UCSD Physics 10

Spring 2008 1

UCSD Physics 10

The Electric ForceThe Electric Force

Electric ChargeElectric Charge

Electric FieldsElectric Fields

Electron BeamsElectron Beams

UCSD Physics 10

Spring 2008 3

Electric ChargeElectric Charge

• Recall that fundamental particles carry something Recall that fundamental particles carry something called electric chargecalled 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 Electromagnetic force is one of the basic interactions in natureinteractions 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 2008 4

Charge BalanceCharge Balance

• Neutral atoms are made of equal quantities of Neutral atoms are made of equal quantities of positive and negative chargespositive 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 Usually charge flows in such a way as to maintain neutralityneutrality– Excess positive charge attracts excess negative charge

– Your body has 51028 positive charges and 51028 negative charges, balanced within millions or billions

UCSD Physics 10

Spring 2008 5

Charge SeparationCharge 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 2008 6

Induced ChargeInduced Charge

• Charge can also be coaxed to redistribute itself Charge can also be coaxed to redistribute itself within an objectwithin an object

– –

––

––

––

––

–

–

– –

+–

––

––

–

+++

++

Charged rod approaches sphere

+ charge attracted to – charge in rod

– charge repelled by rod

Result: Attraction!

UCSD Physics 10

Spring 2008 7

Static ElectricityStatic Electricity

• Rubbing action redistributes charge (unbalanced)Rubbing action redistributes charge (unbalanced)• If enough charge builds up, we get If enough charge builds up, we get dischargedischarge• 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 2008 8

LightningLightning

• Lightning is an unbelievably huge Lightning is an unbelievably huge dischargedischarge

• Clouds get charged through air frictionClouds get charged through air friction• 1 kilometer strike means 3 billion volts!1 kilometer strike means 3 billion volts!• Main path forms temporary “wire” Main path forms temporary “wire”

along which charge equalizesalong which charge equalizes– often bounces a few times before equal

• Thunder is bang produced by the Thunder is bang produced by the extreme pressure variations induced by extreme pressure variations induced by the formation and collapse of the the formation and collapse of the plasma conduitplasma conduit

• www.stormchasing.nl/lightning.html

UCSD Physics 10

Spring 2008 9

Lightning RodsLightning 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 concentratedthere.

Charges “leak” away, diffusing chargein what is sometimes called “St. Elmo’sFire”, or “coronal discharge”

UCSD Physics 10

ST. ELMO’S FIREST. ELMO’S FIRE

Spring 2008 10

UCSD Physics 10Static Electricity LabStatic Electricity Lab• Tear a piece of paper into small pieces, starting about 15 Tear a piece of paper into small pieces, starting about 15

cm away bring an unrubbed ebonite (gray) rod close until cm away bring an unrubbed ebonite (gray) rod close until almost touching. almost touching. Record resultsRecord results. Repeat for iron filings . Repeat for iron filings and small stream of water flowing from the beaker (over and small stream of water flowing from the beaker (over the pan).the pan).

• Repeat the process with an ebonite (gray) rod rubbed with Repeat the process with an ebonite (gray) rod rubbed with the fur material.the fur material.

• Repeat the rubbed procedures with the cotton material and Repeat the rubbed procedures with the cotton material and the nylon material.the nylon material.

• Then repeat all procedures (rubbed and unrubbed) using Then repeat all procedures (rubbed and unrubbed) using the clear plastic rod, metal rod and the comb.the clear plastic rod, metal rod and the comb.

• Summarize and explain all results!Summarize and explain all results!

Spring 2008 11

UCSD Physics 10

Spring 2008 12

““Electrostatic” ForceElectrostatic” Force

• Two charges, Two charges, QQ11 and and QQ22, separated by distance , separated by distance rr exert exert

a force on each other:a force on each other:

FF = ( = (kk··QQ11··QQ22) / ) / rr22

• kk is a constant (9 is a constant (9101099), ), QQ is in Coulombs, is in Coulombs, rr in meters in meters– One unit of charge (proton) has Q = 1.610-19 Coulombs

• Looks a lot like Newton’s gravitation in formLooks a lot like Newton’s gravitation in form• Electron and proton attract each other 10Electron and proton attract each other 104040 times times

stronger electrically than gravitationally!stronger electrically than gravitationally!– Good thing charge is usually balanced!

UCSD Physics 10

Spring 2008 13

Coulomb Law IllustratedCoulomb Law Illustrated

• Like charges Like charges repelrepel• Unlike charges Unlike charges attractattract

– +

+ +r

If charges are of same magnitude (and same separation),all the forces will be the same magnitude, with differentdirections.

––

UCSD Physics 10

Spring 2008 14

Coulomb Force Law, QualitativelyCoulomb 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 Change sign of bothboth charges 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 2008 15

Electric Force a lot like GravityElectric Force a lot like Gravity

• Same 1/Same 1/rr22 dependence; charge takes place of mass. dependence; charge takes place of mass.• Does this mean electricity is product of geometry, Does this mean electricity is product of geometry,

just like gravity (general relativity)?just like gravity (general relativity)?• no because gravity is always pulling down, but no because gravity is always pulling down, but

electrostatic force can act in any directionelectrostatic force can act in any direction

UCSD Physics 10

Spring 2008 16

Electric FieldElectric Field• Can think of electric force as establishing “field” Can think of electric force as establishing “field”

telling particles which way to move and how fasttelling particles which way to move and how fast

+

Electric “field lines” tell a positivecharge which way to move.

For example, a positive charge itselfhas field lines pointing away from it,because this is how a positively-charged“test-particle” would respond if placedin the vicinity (repulsive force).

+

Run Away!

UCSD Physics 10

Electric FieldElectric Field

• Calculating Electric Field (unit of Calculating Electric Field (unit of Newton/coulomb)Newton/coulomb)

• Think of it as the ability to pull negative charge Think of it as the ability to pull negative charge towards it.towards it.

• If you know the force and the charge you are If you know the force and the charge you are attracting, solve for it usingattracting, solve for it using

• E = F / q E = F / q

Spring 2008 17

UCSD Physics 10

Spring 2008 18

Example Electric Fields Around ChargesExample Electric Fields Around Charges

UCSD Physics 10

Spring 2008 19

Electric Fields in CircuitsElectric 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 2008 20

Electron Beams; Cathode Ray Tubes (CRTs)Electron Beams; Cathode Ray Tubes (CRTs)

• Televisions, Oscilloscopes, Monitors, etc. use an Televisions, Oscilloscopes, Monitors, etc. use an electron beam steered by electric fields to light up electron beam steered by electric fields to light up the (phosphorescent) screen at specified pointsthe (phosphorescent) screen at specified points

E-field

metal plates

- - - - - - -

+ + + + + + +

electron beam

screen

cathode emitter

UCSD Physics 10

Spring 2008 21

Magnet, Static and Electric Field Quiz Name: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 2008 22

AssignmentsAssignments

• Selected readings from Hewitt Chaps. 23, 24, 25, Selected readings from Hewitt Chaps. 23, 24, 25, 26 (specific pages listed on assignments page)26 (specific pages listed on assignments page)

• HW 6 due 5/23: 22.E.1, 22.E.5, 22.E.11, 22.E.16, 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, 22.E.20, 22.E.30, 22.E.33, 22.P.1, 23.E.3, 26.E.7, 26.E.9, 26.E.1126.E.9, 26.E.11