Transcript Electricity

Electrostatics :
Charges at rest
Electric Charge
A
property of matter that creates a
force between objects.



Can be positive or negative
Like charges REPEL
Opposite charges ATTRACT
 What
objects (particles) do we know
about that carry a charge?
Electric Charge
An object’s charge depends on imbalance of
protons (+) and electrons (‐)


More protons than electrons  positive
More electrons than protons  negative
Units of charge: Coulombs (C)

Protons and electrons have exactly the same
amount of charge: 1.6 x 10‐19 C
Differ only in sign (+ or -)
Electrostatics – Elementary Charge

Since protons and electrons are the
smallest whole particles, the charge on
any object is a multiple of 1.6 x 10-19 C

Elementary Charge (e) = 1.6 x 10-19 C
1C of charge is made up of 6.25 x1018
electrons.
Charging an Object
Objects become charged if they have an
imbalance of protons and electrons.
 Can

(Think: Can protons MOVE?)
 Can

an object gain or lose protons?
an object gain or lose electrons?
(Think: Can electrons MOVE?)
The total sum of the charge a body has is
the result of a loss or gain of electrons
Electrostatics
 When
there is an imbalance in protons
and electrons the atom will have a total
sum of charge

Total positive sum of the charges – more
protons than electrons
+
-++

Total negative sum of the charges – more
electrons than protons
+-
-- +
-
Electrostatics
 For
an object to gain a total sum of
charge, there must be an imbalance in the
number of protons and electrons.
 For
the imbalance to occur, the atom can
only gain or lose electrons.
 If the atom lost a proton, it would change
what it is.
Charging an Object

Conductors: materials that transfer and redistribute
charge easily.



Insulators: materials that do not transfer charge easily;
retain charge within a localized region.



Excess charge can move through the material.
Examples: ???
Excess charge stays on the surface of the material.
Examples: ???
·Semiconductors: materials that behave as either
insulators or conductors, depending on temperature

Examples: ???
Electrostatics and Charging
 Charge

can move by many methods
Methods:
• Friction
• Conduction
• Induction
 For
charge to move, electrons must move
from atoms. Positive charge does not
move!
Charging by Friction
 The
electrons are literally rubbed off
of one object and move to the other

Charging a balloon
• Hair  positive

Using the fur to charge the plastic rod.
• Fur  positive

Balloon  Negative
balloon charging
Rubber rod  Negative
Charging by Conduction
 Charge
is transferred from one object to
another object by direct contact
 Walk
across the carpet in socks and touch
the doorknob… ZAP!

Charge is transferred and you experience a
shock.
Charging by Conduction: Demo

Demonstration: Touch the negatively
charged magic wand to the mylar butterfly.


Some electrons move from wand to butterfly.
Butterfly repels wand (both slightly negative).
 Static
Discharge: movement of charge
from one object to another by conduction
using the air. Charge can JUMP!
(Lightning)
Charging by Induction

A temporary charge can be induced in a
neutral object by bringing a charged object
close to it.

 If
The charges in the neutral object move in
response to the external charge. Result:
induced charge (POLARIZATION!!!)
a path is provided, this moved charge
will escape. Result: body has a total sum
of charge
Charging by Induction: Demo
 Demonstration:
Bring negatively charged
magic wand near soda can.


Electrons in soda can repel.
Protons in soda can are attracted and cause
the can to move forward
 Demonstration:
Bring negatively charged
balloon near wall.


Electrons in wall repel.
Protons in wall attract to balloon, and it sticks
to wall
Charging by Induction

If a charged object is
brought near the
spheres, the charges
will POLARIZE.

The negative charges
move towards the
positive charges and
away from other
negatives.
Measuring Charge
 Electroscope


Movement of leaves is a “rough estimate” of
amount of charge
Other electroscopes are
more precise.
• When a charge is present,
the straw rotates.
 More rotation = more charge
 Can detect + and – charge,
cannot differentiate
http://www.youtube.com/watch?v=vzFnUtP_wEg
Electric Force
 Electric
Force: force of attraction or
repulsion between objects due to charge

Depends on CHARGE and DISTANCE
• Increase charge  force increases
• Increase distance  force decreases
 Forces
can be exerted by one charge on
another from a distance through a FIELD

Electric Field: region around a charged object
in which other charged objects experience an
electric force
Coulomb’s Law
F = k q 1q 2
2
r
F
= electric force in Newtons
 k = constant (just a #) = 9.0x109 Nm2/C2
 q1 = charge of object #1 in Coulombs (C)
 q2 = charge of object #2 in Coulombs (C)
 r = radius between two charges in meters
Coulomb’s Law calculates a
force
If the calculated force is:
• Negative

The force is attractive between
particles
• Positive

The force is repulsive between
particles
Effect of Coulomb’s Law
 If
the charge of one object doubles…
Force doubles (x2)
 If the charges of both objects double…
Force quadruples (x4)
 If the distance between the charges
doubles…
Force is quartered (divided by 4)
EXAMPLE
 Find
the force exerted by one electron on
another separated by a distance of 2.0 m.



Draw a picture
Table of Knowns
(What is the charge of each electron?)
(What is a constant? k= 9x109 N*m2 /C2)
Is the force repulsive or attractive?
Force (N)
Q1
Q2
D (m)
+2C
+2C
0.5
-3C
-3C
0.1
-1C
+2
2
1 x 10-3 C
2 x 10-3 C
0.5
-4 x 10-3 C
-3 x 10-3 C
0.75
Attractive or Repulsive?
Electric Force
Force – the force of attraction
or repulsion between objects due to
charge.
 Electric




Like charges repel.
Unlike charges attract.
Depends on size of charge and distance
Act over a distance through an electric
field
Coulomb’s Law

Electric force is given by Coulomb’s Law
2
N * m q1q2
F  (9 x10
) 2
2
C
d
9

Where :
 q1 and q2 are the charges
 r is the radius between the charges

Force decreases as r gets bigger but never
will be zero – Like gravity
Electric Field
Field – An electric field is the
region around a charge in which the
electrostatic force is felt by other
charges.
 Electric
 Electric
fields are drawn and 'mapped'
in diagrams using “field lines”..
Electric Field Lines
 Always
extend from a positively charged
object to a negatively charged object
 From
 From
a positively charged object to infinity
infinity to a negatively charged
object.
Electric Field Lines
 Electric
 At
field lines never cross each other.
locations where electric field lines meet
the surface of an object, the lines are
perpendicular to the surface.
Electric Field Lines
 The
number of field lines that meet a
charged particle represents the size of the
charge, higher charge amounts have more
field lines.
Electric Field Lines
 The
closer together field lines are to each
other, the greater the strength of the field
at that point.
Electric Field Lines
 Between
Particles
Electric Force
 Electric
field is ‘seen’ by the force on a
charged body, test charge.
 Electric field is depicted through Electric
Field lines.




Field lines point in direction of force on
positive charge
Field lines never cross
Field lines travel away from positive charge
Field lines travel toward negative charge
Electric Field
Electric Field Lines
 When
two opposite charges are
brought close together,
 The field lines travel from positive to
negative.

The lines connect the charges showing
attraction
Fig 15.29a, p.551
Slide 36
Electric Field Lines
 When
like charges are brought near
each other,
 The fields lines repel each other.

Reason for the repelling force between
like charges.
 http://www.falstad.com/vector3de/index.ht
ml
Electric Field Equations

E = F/qtest
 E = kqsource/r2

The electric field only
depends on the source
charge
 It does NOT depend on
the test charge – see how
the field is only created by
the extra protons and
electrons, NOT the puck