UCSD Physics 10 Spring 20081

UCSD Physics 10 The Electric Force Electric Charge Electric Fields Electron Beams

UCSD Physics 10 Spring Electric Charge Recall that fundamental particles carry something called electric chargeRecall that fundamental particles carry something called electric charge –protons have exactly one unit of positive charge –electrons have exactly one unit of negative charge Electromagnetic force is one of the basic interactions in natureElectromagnetic force is one of the basic interactions in nature –like charges experience repulsive force –opposite charges attracted to each other (like gravity) Electrical current is flow of charge (electrons)Electrical current is flow of charge (electrons)

UCSD Physics 10 Spring Charge Balance Neutral atoms are made of equal quantities of positive and negative chargesNeutral atoms are made of equal quantities of positive and negative charges –Neutral carbon has 6 protons, 6 electrons, (& neutrons) Electrons can be stripped off of atomsElectrons can be stripped off of atoms –Electrons occupy the vulnerable outskirts of atoms Usually charge flows in such a way as to maintain neutralityUsually charge flows in such a way as to maintain neutrality –Excess positive charge attracts excess negative charge –Your body has 5  positive charges and 5  negative charges, balanced within millions or billions

UCSD Physics 10 Spring Charge Separation Can separate charges by rubbing:Can separate charges by rubbing: –feet on carpet –atmosphere across ground –silk on glass –balloon on hair! Insulators keep charges where they are (no flow)Insulators keep charges where they are (no flow) Conductors distribute charge equally on surfaceConductors distribute charge equally on surface –charge is free to “move about the cabin” –why do the charges collect on the surface?

UCSD Physics 10 Spring Induced Charge Charge can also be coaxed to redistribute itself within an objectCharge can also be coaxed to redistribute itself within an object – – – – – – – – – – – – – – + – – – – – – Charged rod approaches sphere + charge attracted to – charge in rod – charge repelled by rod Result: Attraction!

UCSD Physics 10 Spring Static Electricity Rubbing action redistributes charge (unbalanced)Rubbing action redistributes charge (unbalanced) If enough charge builds up, we get dischargeIf enough charge builds up, we get discharge Air spark is actually due to “breakdown” of airAir spark is actually due to “breakdown” of air –neutral air molecules separate into ions (electrons are stripped away) –current can then flow through the “plasma-field” air –In essence, air becomes a “wire” for a short bit –this happens at 3 million volts per meter 1 cm spark then at 30,000 volts typical finger-spark may involve a few billion electrons hold onto key to reduce pain of spark

UCSD Physics 10 Spring Lightning Lightning is an unbelievably huge discharge Clouds get charged through air friction 1 kilometer strike means 3 billion volts! Main path forms temporary “wire” along which charge equalizes –often bounces a few times before equal Thunder is bang produced by the extreme pressure variations induced by the formation and collapse of the plasma conduit

UCSD Physics 10 Spring Lightning Rods Perform two functionsPerform two functions –provide safe conduit for lightning away from house –diffuse situation via “coronal discharge” Charges are attracted to tip of rod, and “electric field” is highly concentrated there. Charges “leak” away, diffusing charge in what is sometimes called “St. Elmo’s Fire”, or “coronal discharge”

UCSD Physics 10 ST. ELMO’S FIRE Spring

UCSD Physics 10 Static Electricity Lab Tear a piece of paper into small pieces, starting about 15 cm away bring an unrubbed ebonite (gray) rod close until almost touching. Record results. Repeat for iron filings and small stream of water flowing from the beaker (over the pan).Tear a piece of paper into small pieces, starting about 15 cm away bring an unrubbed ebonite (gray) rod close until almost touching. Record results. Repeat for iron filings and small stream of water flowing from the beaker (over the pan). Repeat the process with an ebonite (gray) rod rubbed with the fur material.Repeat the process with an ebonite (gray) rod rubbed with the fur material. Repeat the rubbed procedures with the cotton material and the nylon material.Repeat the rubbed procedures with the cotton material and the nylon material. Then repeat all procedures (rubbed and unrubbed) using the clear plastic rod, metal rod and the comb.Then repeat all procedures (rubbed and unrubbed) using the clear plastic rod, metal rod and the comb. Summarize and explain all results!Summarize and explain all results! Spring

UCSD Physics 10 Spring “Electrostatic” Force Two charges, Q 1 and Q 2, separated by distance r exert a force on each other: F = (k·Q 1 ·Q 2 ) / r 2Two charges, Q 1 and Q 2, separated by distance r exert a force on each other: F = (k·Q 1 ·Q 2 ) / r 2 k is a constant (9  10 9 ), Q is in Coulombs, r in metersk is a constant (9  10 9 ), Q is in Coulombs, r in meters –One unit of charge (proton) has Q = 1.6  Coulombs Looks a lot like Newton’s gravitation in formLooks a lot like Newton’s gravitation in form Electron and proton attract each other times stronger electrically than gravitationally!Electron and proton attract each other times stronger electrically than gravitationally! –Good thing charge is usually balanced!

UCSD Physics 10 Spring Coulomb Law Illustrated Like charges repelLike charges repel Unlike charges attractUnlike charges attract –+++ r If charges are of same magnitude (and same separation), all the forces will be the same magnitude, with different directions. ––

UCSD Physics 10 Spring Coulomb Force Law, Qualitatively Double one of the chargesDouble one of the charges –force doubles Change sign of one of the chargesChange sign of one of the charges –force changes direction Change sign of both chargesChange sign of both charges –force stays the same Double the distance between chargesDouble the distance between charges –force four times weaker Double both chargesDouble both charges –force four times stronger

UCSD Physics 10 Spring Electric Force a lot like Gravity Same 1/r 2 dependence; charge takes place of mass.Same 1/r 2 dependence; charge takes place of mass. Does this mean electricity is product of geometry, just like gravity (general relativity)?Does this mean electricity is product of geometry, just like gravity (general relativity)? no because gravity is always pulling down, but electrostatic force can act in any direction no because gravity is always pulling down, but electrostatic force can act in any direction

UCSD Physics 10 Spring Electric Field Can think of electric force as establishing “field” telling particles which way to move and how fastCan think of electric force as establishing “field” telling particles which way to move and how fast + Electric “field lines” tell a positive charge which way to move. For example, a positive charge itself has field lines pointing away from it, because this is how a positively-charged “test-particle” would respond if placed in the vicinity (repulsive force). + Run Away!

UCSD Physics 10 Electric Field Calculating Electric Field (unit of Newton/coulomb)Calculating Electric Field (unit of Newton/coulomb) Think of it as the ability to pull negative charge towards it.Think of it as the ability to pull negative charge towards it. If you know the force and the charge you are attracting, solve for it usingIf you know the force and the charge you are attracting, solve for it using E = F / qE = F / q Spring

UCSD Physics 10 Spring Example Electric Fields Around Charges

UCSD Physics 10 Spring Electric Fields in Circuits Point away from positive terminal, towards negativePoint away from positive terminal, towards negative Channeled by conductor (wire)Channeled by conductor (wire) Electrons flow opposite field lines (neg. charge)Electrons flow opposite field lines (neg. charge) E E E electrons & direction of motion E Electric field direction

UCSD Physics 10 Spring Electron Beams; Cathode Ray Tubes (CRTs) Televisions, Oscilloscopes, Monitors, etc. use an electron beam steered by electric fields to light up the (phosphorescent) screen at specified pointsTelevisions, Oscilloscopes, Monitors, etc. use an electron beam steered by electric fields to light up the (phosphorescent) screen at specified points E-field metal plates electron beam screen cathode emitter

UCSD Physics 10 Spring Magnet, Static and Electric Field QuizName: Physics- Kania 1. If you wrap a current around a vertical metal bar and the current is going around the bar to the right as you view it, what direction is the north pole of the magnetic field created? 2.Why can iron be made to behave as a magnet but wood cannot? 3.Explain how a compass works. 4.Describe charging by induction. 5.Describe how static electricity is created in clouds. 6.In the static lab, why was the metal bar not able to be induced into picking up paper, but the ebonite bar was? 7.What happens to the Electrostatic force between two charges if the distance between them is doubled? 8.What happens to the Electrostatic force between two charges if one of the charges is doubled and the distance between them is doubled? 9.Describe the main difference between electric field lines and magnetic field lines. 10.Describe St. Elmo’s Fire.

UCSD Physics 10 Spring Assignments Selected readings from Hewitt Chaps. 23, 24, 25, 26 (specific pages listed on assignments page)Selected readings from Hewitt Chaps. 23, 24, 25, 26 (specific pages listed on assignments page) HW 6 due 5/23: 22.E.1, 22.E.5, 22.E.11, 22.E.16, 22.E.20, 22.E.30, 22.E.33, 22.P.1, 23.E.3, 26.E.7, 26.E.9, 26.E.11HW 6 due 5/23: 22.E.1, 22.E.5, 22.E.11, 22.E.16, 22.E.20, 22.E.30, 22.E.33, 22.P.1, 23.E.3, 26.E.7, 26.E.9, 26.E.11