Transcript Chapter 32 * electrostatics

Chapter 32 –
Electric Forces
32.1 – Electric Forces & Charges
 Electrostatics – nonmoving
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electric charges
All atoms composed of protons –
positively charged, electrons –
negatively charged & neutrons –
no charge
Electrons (e) held on atom
because of attraction to positive
protons in nucleus
mproton~ 2000melectron
Neutral atom  # protons = #
electrons
The Simple Model of the Atom
 Electricity has attraction & repulsion (forces)
 Like charges  repulsive, unlike  attractive
 Pith balls and the electroscope are often used to
demonstrate this
32.2 – Conservation of Charge
 Atoms (& objects) become charged
when they gain or lose electrons
 Gain e  negative charge (ion)
 Lose e  positive charge (ion)
 Objects become charged because of
exchanges of e, gain of e means loss
somewhere else – conservation of
charge
 Therefore, objects become charged as
multiples of e (fundamental electric
charge)
 The outer e (valence) in atoms are
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not “held” very tight
Different materials can hold onto e
stronger than others (a larger
affinity, ie: rubber, plastic)
They can strip away e of another
material (fur, glass) – giving them
extra e & negative charge
The other material has absence of
e, & is positive
Protons do not move to charge
objects positively, they are “locked
inside the nucleus
Triboelectric Series
32.5 – Charging by Friction & Contact
 Objects become charged in two fundamental ways:
 By conduction – friction & contact
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Induction - no contact
 Rubbing two materials together can cause e to be pulled
from one and placed on the other, ie: charged
 Once an object is charged, touching something will cause
some of the charge to transfer
32.3 – Coulomb’s Law
 Charles Coulomb discovered the math. Relationship
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between charges – another inverse square
q  magnitude of electric charge, unit = coulomb (C)
k  constant of proport., k = 8.99 x 109 Nm2/C2
This makes electrical forces HUGE, 1 C is HUGE
Can be attractive or repulsive
 Since everyday objects are neutral (protons = electrons),
there are no electrostatic forces
 At the atomic level, atoms bond together because of
giving up/taking e (ionic) or sharing e (covalent &
metallic)
32.4 – Conductors & Insulators
 Some materials allow e to flow easier than others –
conductors – e are held weakly to nucleus
 Generally metals
 Others materials, e have difficult time moving –
insulators – e held strongly to nucleus
 Rubber, glass
 All charges placed on conductors spread out evenly
 Those placed on insulators stay in location of placement
 Some materials are good insulators & then when
energy is added to the material, become conductors –
semiconductors
 Used to make transistors that act as tiny switches or
amplify electric signals
32.6 – Charging by Induction
 Because of electrostatic forces, charged objects brought
near conductors move charges on surface – they are
induced to move
 Object is still neutral, but charge is nonuniform on
surface
 Contact with another object causes charges to leave first
– giving a charge to it
 Charge can leave one side of induced object by grounding
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it – providing a path for charges to, go to/come from, earth
Can be a wire connected to earth or touching it
Clouds can become charged (friction) inducing charge on
the surface - Franklin
Also discovered charge collects at points – lightning rod
Allows for easier discharge from surface to cloud
Lightning strikes rod instead of building
32.7 – Charge Polarization
 It is possible for an
insulator to pretend to
become positive
 Since charges are not
allowed to move, they can
only realign
 Sides of atoms/molecules
have a temporary charge –
electrically polarized
 Some molecules come
polarized (water) – electric
dipoles