Transcript Electrostatics - Roden's Physics


Effect static electricity charging

Protons (p+)
› in nucleus (center) of an atom
› positively charged

Neutrons (n0)
› in nucleus (center) of an atom
› neutral (no effect on charge)

Electrons (e-)
› move freely outside of the atom
› negatively charged

Electrical Charge – a force that one charge exerts on another

An amount of static electricity can ignite volatile substances
(gasoline)

A large difference in the charges in the air can trigger a
lightning.

Opposite charges attract each other
›

protons are attracted to electrons because they have different electric
charges
Like charges repel
›
one electron would repel another electron because they have the same
electric charge.
Remember, an electron (e-) is free to
move but a proton is “locked in” to the
nucleus of an atom.
 This means that when charging occurs,
the electrons either move from one
physical body to another, OR are
rearranged within an existing body.
DON’T FORGET THIS!


Electrons are opportunistic migrants
› for electrons to make a move from the
atoms of one material to the atoms of
another material, there must be
 an energy source
 a motive
 a low-resistance pathway



Charged objects (ions) have an unequal number
of protons and electrons
Charge is the difference between number of
electrons and protons
Coulomb (C)
› The charge on a single electron is -1.6 x 10-19 C
› The charge on a single proton is +1.6 x 10-19 C

The amount of charge carried by a
lightning bolt is estimated at 10 C. What
quantity of excess electrons is carried by
the lightning bolt?
› Answer: 6.25 x 1019electrons
When one body loses an electron, the
other body will gain that electron so that
there is a net charge on each body
 no electrons are destroyed in the
process.

The farther apart opposite charges are,
the weaker the attraction force between
them.
 The closer the charges get, the stronger
the force becomes.

›
›
›
›
›
F = force
kc = Coulomb’s constant (9x109 Nm2/C2 )
q1 = charge of particle 1
q2 = charge of particle 2
r = distance

One charge of 2.0 C is 1.5 m away from a – 3.0 C charge.
Determine the force they exert on each other.

The negative sign just means that one charge is positive,
the other is negative, so there is an attractive force
between them.

Two balloons are charged with an identical
quantity and type of charge of -6.25x10-9 C.
They are held apart at a separation distance of
.617 m. Determine the magnitude of the
electrical force of repulsion between them.

Two balloons with charges of +3.37 μC
and -8.21 μC attract each other with a
force of 0.0626 Newton. Determine the
separation distance between the two
balloons.

Workbook problems
› Pages 197-200, exercices 1-5
Any material that does not conduct
electricity
 store electrons on their surface.
 Ex: Plastics, dried wood, glass, fabric,
and other non-metals


Metals are CONDUCTORS, and do not
store charges (wires in your home,etc)

Friction

Conduction

Induction

When 2 uncharged objects come into
contact, electrons are stripped from one
object onto the other and they receive
opposite charges. Usually happens when
objects rub together.

When a charged object touches an
uncharged object, some of the charge
moves so that both objects have the
same charges. The new charge will be
weaker than the original one.

When a charged object approaches,
but does not touch an uncharged
object, a migration of electrons creates
an “apparent” charge on the object
even though the net charge remains
zero
Charging by Induction: Charged
object drives off like charges,
leaves charged object with
opposite charge

Field Lines -Invisible lines surrounding a
charge manifesting a force.

Number of lines per area represents field
strength

As lines diverge, field strength diminishes

More charges means greater number of
field lines

Which region
experiences
greater electric
field strength?
› region A

Where is that
region near at?
› near the charge
Two point charges of
equal magnitudes and
opposite signs.
 Near the charge the field lines are
radial
 All lines that emerge will terminate
 Emerge on positive charge and
terminate on negative.

Field lines do not cross
 Repulsion
 Like Charges
 For Negative charges,
reverse the direction
of the field lines.


Given a positive charge
that is 2X that of
negative, how will the
lines terminate?
› Only half the lines that
emanate from the
positive charge will
terminate in negative
charge
› Rest of lines terminate at
infinity.