Transcript Electrostatics

Electrostatics
Structure of the atom
• All matter is made up of atoms
• Atoms consist of a small central nucleus,
containing positively charged protons and
neutral neutrons, surrounded at relatively
large distances by negative electrons
Atom
Nucleus
Protons(+)
Neutrons
Electrons(-)
Electrical charge
• In an uncharged system/atom
there must be the same no. of +
protons and – electrons
• In any system it is only the
electrons which are free to
move
• Thus to give an object an
electrical charge it is necessary
to gain or lose electrons.
• Electric charge is measured in
coulombs (C)
• If an object is negatively
charged it means it has gained
electrons
• If an object is positively
charged, it means that it has
lost electrons and so is left with
more protons than electrons,
and so has a positive charge
Conductors and Insulators
• Conductors = substances
through which electric charge
can flow (i.e. when you place a
charge on the object the charge
can move around)
• Insulators=substances through
which electric charge cannot
flow (i.e. when you place a
charge on the object the charge
stays where you put it)
To charge an object
• An object becomes electrically
charged when it either gains or
loses electrons
• There are two ways to charge
an object
(a) by contact
(b) by induction
Charge by Contact
• If a polythene rod is rubbed with a
woollen cloth some of the electrons
from the cloth rub onto the rod.
• Thus the rod has acquired extra
negative charges and is negatively
charged.
• The cloth has lost an equal no. of
electrons and so is positively
charged to the same extent
(a)A and B are initially neutral. (same no. of
protons and electrons)
(b)A and B are brought into contact. Some of the
loosely held electrons from A transfer to B.
(c)When A and B are separated A now contains
more protons than electrons and so A is
positively charged. Similarly B is negatively
charged
Charging by induction
4 step process
1 Bring a rod of the
opposite charge to the
one desired near, but
not touching the object.
(this causes the +
charges in the object to
be attracted and the –
charges to be repelled)
Charging rod
+++
- - -
Object
for
charging
2. Connect the
object to earth.
(the negative
charges in the
object, which want
to get as far away
from the charging
rod as possible,
travel down the
earth connection
to ground)
Charging rod
+++
- - -
Object for
charging
3.Break the earth
connection –
without removing
the charging rod.
Charging rod
+++
4. Remove the
charging rodobject is charged
in the opposite way
+++
Object
charged by
induction
NOTE:
IT IS ALWAYS THE ELECTRONS WHICH MOVE
When charging an object positively the electrons
from the object flow away through the earth
connection to ground.
When charging an object negatively electrons
from the ground travel up through the earth
connection to neutralise (i.e. effectively remove)
the positive charges in the object
The gold leaf electroscope
• Used to detect the presence of
electric charge, and indicate its
size.
Metal cap
Insulator
Metal rod
Metal Case
Gold leaf
Glass window
Note:
• The divergence of the leaves is due to 2
factors
− The repulsion of the same charges
on the two leaves
− The attraction between the leaves
and the opposite charge induced on
the inside of the case
The divergence of the leaves measures the
potential difference between the leaves
and the case
To detect charge using
an electroscope
----
• If a charged rod is brought near the cap
of the electroscope the leaves diverge
-the neg. charges
in the rod repel the
neg. charges in the
++++++
electroscope, which travel
down the leaves which repel
-the size of the divergence
shows the size of the charge
To test the sign of a
charge
• Charge the electroscope either
+ or –
• Bring the object to be tested
near to the cap
• If the leaves collapse then the
object has the opposite charge
• If the divergence increases the
charge is the same sign
Distribution of charge on
a conductor
• If positive (or negative) charges are placed
on the surface of a conductor, they will
move until they are as far away from each
other as possible
• When they have stopped moving (i.e. are
static) it is found that
All static charges reside on the outside
of a conductor (where they are furthest
apart)
Static charges tend to accumulate
where the conductor is most pointed
Van de Graaff generator
• A machine for storing large
static charges
• Charge from a point travels up
the belt and is picked off by
another point to be stored on
the dome.
• If the dome is then connected
to earth a current will flow to
earth
Point Effect
• Charge tends to accumulate around
pointed objects
• This generates a large electric field
in the air around the point
• This causes the air to ionise and
then the ions in the air are attracted
or repelled by the point -as these
move they hit other atoms and cause
further ionisation
Oppositely charged ions move
towards the point and neutralise it.
Thus a pointed object will not retain
as big a charge as a rounded object.
This loss of charge is called the
point effect
Same charged ions move away from
the point, creating an electric wind.
e.g. Put a candle near a point on a
charged van de Graaff-gets deflected
Effects of static
electricity
• Lightning
• When clouds pass over each other
they build up static charge
• Lightning occurs when these
charges discharge, either to each
other or to earth
• Lightning will choose the easiest
path to earth-through a tall object
Lightning conductor
• Long metal rod attached to the
side of the building. The top of
the rod ends in a point (which is
higher than the building) and
the bottom ends in a large
metal plate buried in the earththis rod provides the easiest
path to earth.
•The cloud induces an opposite charge
• on the rod.
•This charge accumulates
•around the point and so a big electric
•field builds up
•around the point.
•This ionises the air around the point
•and so provides an easy path for the
•cloud to discharge
Other effects of static
electricity
• TV screen collects dust-picture
on screen is caused by beams
of electrons, so screen
becomes charged, so attracts
dust particles.
• Aeroplane gets charged in
flight-by friction with air. Must
be discharged before refuelling
in case a spark would ignite
fuel-usually discharged via the
conducting rubber of the wheels
Force between charges
(Coulomb’s Law)
• Coulomb’s law states that the
force of attraction or repulsion
between two point charges is
directly proportional to the
product of the charges and
inversely proportional to the
square of their distance apart
F Q1Q2 / d2
Coulomb’s Law
F Q1Q2 / d2
F= constant Q1Q2 / d2
Constant =1/4
F= (1/4 ) Q1Q2 / d2
Inverse square law-force proportional
to 1/square of distance
Coulomb’s Law
calculations
• Example
• Three charges, each of +100 C, are
equally
• spaced along a straight line, successive
charges
• being 3 m apart. Calculate (i) the
• resultant force acting on one of the end
charges,
• (ii) the resultant force on the central
charge.
Permittivity 
•  = permittivity of the medium –
this is a constant for a
particular medium.
• Usually the medium is air or
free space, in which case the
permittivity is given by
0 = 8.9x10-12 F m-1
Relative permittivity r
• If the medium is not free space
the permittivity is sometimes
given in terms of relative
permittivity r which relates the
permittivity of the medium to
that of free space. The
permittivity of the medium can
then be calculated from
 = r 0
Do questions p229
Electric Field
• An electric field is a region
where an electric charge
experiences a force-this force is
caused by the presence of other
static charges in the vicinity
• Electric field can be
represented by electric field
lines –Imaginary lines showing
the direction in which a positive
charge would move if placed in
the field
Electric field lines
• Always point out from + charges
• Always point into negative
charges
+
-
Field lines due to
combinations of charges
• Field line pattern due to unlike point
charges
Field pattern due to like point charges
Field pattern due to parallel plates
+
+
-
To show electric field
patterns (Experiment)
Place some cooking oil in a
container with 2 electrodes
Sprinkle fine powder (semolina)
on the oil
The semolina particles will align
themselves along the field
pattern
Electric Field Strength
• The electric field strength (E) at
a point is defined as the force
per unit positive charge at that
point (measured in volts/m)
E = F/Q
Since F is given by coulomb’s
law
E = Q/4d2
See text p232
Electric field intensity
calculations
Applications
• Photocopier- The drum is
charged electrostatically. Light
is reflected off the blank bits of
the page, and this reflected
light knocks charge off the
drum  the charge pattern on
the drum represents the print
pattern on the page
Toner is then sprinkled over the drum
and sticks to the charged bits
- The pattern thus gets transferred to
the new page
Applications (ctd)
•
Electrostatic precipitators
-device for removing dust particles
from air. It charges the dust
particles using the point effect.
These are then attracted to
oppositely charged metal plates.
Once a certain amount of dust has
built up on these plates, they can
then be removed and cleaned.-used
for smoke removers etc.