Transcript Electricity
Electricity & Magnetism Where science and magic meet What about it? • While the study of electricity is relatively new (less than 200 years), a great deal of investigation has been done recently • Due to innovations in the modern electrical world (1920 –) Wringers… Electricity! • is the flow of electrons, described as an electrical current. Charges • A charge is a point source of electrical force. • There are only two charges; Positive and Negative. Rule of Charge • The Rule of Charges is stated: “Opposites Attract, Likes Repel” Electricity • Electrons are negative, so the electrons move from a negative pole towards a positive pole. • Electricity can be described as the forces between charges. Electrical Field • Area over which an electrical charge can apply a force SQUIDS? SQUIDS? Charges in the world • Most common objects are neutral (having the same number of + and – charges) • Law of Conservation of Charge • An object which has more of one charge than another is said to be charged. Shocking! • The buildup of charges on an object is static electricity. • When the static charges do move, it is called an electrical discharge (the charges move along the path of least resistance). How to charge • Induction – Bringing a charged object near an uncharged object causes the like charges to run away and attracts the opposite charges – Give the like charges an “out”, leaving the opposites Inducing • A charged object is brought near a neutral object • The Rule of Charge causes like charges to “flee” from their equals • If the neutral object is “grounded” the like charges leave • The grounding path is severed after the like charges flee • The “neutral” object now has the opposite charge How to charge • Conduction – Touching a charged object to an uncharged object causes the charges on the charged object to spread through the two objects Conducting • A charged object is brought into contact with a neutral object • The Rule of Charge causes like charges to “flee” from their equals • The like charges spread equally over the two objects, charging the previously neutral object Shazam! • Lightning is a form of electrical discharge. • The static charges are formed when the particles inside the storm cloud rub against each other, separating charges. Lightning Lightning Get motivated! • The voltage difference between two points in a circuit causes the electrons to flow • The larger the electric potential difference, or voltage, the more “push” for the electrons to flow • Unit and formula symbol: V Get moving! • The electric current is the amount or number of electrons that flow through a point in a circuit • Measured in Amperes, or Amps, symbolized by an “A” • Symbol “I” in a formula – Originally called “Electrical Intensity” Comparing V and I • Voltage and Current can be changed in varying amounts depending on the load to be accomplished Get out of the way! • The electric resistance is the resistance to the flow of electricity due to the nature of the material – Conductors have lower resistance than insulators • Measured in Ohms,, symbolized by an omega “Ω” • Symbol “R” in a formula Get connected! • The wires or leads in a circuit have some resistance – Length • Shorter wires have less resistance than longer wires – Thickness • Thicker wires have less resistance than thinner wires – Temperature • Cooler wires have less resistance than hotter wires Ohm’s Law • Ohm’s law states that the current flowing through a wire is dependent on the ratio of voltage to resistance – How much “push” divided by how much resistance V I current = voltage R resistance EMF • Any source of voltage can be called an “Electromotive Force” • EMF • Portable sources are usually “cells” or “batteries” Know the difference! • A “cell” is a reactor designed to create electric potential difference due to a chemical reaction Know the difference! A “battery” is a series of cells that work together to produce higher voltages Wet or dry? • A “wet cell” battery uses a liquid electrolytic solution to conduct electricity and ions – Car, motorcycle, lawnmower So dry, it’ll blow away… • A “dry cell” battery uses an alkaline paste to conduct electricity and ions – Alkaline batteries, certain rechargeable batteries Electrons freed from the Zinc casing flow through the circuit and are absorbed by the Carbon. How to tell… • An electroscope is a device that can detect whether or not an object is charged • As charges spread over the leaves (sing. leaf), likes repel and the leaves separate Bringing a charged object near an electroscope causes like charges to repel, separating the leaves Circuits • A circuit describes the path that electrons flow through. • If there is a break in the circuit, the electric current will not flow. Circuits • An incomplete or broken circuit is said to be an “open circuit”. • A complete circuit is called a “closed circuit”. • Switches are devices designed to make a circuit open or close. Oops! • A short circuit occurs when the electrical current finds a “short cut” across the circuit. • When this happens, the electricity jumps across the path of least resistance, and the designated work, or load, is not accomplished. A complete circuit • A voltage source • Wires or leads • A load • Optionally, • Switches – Multiples… Circuit Symbols Wire EMF Fuse Speaker Switch Connection Transformer Capacitor Resistor Variable Resistor Ammeter Voltmeter Fuse 2 Ground Lamp Circuit Breaker Galvanometer All in a row… • A series circuit has all the parts of the circuit in one line • If any break opens the circuit NO CURRENT will flow • “If one goes out, they all go out.” Measuring electricity • Voltmeters must be linked across a part of a circuit to measure voltage • Ammeters must be linked in the circuit to measure current EMF Which way does the circuit operate? Resistor or LOAD Ammeter EMF Which way does the circuit operate? Voltmeter Resistor or LOAD Ammeter Series calcs… • The resistance of any resistors or loads in a series circuit simply add together… • Rtotal = R1 + R2 + R3 + R4… All in a row… • A parallel circuit has multiple paths between the EMF and the loads • Current will flow though any part that can get back to the EMF • How houses and such are wired • Otherwise, you would have to have something in and turned on in every outlet Parallel calcs… • The resistance of any resistors or loads in a parallel circuit actually reduce reisistance… • 1 1 1 1 1 • Rtotal = R1 + R2 + R3 + R4… • More parallel circuits reduce resistance, increase current… Safety First! • Because buildings and vehicles contain so many parallel connections, safety devices had to be designed to limit the flow of current Fuses • Fuses are the simplest of electrical safety devices. • A fuse consists of a thin strip of metal between connections • When too much current flows, the strip melts, opening the circuit • Once a fuse has “blown”, it can not be reset, and must be replaced Breaker, breaker… • A circuit breaker is a spring-loaded switch that will “trip” and open the circuit when a metallic strip inside gets hot from excess current • Circuit breakers are resettable after they are tripped WARNING • Do not continue resetting a circuit breaker – find out why it is tripping GFCI • A Ground-Fault Circuit Interruptor, or GFCI, is a circuit breaker built into an outlet in your home. • GFCIs are required in newer homes, usually in areas that experience excess moisture WARNING • Do not continue resetting a GFCI breaker– find out why it is tripping ELECTRICITY! • • • • • • • Charge Flow Rule of Charge Electric Field Static Electricity Electric Discharge Charging by: – Conduction – Induction • Lightning • Voltage • Current • • • • • • • • • • Resistance Ohm’s Law Cells Batteries Electroscope van de Graff Generator Circuit Series Circuit Parallel Circuit Safety Devices