Unit 14: Electrostatics

Units of Chapter 16• Static Electricity; Electric Charge and Its Conservation• Electric Charge in the Atom• Insulators and Conductors• Induced Charge; the Electroscope• Coulomb’s Law• Solving Problems Involving Coulomb’s Law and Vectors• The Electric Field

Units of Chapter 16• Field Lines• Electric Fields and Conductors

Do Now1. How do protons and electrons differ in their

electrical charge?2. Is an electron in a hydrogen atom the same as

an electron in a uranium atom?

3. Which has more mass – a proton or an electron?

4. In a normal atom, how many electrons are there compared to protons?

Do Now1. How do protons and electrons differ in their

electrical charge?Same magnitude, but opposite charge.

2. Is an electron in a hydrogen atom the same as an electron in a uranium atom?Yes.

3. Which has more mass – a proton or an electron?Proton

4. In a normal atom, how many electrons are there compared to protons? Same number, no net charge.

Atomic Structure

16.1 Static Electricity; Electric Charge and Its Conservation

Objects can be charged by rubbing

Triboelectric Series• Friction can cause electrons to transfer

from one material to another• Different materials have a different degree

of attraction for electrons• The triboelectric series determines which

materials have a greater attraction.• When two materials are rubbed together,

the one with the higher attraction will end up getting some of the electrons from the other material

Triboelectric Series

Human Hands (if very dry) Leather Rabbit Fur Glass Human Hair Nylon Wool Fur Lead Silk Aluminum Paper Cotton Steel (neutral) Wood Amber

Hard Rubber Nickel, Copper Brass, Silver Gold, Platinum Polyester Styrene (Styrofoam) Saran Wrap Polyurethane Polyethylene (scotch tape) Polypropylene Vinyl (PVC) Silicon Teflon 

MORE POSITIVE            

MORE NEGATIVE

If two materials are rubbed together, the one that is higher in the series will give up electrons and become more positive.

Question

• If fur is rubbed on glass, will the glass become positively charged or negatively charged?

• Glass – positive• Fur -negative

16.1 Static Electricity; Electric Charge and Its

Conservation

Charge comes in two types, positive and negative; like charges repel and opposite charges attract

Interaction Between Charged and Neutral Objects

• Any charged object - whether positively charged or negatively charged - will have an attractive interaction with a neutral object.

16.1 Static Electricity; Electric Charge and Its Conservation

Electric charge is conserved – the arithmetic sum of the total charge cannot change in any interaction.

16.2 Electric Charge in the Atom

Atom: Nucleus (small, massive, positive charge)Electron cloud (large, very low density, negative charge)

16.2 Electric Charge in the Atom

Atom is electrically neutral.Rubbing charges objects by moving electrons from one to the other.

16.2 Electric Charge in the Atom

Polar molecule: neutral overall, but charge not evenly distributed

16.3 Insulators and Conductors

Conductor:Charge flows freelyMetals

Insulator:Almost no charge flowsMost other materials

Some materials are semiconductors.

How Charge Is Transferred

• Objects can be charged by rubbing• Metal objects can be charged by

conduction• Metal objects can be charged by• induction

16.4 Induced Charge; the Electroscope

Metal objects can be charged by conduction:

Charging by Induction• When an object gets charged by induction, a

charge is created by the influence of a charged object but not by contact with a charged object.  The word induction means to influence without contact. 

• If a rubber balloon is charged negatively (perhaps by rubbing it with animal fur) and brought near (without touching) the spheres, electrons within the two-sphere system will be induced to move away from the balloon.

Charging By InductionWith Negatively Charged Object

In step iii, why is the charge on the right sphere almost uniformly distributed?

Charging By InductionWith Negatively Charged Object

What was the source of positive charge that ended up on sphere B?

Source of charge in induction

• In induction, the source of charge that is on the final object is not the result of movement from the charged object to the neutral object.

Ground

• An infinite source or sink for charge

16.4 Induced Charge; the Electroscope

They can also be charged by induction:

16.4 Induced Charge; the Electroscope

The electroscope can be used for detecting charge:

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

16.4 Induced Charge; the Electroscope

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

16.4 Induced Charge; the Electroscope

Nonconductors won’t become charged by conduction or induction, but will experience charge rearrangement. The atoms or molecules become polarized.

:

16.4 Induced Charge; the Electroscope

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

Do Now

True or False? Explain your reasoning.

1. An object that is positively charged contains all protons and no electrons.

False Positively charged objects have electrons; they simply possess more protons than electrons.2. An object that is electrically neutral contains only neutrons.FalseElectrically neutral atoms simply possess the same number of electrons as protons.

Coulomb’s Law

• The French physicist Charles Coulomb (1736 – 1806) investigated electric forces in the 1780s using a torsion balance.

16.5 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 Law1. If two point charges and are a distance r apart, the charges exert forces on each object of magnitude:

These forces are an action/reaction pair, equal in magnitude but opposite in direction.

1Q 2Q

221

1221 rQQ

kFF onon

16.5 Coulomb’s Law2. The forces are along the line connecting the

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

16.5 Coulomb’s Law

Unit of charge: coulomb, CThe proportionality constant in Coulomb’s law is then:

Charges produced by rubbing are typically around a microcoulomb:

229 /100.9 CmNk When we only need two significant figures:

16.5 Coulomb’s Law

Charge on the electron:

Electric charge is quantized in units of the electron charge.

This is the smallest charge in nature – fundamental or elementary charge.

The net charge of any object must be a multiple of that charge.

16.5 Coulomb’s Law

The proportionality constant k can also be written in terms of , the permittivity of free space:

(16-2)

16.5 Coulomb’s LawCoulomb’s law strictly applies only to point charges.Superposition: for multiple point charges, the forces on each charge from every other charge can be calculated and then added as vectors.

16.6 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.

16.6 Solving Problems Involving Coulomb’s Law and VectorsVector addition review:

Example 1

Suppose that two point charges, each with a charge of +1.00 Coulomb are separated by a distance of 1.00 meter. Determine the magnitude of the electrical force of repulsion between them.Given: Find: F - ?IQI I= 1.00 C Q2 I= 1.00 Cr = 1.00 m

Solve:

221

rQQ

kF

NF 92

9

100.91

)1)(1)(100.9(

The force of repulsion of two +1.00 Coulomb charges held 1.00 meter apart is 9 billion Newton. This is an incredibly large force that compares in magnitude to the weight of more than 2000 jetliners.

Example 2

Determine the magnitude and direction of the electric force on the electron of a hydrogen atom exerted by the single proton that is the atom’s nucleus. Assume the average distance between the revolving electron an the protonGiven: Find: F-?

mr 101053.0

CQQ 1921 106.1

mr 101053.0

Solution:

Nm

CCCmNrQQ

kF 8210

1919229

221 102.8

)1053.0()106.1)(106.1)(/100.9(

Problem 1

A balloon with a charge of is held a distance of 0.10m from a second balloon having the same charge. Calculate the magnitude of the electrical force between the charges. Draw a diagram.

C5100.4

Nm

CCCmNrQQ

kF 14401014400)10.0(

)100.4)(100.4)(/100.9( 12

55229

221

Problem 2

• Calculate the electrical force exerted between a 22-gram balloon with a charge of -2.6μC and a wool sweater with a charge of +3.8μC; the separation distance is 75cm. (Note: ) CC 61011

NNm

CCCmNrQQ

kF 158.01008.158)75.0(

)106.2)(106.2)(/100.9( 32

66229

221