Objectives: The students will be able to:

1. Demonstrate that charged objects exert forces, both attractive and repulsive.

2. Explain that charging is the separation, not the creation, of electric charge.

3. Distinguish between an insulator, a conductor, and a semi conductor and give examples of each.

Induced Charge; the Electroscope

Metal objects can be charged by conduction:

Electric Charges and Forces

Explain why?

Induced Charge; the Electroscope

They can also be charged by induction:

Induction: The production of a charge in an uncharged body by bringing a charged object close to it

When negatively charged rod is put near a metal can...

electrons of the can are pushed away from the rod.

top of the can: positive

& attraction > repulsion+++ ++

metal can - -

---

----- --

buttom of the can: negative

induced charges

attraction

repulsion

Attraction of uncharged objects

Similarly, when charged rod is close to paper scrap...

----- --molecules of paper align.

attraction between the rod and + charge > repulsion between the rod and - charge.paper +–+–+–+–

+–+–+–+–

attraction

repulsion

Methods of Charging

Charging by Induction: bringing a charged object close to a neutral object will induce the same charge in the neutral object

-when moved away, the neutral object goes

back to being neutral

negative

Induction & Polarization

The displacement of charge in an isolated conductor when placed near by an electrically charged body

Separation of positive and negative charges

Polarization

Uncharged insulator the polarized insulator

Conservation & Quantization of Charge

Conservation: Total electric charges in any closed system is a constant. Charge can be transferred from on object to another.

Quantization: minimum amount of charge: e, the basic unit, the charge of the electron or proton. Any Q = integer x e

e = 1.602x10-19 C or 1 C ~ 6x1018 protons(+) or electrons(-)!

mC, C, nC…

Induced Charge; the Electroscope

Nonconductors won’t become charged by conduction or induction, but will experience charge separation:

Induced Charge; the Electroscope

The electroscope can be used for detecting charge:

Methods of Charging

Electroscope: a device used to detect and identify static electric charges

-metal conductors insulated by a rubber collar

Methods of Charging

Electroscope: a device used to detect and identify static electric charges

-metal conductors insulated by a rubber collar-charges spread throughout metal parts so both leaves have the same charge-like charges repel so the leaves move apart

Induced Charge; the ElectroscopeThe electroscope can be charged either by conduction or by induction.

When a + charged rod is put near neutral object, ______________ is induced on the side of the object near the rod and _____________ is induced on the side away from the rod. The rod can attract the netural object because _________ between rod and – induced charge > the ________ between rod and + induced charge.

How does a positively charged rod attract a neutral object?

negative chargepositive charge

attractionrepulsion

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

Triboelectric series => Triboelectric EffectWhen two of the following materials are rubbed together under ordinary circumstances, the top listed material becomes positively charged and the lower listed material becomes negatively charged.MORE POSITIVE rabbit's fur glass mica nylon wool cat's fur silk paper cotton wood acrylic cellophane tape polystyrene polyethylene rubber balloon saran

wrap MORE NEGATIVE

Induced Charge; the Electroscope

The charged electroscope can then be used to determine the sign of an unknown charge.

Summary: How Can You Charge Objects?

• There are 3 ways objects can be charged:1. Friction2. Conduction3. Induction

**In each of these, only the electrons move. The protons stay in the nucleus**

Friction• Charging by friction occurs when electrons are

“wiped” from one object onto another.

Ex. If you use a cloth to rub a plastic ruler, electrons move from the cloth to the ruler. The ruler gains electrons and the cloth loses electrons.

Conduction

• Charging by conduction happens when electrons move from one object to another through direct contact (touching).

Ex. Suppose you touch an uncharged piece of metal with a positively charged glass rod. Electrons from the metal will move to the glass rod. The metal loses electrons and becomes positively charged. 

Period 3 stopped here

Induction• Charging by induction happens when charges in

an uncharged object are rearranged without direct contact with a charged object. Ex.If you charge up a balloon through friction and

place the balloon near pieces of paper, the charges of the paper will be rearranged and the paper will be attracted to the balloon.

Conservation of Charge• When you charge something by any

method, no charges are created or destroyed.

• The numbers of electrons and protons stay the same. Electrons simply move from one atom to another, which makes areas that have different charges.

Objectives: The students will be able to:

Apply Coulomb's law to determine the magnitude of the electrical force between point charges separated by a distance r and state whether the force will be one of attraction or repulsion.

Two types of charge:

Positive Charge: A shortage of electrons.

Negative Charge: An excess of electrons.

Conservation of charge – The net charge of a closed system remains constant.

+n + +

+ +

+n

nn

n n

-

-

-

-

-

-

Neutral Atom

Number of electrons = Number of protons

Nucleus

Negative Atom

Number of electrons > Number of protons

-2e = -3.2 x 10-19C

-

-

Positive Atom

Number of electrons < Number of protons

+2e = +3.2 x 10-19C

Inquiry ActivityElectric Field Hockey phET

Electric Forces

Like Charges - Repel

Unlike Charges - Attract

- +F F

+ + FF

Coulomb’s Law

Experiment shows that the electric force between two charges is proportional to the product of the charges and inversely proportional to the distance between them.

Coulomb’s Law – Gives the electric force between two point charges.

221

rqqkF

k = Coulomb’s Constant = 8.988x109 Nm2/C2

q1 = charge on mass 1

q2 = charge on mass 2

r = the distance between the two charges

The electric force is much stronger than the gravitational force.

Inverse Square

Law

Coulomb’s Law

Coulomb’s law:

This equation gives the magnitude of the force.

Coulomb’s LawThe force is along the line connecting the

charges, and is attractive if the charges are opposite, and repulsive if they are the same.

Coulomb’s Law

Unit of charge: coulomb, C

The proportionality constant in Coulomb’s law is then:

Charges produced by rubbing are typically around a microcoulomb:

Coulomb's Law

• The force between two charges gets stronger as the charges move closer together.

• The force also gets stronger if the amount of charge becomes larger.

Coulomb's Law• The force between two

charges is directed along the line connecting their centers.

• Electric forces always occur in pairs according to Newton’s third law, like all forces.

Coulomb's Law

• The force between charges is directly proportional to the magnitude, or amount, of each charge.

• Doubling one charge doubles the force.

• Doubling both charges quadruples the force.

Coulomb's Law

• The force between charges is inversely proportional to the square of the distance between them.

• Doubling the distance reduces the force by a factor of 22 = (4), decreasing the force to one-fourth its original value (1/4).

• This relationship is called an inverse square law because force and distance follow an inverse square relationship.

Coulomb’s Law

Charge on the electron:

Electric charge is quantized in units of the electron charge.

Unit of charge is a Coulomb (C)

221

rqqkF

If r is doubled then F is :

If q1 is doubled then F is :

If q1 and q2 are doubled and r is halved then F is :

¼ of F

2F

16F

Two charges are separated by a distance r and have a force F on each other.

q1 q2

r

F F

Example 1

Example 2

Two 40 gram masses each with a charge of 3μC are placed 50cm apart. Compare the gravitational force between the two masses to the electric force between the two masses. (Ignore the force of the earth on the two masses)

3μC40g

50cm

3μC40g

221

rmmGFg

211

)5.0()04)(.04(.1067.6 N131027.4

221

rqqkFE

2

669

)5.0()103)(103(100.9

N324.0

The electric force is much greater than the gravitational force

Homework

Objectives: The students will be able to:

• Apply Coulomb's law to determine the magnitude of the electrical force between point charges separated by a distance r and state whether the force will be one of attraction or repulsion.

Summer July 06 PHYS632 E&M 44

Coulombs Law

In 1785 Charles Augustin Coulomb reported in the Royal Academy Memoires using a torsion balance two charged mulberry pithballs repelled each other with a force that is inversely proportional to the distance.where k=8.99*109 Nm2/C2 in SI unit

k ~ 1010 Nm2/C2

q2q1 r RepulsionRepulsion

RepulsionRepulsion

AttractionAttraction

++ ++

++

-- --

Lab ExperimentLab Experiment

Spheres sameSpheres same as pointsas points

----Point chargesPoint charges

Superposition of electric forces

Net force is the vector sum of forces from each charge

q1

q2

q3

q

F3

F2

F1

Net force on q: F = F1 + F2 + F3

F

Principle of SuperpositionThree charges In a line

• In the previous example we tacitly assumed that the forces between nuclei simply added and did not interfere with each other. That is the force between two nuclei in each penny is the same as if all the others were not there. This idea is correct and is referred to as the Principle of Superposition.

• Example of charges in a line

– Three charges lie on the x axis: q1=+25 nC at the origin, q2= -12 nC at x =2m, q3=+18 nC at x=3 m. What is the net force on q1? We simply add the two forces keeping track of their directions. Let a positive force be one in the + x direction.

1 2x

3

ExampleThree charges in a line. Three charged particles are arranged in a line, as shown below. Calculate the net electrostatic force on particle 3 (the -4.0μC on theright) due to the other two charges.

ExampleThree charges in a line. Three charged particles are arranged in a line, as shown below. Calculate the net electrostatic force on particle 3 (the -4.0μC on theright) due to the other two charges.

Solving Problems Involving Coulomb’s Law and Vectors

The net force on a charge is the vector sum of all the forces acting on it.

Homework

Closure

Kahoot

Elaboration

Assignments Determination of Charge P.552 9-13 Charging by Induction