ElectrostaticsElectrostaticsA PowerPoint Presentation byA PowerPoint Presentation by

Paul E. Tippens, Professor of Paul E. Tippens, Professor of PhysicsPhysics

Southern Polytechnic State Southern Polytechnic State UniversityUniversity© 2007

Objectives: After finishing Objectives: After finishing this unit, you should be this unit, you should be able to:able to:

• Explain and demonstrate the Explain and demonstrate the first law of electro- first law of electro- staticsstatics and discuss charging by and discuss charging by contact contact and by and by inductioninduction..

• Understand Understand Coulomb’s LawCoulomb’s Law and its symmetry with gravityand its symmetry with gravity

• Define the Define the electronelectron and and coulomb coulomb as units of electric as units of electric charge.charge.

Electric ChargeElectric ChargeWhen a rubber rod is rubbed against fur, electrons When a rubber rod is rubbed against fur, electrons are removed from the fur and deposited on the rod.are removed from the fur and deposited on the rod.

The rod is said to be The rod is said to be negatively chargednegatively charged because of an because of an excessexcess of electrons. The fur is said to be of electrons. The fur is said to be positively positively chargedcharged because of a because of a deficiencydeficiency of electrons. of electrons.

Electrons Electrons move from move from fur to the fur to the rubber rod.rubber rod.

positive

negative

+ + + +

--

--

Glass and SilkGlass and SilkWhen a glass rod is rubbed against silk, electrons are When a glass rod is rubbed against silk, electrons are removed from the glass and deposited on the silk.removed from the glass and deposited on the silk.

The glass is said to be The glass is said to be positivelypositively chargedcharged because of because of a a deficiencydeficiency of electrons. The silk is said to be of electrons. The silk is said to be negatively chargednegatively charged because of a because of a excessexcess of electrons. of electrons.

Electrons Electrons move from move from glass to the glass to the silk cloth.silk cloth.

positive

negative- - - -

+ +

+ +

silk

glass

The ElectroscopeThe Electroscope

Pith-ball Electroscope

Gold-leaf Electroscope

Laboratory devices used to study the existence of two kinds of electric charge.

Laboratory devices used to study the existence of two kinds of electric charge.

Two Negative Charges Two Negative Charges RepelRepel

1. Charge the rubber rod by rubbing against fur.1. Charge the rubber rod by rubbing against fur.

2. Transfer electrons from rod to each pith ball.2. Transfer electrons from rod to each pith ball.

The two negative charges repel each other.The two negative charges repel each other.

Two Positive Charges Two Positive Charges RepelRepel

1. Charge the glass rod by rubbing against silk.1. Charge the glass rod by rubbing against silk.

2. Touch balls with rod. Free electrons on the balls 2. Touch balls with rod. Free electrons on the balls move to fill vacancies on the cloth, leaving each of move to fill vacancies on the cloth, leaving each of the balls with a deficiency. (Positively charged.)the balls with a deficiency. (Positively charged.)

The two positive charges repel each other.The two positive charges repel each other.The two positive charges repel each other.The two positive charges repel each other.

The Two Types of ChargeThe Two Types of Charge

fur

Rubber

Attraction

Note that the negatively charged Note that the negatively charged (green)(green) ball is ball is attractedattracted to the positively charged to the positively charged (red)(red) ball. ball.

Opposite Charges Attract!Opposite Charges Attract!

silk

glass

The First Law of The First Law of ElectrostaticsElectrostatics

Like charges repel; unlike charges attract.Like charges repel; unlike charges attract.

NegNeg PosNegPosPos

Charging by ContactCharging by Contact1. Take an uncharged electroscope as shown below.1. Take an uncharged electroscope as shown below.

2. Bring a negatively charged rod into contact with knob.2. Bring a negatively charged rod into contact with knob.

3. Electrons move 3. Electrons move downdown on leaf and shaft, causing them on leaf and shaft, causing them to separate. When the rod is removed, the scope to separate. When the rod is removed, the scope remains remains negativelynegatively charged. charged.

------

--

-

------

--- - -

Charging Electroscope Charging Electroscope Positively by Contact with a Positively by Contact with a

Glass Rod:Glass Rod:

+++

+++ ++

+++

+++

++

+

+++

Repeat procedures by using a positively charged glass rod. Electrons move from the ball to fill deficiency on glass, leaving the scope with a net positive charge when glass is removed.

Charging Spheres by Charging Spheres by InductionInduction

--- - -

Uncharged Spheres Separation of Charge--- - -

Isolation of Spheres Charged by Induction

----

++++

----

++++ +

++ +

-

-- -

Induction

Electrons Electrons RepelledRepelled

Induction for a Single Induction for a Single SphereSphere

--- - -

Uncharged Sphere Separation of Charge

Electrons move to ground.

Charged by Induction

+

++ +

Induction

----

--- - -

++++

----

- - - -

----

++++

----

The Quantity of ChargeThe Quantity of ChargeThe The quantity of chargequantity of charge (q)(q) can be defined can be defined in terms of the number of electrons, but in terms of the number of electrons, but the the Coulomb (C)Coulomb (C) is a better unit for later is a better unit for later work. A work. A temporarytemporary definition might be as definition might be as given below:given below:

The Coulomb: 1 C = 6.25 x 1018 electronsThe Coulomb: 1 C = 6.25 x 1018 electrons

Which means that the charge on a single Which means that the charge on a single electron is:electron is:

1 electron: e- = -1.6 x 10-19 C1 electron: e- = -1.6 x 10-19 C

Coulomb’s LawCoulomb’s Law

The force of attraction or repulsion between two point charges is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them.

The force of attraction or repulsion between two point charges is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them.

F

r

FF

q

q q’

q’- +

- -

2

'qqF

r 2

'qqF

r

COULOMB’S LAWCOULOMB’S LAWThe force between two charged objects isThe force between two charged objects is

directly proportional to the product of theirdirectly proportional to the product of theircharges and inversely proportional to theircharges and inversely proportional to their

separation distance squared.separation distance squared.

link1, link2, link3, link4, link4

In equation form:In equation form:

FFF is the is the force of attractionforce of attraction, measured in , measured in NEWTONSNEWTONS,,between charges qbetween charges q11 and q and q22

k

kk is the is the Universal Electrostatic ConstantUniversal Electrostatic Constant, equal to , equal to

9.00 x 109.00 x 1099 N N mm22/coul/coul22

q1 q2

qq11 and and qq22 are the are the attracting chargesattracting charges, measured in, measured inCoulombsCoulombs

d2

dd is the is the distance between the chargesdistance between the charges, , and is measured in and is measured in METERSMETERS

Electric FieldsElectric FieldsAn electric field exists in a regionAn electric field exists in a regionif space if a charge placed in thatif space if a charge placed in that

region experiences an electric force.region experiences an electric force.

The magnitude of an electric field at any The magnitude of an electric field at any given point is defined to be the ratio ofgiven point is defined to be the ratio ofthe force on athe force on acharge at thatcharge at thatpoint to the point to the amount of charge.amount of charge.

E = F/QElectric field strength has unitsElectric field strength has units

of Newtons/Coulomb (N/C).of Newtons/Coulomb (N/C).

Properties of SpaceProperties of Space

E

Electric Field

++++ +

+++Q

.

r

An electric field is a property of space allowing prediction of the force on a charge at that point.

; F

E F qEq

; F

E F qEq

The field E exist independently of the charge q and is found from:

2 :

kQElectric Field E

r 2

: kQ

Electric Field Er

E is a Vector

The The directiondirection of the electric fieldof the electric field at any point is defined at any point is definedto be the same direction as the to be the same direction as the direction of force on a direction of force on a positive test chargepositive test charge placed in the region at that point. placed in the region at that point.

Field lines Field lines point away from positivepoint away from positive and and toward negative chargestoward negative charges..

CONCLUSION: CONCLUSION: Static ElectricityStatic Electricity