induction – charging without contact Object is brought near a charged object Electrons move Object is grounded An electroscope measures if an object has a charge on it

10.3 Coulomb’s Law

Electric charges apply forces to each other From experiments Force is proportional to charge Inversely proportional to square of distance

10.3 Coulomb’s Law Equation – gives magnitude of force Opposite charges – force directed toward each other Like charges – force directed away from each other Charge is measured in Coulombs

10.3 Coulomb’s Law 1 Coulomb is the amount of charge, that if placed 1 m apart would result in a force of 9x10 9 N Charges are quantized – that is they come in discrete values The constant k relates to the constant called the permittivity of free space

10.3 Coulomb’s Law These are forces, so be sure to use vector math, draw free body diagrams For multiple objects, require multiple free body diagram

10.4 The Electric Field

Electrical forces act over distances Field forces, like gravity Michael Faraday electric field – extends outward from every charge and permeates all of space The field is defined by the force it applies to a test charge placed in the field

10.4 The Electric Field The Electric field would then be Or q is the test charge We can also say that Remember that E is independent of the test charge. The electric field is also a vector (free body diagrams are probably a good idea)

10.5 Electric Field Lines

To visualize electric fields Draw electric field lines Direction of the lines is the direction of force on a positive test charge The density of the lines indicates relative strength of the field Note: the field density increase as you get closer

10.5 Electric Field Lines For multiple charges, keep in mind 1.Field lines indicate the direction of the field The actual field is tangent to the field lines 2.The magnitude of the field is relative to the field line density 3.Fields start at positive and end at negatives Field Lines

10.5 Electric Field Lines If the field is produced by two closely spaced parallel plates The field density is constant So the electric field is constant Electric Dipole – two point charges of equal magnitude but oppsite sign

10.7 Potential Difference and Electric Potential

Electricity can be viewed in terms of energy The electrostatic force is conservative because it depends on displacement Now We can calculate this value for a uniform electric field

10.7 Potential Difference and Electric Potential Positive test charge – increases when moved against the field Negative test charge – increases when moved with the field Electric Potential (Potential) – electric potential energy per unit charge

10.7 Potential Difference and Electric Potential Only difference in potential are meaningful Potential Difference (Electric Potential Difference) – is measureable Measured in volts (after Alessandro Volta)

10.7 Potential Difference and Electric Potential If we want a specific potential value at a point, we must pick a zero point. That point is usually either A. The ground B. At an infinite distance

10.8 Electric Potential & Potential Energy

Using calculus it can be shown that the electric potential a distance r from a single point charge q is Assuming that potential is zero at infinity Like Potential Difference, this value is a scalar So