- Slides: 53
Electric Charge § Atoms are made up of 3 particles – – – Neutrons have a neutral charge. Protons have a positive charge. Electrons have a negative charge. § An atom is electrically neutral if the number of electrons EQUALS the number of protons § However, atoms sometimes LOSE or GAIN electrons. § When excess (or extra) electrons are stored, an overall NEGATIVE (-) charge is achieved
Charges § Handy rule : Opposites attract And likes repel
Electric fields § Every electric charge is surrounded by an electric field which can exert a force on objects which come into it. These fields can push or pull over distances
2 types of electricity § Static electricity §Electric current
§ Static electricity is an accumulation of electric charges on an object. Neutral Charge Negative Charge transferred -
Charging objects § Objects can become charged by a variety of methods § 2 ways are – Charging by CONTACT – Charging by INDUCTION
Charging by Contact § Objects touch or rub against each other (contact) and charge is transferred
Charging by Induction § Occurs when a charged object is brought near a neutral object. The charges in the neutral object rearrange themselves and present the illusion of a charge.
§ Once objects are charged, they can then transfer their charge…sometimes, suddenly, in what is called a STATIC DISCHARGE.
“Balloons on the Wall” clip
Conductors and Insulators Conductors = materials which allow excess electrons to easily move through them. Examples : metals, water The Earth also functions as a conductor.
Conductors and Insulators Insulator = a material that doesn’t allow electrons to move through it easily Examples : plastic around wires, wood doors, fiberglass, rubbers Notice what the handles are made of. Why?
“Conductors and insulators” clip
§ A device made up of two thin metal leaves attached to a knobbed metal rod. The leaves can move and hang straight down when not charged. This device is used to detect electric charges. Electroscope
Lightning Large static discharge Air masses move and swirl Top + Bottom - Negative charge induces a + charge on ground (repel e-)
When electrons get close to ground Bottom accumulates enough Electrons in cloud attracted to ground Positive charges are attracted Connection made = lightning
Electric charges move through air collide with atoms/molecules Collisions cause atoms/molecules to emit light
Electrons are ripped off atoms in the air Produce great amounts of heat Powerful sound waves
Warms surrounding air to 25, 000 C (2. 5 x hotter than Sun surface)
Heat causes air in path to expand rapidly sound waves = Thunder
Earth is a large neutral object Connection (conductor) will transfer any excess electric charge “Grounding” – the act of making a connection for an electric charge to go to the Earth (Lightning rods)
“Van de Graff” clip with safety from static discharge
Video quiz static electricity
Resistance § RESISTANCE is the tendency for a material to oppose the flow of electrons. § Causes the ELECTRICAL energy to change into THERMAL energy and LIGHT. § Filament in light-bulb has high resistance so it heats up and glows
§ The amount of resistance is different for each type of conductor. § Resistance is measured in ohms. Its symbol is Ω. This is the Greek letter omega
Resistance in wires § Resistance is caused by electrons running into each other or the walls of the wire. § Resistance is caused by friction which means heat will be present. Longer, thin wires have MORE resistance than short, thick wires
§ The resistance of a material will increase (↑) as the temperature increases (↑) in most conductors. § This is why electrical equipment does not work as well if it has been on for an extended time.
Electric Current § Similar to HEAT which flows from areas of high temps to areas of low temps, ELECTRONS flow from places of high potential energy to places of lower PE. § A negatively charged object has more PE than an uncharged one § When electrons do get to move (like in a spark of static discharge) the potentials become equal and the flow stops.
Comparison of GPE to potential difference Potential Energy (height) 1. 5 V Potential Difference 1. 5 Meters 0. 0 V To Lower Energy The potential difference (voltage) is 1. 5 V.
Electrical Energy Moving electrons in a path is electricity § Electrical Potential Difference (v) is measured in Volts § The rate of moving electric charges, Electric Current (I), is measured in Amperes § Resistance or opposition to the movement of the energy is called Resistance (R). § I=V R
Circuits – 2 types § Series circuits are the most simple. § One COMPLETE (1) path for the current to travel. § Contains an energy source, a path, and a load (something for it to do, like a lamp)
Circuits – 2 types § Parallel circuits provide more than one path for the current to travel. § Most circuits are parallel, since if one lamp goes out, the others can stay lit.
Which switches, if opened, will cause the light bulb to stop glowing? F. Q G. R It is the only H. S switch in J. T series to both the battery and light.
USE THE FORMULA SHEET!! § What is the current in a copper wire that has a resistance of 2 ohms and is connected to a 9 volt electrical source? A. B. C. D. 0. 22 amp 4. 5 amps 11. 0 amps 18. 0 amps V = I R so, 9 V = I x 2 ohms or 4. 5 amps
Historical perspective on Electricity § 600 B. C. Thales of Miletus writes that the Greeks had discovered that rubbing fur on certain substances such as amber would cause an attraction between the two. The amber would attract light things such as hair and if rubbed long enough produced a spark.
250 B. C. § Baghdad Battery – Object found in Iraq in 1938 (and dated back to 250 BC) resembles a galvanic cell and is thought to have been used for electroplating – Electroplating is the use of electricity to cover something with certain metals
1550 A. D. § Italian physician Girolamo Cardano in his De Subtilitate distinguishes difference between electrical and magnetic forces
1600 A. D. § English scientist William Gilbert writes De Magnate coined the Latin word electricus from the Greek word ηλεκτρον (elektron), § Ηλεκτρον means “amber” in Greek § English words electric and electricity came from
§ 1660 A. D. Otto von Guericke invents an electrostatic generator § 1675 A. D. Robert Boyle determines that electric forces can attract or repel through a vacuum
§ 1729 A. D. Stephen Gray classifies materials as either conductors or insulators § 1729 A. D. C. F. Du Fay identifies positive and negative charges
1752 A. D. § Benjamin Franklin legend says that he conducted famous, yet extremely dangerous experiment of flying kite with metal key attached during lightning storm linking lightning with electricity. He saw that the fibers on the string were standing up and also received a shock from the key when he touched it.
§ Franklin invented the lightning rod to protect structures by directing the charge to the ground. • Georg Wilhelm Richmann of St. Petersburg, Russia died performing kite experiment in 1753
Nikola Tesla Michael Faraday Thomas Edison