Electrostatics Electrical fields Electrical energy

Electrical forces and Charge Arise from the particles in atoms Protons + Electrons – Much stronger than gravitational force! Over a billion times stronger! Atoms usually have the same # of electrons and protons so they are electrically neutral An atom that looses or gains an electron is called ? Ion Charges: like charges repel; unlike charges attract Attraction and Repelling properties

Charges attracting and repelling

Conservation of Charge Electrons are neither created nor destroyed but are simply transferred from one material to another. Charge is conserved. Examples: Rubber rod is rubbed by a piece of fur, electrons transfer from the fur to the rubber rod Rub a glass or plastic rod with silk, you’ll find the rod becomes positively charged (loses electrons) Remember orbitals in chemistry? Which electrons are transferred? The charge of an object is always a whole number multiple of the charge of an electron Electrons cannot be divided into fractions of electrons

Coulomb’s Law For charged particles or objects that are small compared with the distance between them, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them. F=k q 1 q 2 d 2 9 x 10 9 N·m 2 /C 2 Quantity of charge of 1 st and 2 nd object Distance between them

Compare Gravity and Coulomb’s Law Only attractive G=small magnitude Both are inverse- square laws Attractive or repulsive K=large magnitude Both are inverse- square laws If a pair of charges of 1C each were 1 m apart, the force of repulsion between the two charges would be 9 billion Newtons (10 times the weight of a battleship!

Creating a charge: By friction Drag your shoes across a carpeted floor and touch a doorknob By touch By induction Using a ground; infinite supply of electrons Charge polarization Creating a charge on an insulator

Electric Field Lines aka lines of force Since electric fields have magnitude and direction, it is a ___________ quantity. Lines drawn to represent the electric fields are always drawn pointing away from the positive charge

Electrical Potential Energy Recall gravitational potential energy A charged object can have potential energy by virtue of its location in an electric field. Electrical pe if a charged particle is increased when work is done to push it against the electric field of something else that is charged. Work done is equal to energy gained by the charge.

Electric Potential Explain why we refer to electrical potential per charge You may have multiple charges in one location. Simply discussing the electrical energy would be misleading without talking about how many charges you had

Electrical potential energy in a uniform electric field A is moving to B in a uniform electric field The change in PE depends upon the charge, the strength of the electric field and the displacement problems PE el if – PE el if+ ΔPE

Why is the electrical potential energy not changed if A moves perpendicular to the uniform field lines? That would be the same as moving an object horizontally and saying it has the same gravitational PE

Electrical potential energy with a pair of charges Key points in the formula: Reference point for electrical potential energy starts at infinity (because PE goes to zero as the distance between the charges goes to infinity) PE is + when you have like charges bec/ like charges repel, + work must be done to bring them together PE is – when you have unlike charges

Electricity in Circuits

CapacitoCapacitor Passive Electronic device to store energy in the form of an electrostatic field Consists of 2 conducting plates separated by a dielectric (insulator)

Voltage Since electric potential is measured in volts, it is commonly called ___________. Electric potential = electrical potential energy charge 1 volt = joule coulomb 1-7

Potential difference=Voltage Cell Covert chemical energy to electrical energy AA, AAA, D batteries have the same voltage + end Max Volts -end Zero Volts

A voltage sets charges in motion “voltage supplies electrical pressure” Moving electrons (negative to positive) Current Rate the charges move through a conductor Unit = amp In metals, moving electrons make up the current In gases or liquids, both + and – charges are in motion WHY? Battery = direct current

Current The flow of electric charge

Ohm’s Law Voltage - V Volt Battery Potential difference Current - I Amps Rate of flow Resistance - R Ohms As resistance, current I=V/R

Resistance The resistance of the filament determines the brightness of the bulb

Moving electrons Random movement if circuit is off Marching analogy Where do the electrons come from in a circuit? Free electrons in the wires Why do the wires become hot when a current is flowing through them? Transfer of kinetic energy due to electron speed

Electric circuit An electrical device connected so that it provides one or more complete paths for the movement of charges Batteries add the energy for the electrons to move Electrons move – to + Conventional Current moves + to – Explain an open and closed circuit.

Circuit Breakers and fuses Safety device Respond to an overloaded circuit by opening the circuit Acts like a switch Reset Fuses: Must replace

Electric power Current x voltage Unit Watts Kilowatt-hour Energy delivered in 1 hour at the rate of 1 kW

Series Circuit Parallel Circuit One path Resistors share voltage Current the same throughout circuit More than one path Each path receives maximum voltage Current could be different in different branches

Kirchoff’s Current Law All current flowing into a branch point must flow out

Energy Transformations Motor Electrical Energy Mechanical Energy Generator Mechanical Energy Electrical Energy Hydroelectric Power Plant Mechanical Energy (Water flowing and turning the turbines) Electrical Energy

How does electricity get to your house? THE SPINNING SHAFT turns magnets inside a stationary ring of copper, moving electrons to produce electricity STEP-UP TRANSFORMERS increase the voltage of electricity produced by the generator. TRANSMISSON LINES carry electricity to substations in our communities. The voltage is decreased and the power is distributed to homes and businesses

How does electricity get to your house? THE SPINNING SHAFT turns magnets inside a stationary ring of copper, moving electrons to produce electricity STEP-UP TRANSFORMERS increase the voltage of electricity produced by the generator. TRANSMISSON LINES carry electricity to substations in our communities. The voltage is decreased and the power is distributed to homes and businesses

Calculate resistance using the graph.