Unit 9 Part 1: Electric Force & Coulomb’s Law

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• Vector quantityVector quantity– Magnitude (numerical quantity)– Direction

• Net ForceNet Force– unbalanced forces cause motion– balanced forces keep objects at rest

• Types of Forces Types of Forces – Contact forces like normal or tension– Field forces, like gravity (field = “acts

at a distance”)

All matter is made of atoms Atoms have 3 basic parts:

– positively charged protons (+)+)– negatively charged electron (-)(-)– neutral (no charge) neutrons (0)neutral (no charge) neutrons (0)

Electric charge is a fundamental property of matter!Electrons have the smallest mass, thus they are the only subatomic particle that can move from one atom/object to the next (if the right conditions are present)

• Caused by positive and negative charges

• Electric Force Electric Force is the attraction or repulsion between objects– can cause a change in motion

• Is a field force because it “acts at a distance”

FORCE DIAGRAMSFORCE DIAGRAMS

• Exist in the region surrounding charged particles or objects

• Represented by lines of force– Start at positive

charges– End on negative

charges• Strongest near the

object

Consider the electric field lines drawn at the right for a configuration of two charges. Several locations are labeled on the diagram. Rank these locations in order of the electric field strength - from smallest to largest.

• Quantifies the electric force between 2 charged objects that are at some distance apart.– a Coulomb (C) is the SI unit of charge

• 1 C = 6.24 x 1018 Newtons (N)• The charge on one electron or proton = (+/-)1.6 x 10-19 C

• Called the “elementary charge”• Symbolized by q0

• Formula:Formula:

– k is coulomb’s law constant==8.99x 108.99x 1099 N-m N-m22/C/C22

– Q1 and Q2 are the charges of each object– d is the distance between the objects

Determine the magnitude of electrical force of attraction between two balloons with

separate charges of +3.5 x 10-8 C and 2.9 x 10-8 C when separated a distance of 0.65 m.

A young woman (m=55 kg) accumulates a charge of +2x105 C while sliding out of the front seat of her car. Her boy friend (m=80 kg) has been standing in the wind and has

accumulated a charged of -8 x10-5 C. Calculate the magnitude of electrical force the woman exerts on her boyfriend when

they are 0.5 m apart.

F= kQ1Q2/d2

F= (8.99x109)(+2x10-5)(-8x10-5)/(0.5)2

F= 57.54 NF

How does the magnitude of the force the boyfriend exerts on the woman compare to

the magnitude of the force the woman exerts on her boyfriend.

Determine the magnitude of electrical force between two protons in nucleus of helium

atom when separate by 2.0x10-15 m..

F= kQ1Q2/d2F

F= kQ1Q2/d2

F= (8.99x109)(+1.6x10-19)(+1.6x10-19)/(2x10-15)2

F= 57.5 NF

A cell membrane has a positive charge of 1.6 x10-19 C on the outside wall and negative

charged of -1.6x10-19 C on the inside wall. Calculate the magnitude of electrons of

electrical force between these two ions if the membrane thickness is 8x10-10 m.

F= kQ1Q2/d2

F= (8.99x109)(1.6x10-19)(-1.6x10-19)/(8.0x10-10)2

F= 3.59x10-10 N

• Force between the objects is:– directly proportional to the

product of their charges (QxQ)

– inversely proportional to the distance squared between them

F ∽ C

• Follows the Inverse Follows the Inverse Square LawSquare Law

• If distance between 2 objects is doubled, what happens to the force between them?

• If distance between 2 objects is halved, what happens to the force between them?

F4 2 1

The magnitude of The magnitude of electrical force between electrical force between two particles is 10 N. two particles is 10 N.

• If charge of one object is halved, what happens to the force between the objects?

• If the charge of one object is doubled, what happens to the force?

F ∽ C

2F 2C so C F ∽ ∽

F C so C F 21

21 ∽∽

The magnitude of The magnitude of electrical force between electrical force between two particles is 10 N. two particles is 10 N.

• How are these Laws the same?How are these Laws the same?– Both concern field forces between objects– Both have a constant value– Both follow the inverse square law

• How are these Laws different?How are these Laws different?– Gravitational force can only be attractive– Electrical force can be attractive or

repulsive

Study of electric charge “at rest” within objectsStudy of electric charge “at rest” within objects

Positive electric charges

a. attract both positive and negative charges.b. repel both positive and negative charges.c. attract positive and repel negative charges.d. repel positive and attract negative charges.

A beam of electrons is directed into the electric field between two oppositely charged parallel plates, as shown in

the diagram below.

The electrostatic force exerted on the electrons by the electric field is directeda. into the screen.b. out of the screen.c. toward the bottom of the screen.d. toward the top of the screen.

A beam of electrons is directed into the electric field between two oppositely charged parallel plates, as shown in

the diagram below.

What is the direction of the electric field in the above situation?a. into the screen.b. out of the screen.c. toward the bottom of the screen.d. toward the top of the screen.

Different Question

The diagram below shows two identical metal spheres, A and B, separated by

distance d. Each sphere has mass m and possesses charge q. Which diagram best represents the electrostatic force Fe and

the gravitational force Fg acting on sphere B due to sphere A?

ElectrostaticsElectrostatics is defined as the study of electric charge at rest within objects. (static electricity)

– It is the accumulation of charge, followed by a rapid discharge

How do objects become charged?How do objects become charged?– Charges within objects can become

separated, due to the movement of electrons• Neutral atoms/objects have equal protons and

electrons• Charged atoms/objects have more or less electrons

compared to it’s number of protons

Gas Station Flash Fire

John Travoltage

Remember, the only subatomic particle that can move from one atom/object to another is the electronelectron.

– An object that:• Gaining electrons, will

result in a negative charge

• Loses electrons, will result in a positive charge

Neutral Object Charged Object

--88++88

--66+8+8

+2+2Net Charge:Net Charge: Net Charge:Net Charge:

Neutral sampleNeutral sample Charged sampleCharged sample

The most common way for a negative pith ball to

become neutral is for it to

a. gain some electrons.b. lose some electrons.c. gain some protons. d. lose all its electrons.

Conductors: metals, aqueous solutions of salts (i.e., ionic compounds dissolved in water), graphite, water, and the human body.

Insulators: plastics, styrofoam, paper, rubber, glass, and dry air. Electrons will move differently, depending on the kind of material they are moving within

• In conductors:In conductors:electrons are “free” & can move throughout the materialin good conductors, can move from atom to atom & object to object

• In Insulators:In Insulators:do not allow free movement of electrons throughout the material, or

from the object to other things

Semiconductors are found in microprocessor chips, transistors, and anything that's computerized or uses radio waves depends on semiconductors

In a silicon lattice, all silicon atoms bond perfectly to four neighbors, leaving no free electrons to conduct electric current.

A pure silicon crystal is nearly an insulator

Doping SiliconChanging the behavior of silicon mixing a small amount of an impurity into the silicon crystal.

N-type - phosphorus or arsenic is added to the silicon. Both have five outer electrons, so they're out of place when they get into the silicon lattice.

P-type - boron or gallium are added. Each have only three outer electrons. They form "holes" in the lattice where a silicon electron has nothing to bond to. The absence of an electron creates the effect of a positive charge, hence the name P-type. Holes can conduct current. A hole happily accepts an electron from a neighbor, moving the hole over a space.

• Within objects:

– If part of an object becomes positive (or negative), the other part of an object must become equally but oppositely charged

– Example: water Example: water moleculesmolecules

• Between interacting objects

– If an object becomes charged from interacting with another, the other object must become equally, but oppositely charged.

Static Electricity Balloon Phet

Balloon net Balloon net charge: – 10charge: – 10

Sweater net Sweater net charge: +10charge: +10 Net overall Net overall

charge: 0charge: 0

Net charge: 0Net charge: 0

#1: Friction#1: Friction– Electrons transferred between two

neutral objects by rubbing 2 them together

Charging by Friction

• Friction creates oppositely charged objects. The net charge of both together is zero.

Triboelectric SeriesTriboelectric SeriesDifferent materials have different affinities for electrons.

•Rubbing amber with silk causes electrons from the silk to move to the amber

•Rubbing glass with silk causes electrons to move from the glass to the silk

Human hands (when dry)LeatherRabbit furGlassHuman hairWoolSilkPaperAmberBrassSilverPolyesterSytrofoamScotch tapePVC

More likely to More likely to loselose electrons (+) electrons (+)

More likely to More likely to gaingain electrons (-) electrons (-)

Which has the greatest

affinity for electrons?

Teflon

When amber is rubbed on a piece

of silk, what happens?

a. Electrons are transferred from the amber to the silk.b. Electrons are transferred from the silk to the amber.c. Protons are transferred from the amber to the silk.d. Protons are transferred from the silk to the amber.

#2: Conduction#2: Conduction– Occurs when a charged object

transfers it’s electron to a neutral object, by coming into direct contact with it. (touching)

John Travoltage

pith ball simulation

Sharing Charge via Conduction

• A metal sphere (A) with a charge of +4 units is brought in contact with a metal sphere (B) with -2 units of charge, and the spheres are then separated. What is the charge on each sphere after separation?

-+-+-+

-+ -+- ++

#3: Polarization#3: Polarization– Surface charge (object

remains neutral, overall)

– Charges within object shift•in response to being

near a charged object•Only exists while near

the charged object

Lightning

Caused by the buildup of electrical charge in clouds (due to the friction caused by the movement of water droplets)

i. Bottom is usually negative, and the top positive

ii. This accumulation repels negative charges in the ground, leaving the surface of the ground positive

As the charge increases, a bolt of many billions of electrons are transferred at the same time to the Earth.

This is followed by a streamer of positively charged air particles that is repelled by the ground that travels back upwards

Lightning bolts ionize the atoms in the air producing great amounts of heat, causing compressional waves (sound) called thunder.

#4: Induction#4: Induction– Starts similar to polarization– Two neutral objects in contact (a)– A third, charged object brought near to polarize

them (b)– The now polarized objects are separated – And now the objects are themselves charged

Leyden JarLeyden JarStores electrical charge using an insulator between two conductors

Grounding

phet - travolta

Van de Graaff Van de Graaff GeneratorGenerator

Generates large amounts of static electricity, using a moving rubber belt (FRICTION!!!)

Van de Graaff Generator Van de Graaff Generator and Pie Pansand Pie Pans

How a Van de Graaff Generator Works:

Van de Graaff explanation

Van de Graaff videolosing charge

Charge…Charge…

Discharge!!!Discharge!!!

electroscope simulations

Electroscope

The Electroscope

made of two thin metal leaves attached to a metal rod with a knob at the top.

- when the device is not charged, the leaves hang straight down.

- when an electric charge is present, the leaves repel each other, spreading apart.

Closure on Static ElectricityWhat are the charged particles in an atom?What is the rule of electric charges?What causes an electric charge?What is static electricity?What is static discharge?What would happen if a lightning rod were made of an insulator rather than a conductor?

Unit 9 Part 1: Electric Force & Coulomb’s Law

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