Download Electric Field & Force

Electrostatics
Unit 11
Electric Charge
Symbol: q
Unit: Coulomb (C)
Chemistry Review:
Two kinds of Charge:
Positive
Negative
Law of Electrostatics:
Like charges REPEL- they don’t want to be
anywhere near each other.
Unlike charges ATTRACT- they want to be as
close as possible.
Fundamental Amount of Charge for
1 electron or 1 proton:
e = ± 1.6 x 10-19 C
q = n (e)
n = # of particles
e = charge per particle
q = total charge of object
Transfer of Charge
Electroscope: a device that detects the presence of electric charge.
Only ELECTRONS can be transferred from one object to another because protons and
neutrons are fixed in the nucleus.
1. Conductors: material that transfer electrons easily
ex: most metals
2. Insulators: materials that don’t easily transfer electrons
ex: glass, plastic, rubber
3. Semiconductors: in between the properties of conductors and insulator
ex: metalloids
4. Superconductors: perfect conductors at or below a certain temperature.
Methods of Charging
CONDUCTION
Conduction: charging by CONTACT.
• rubbing two objects together, the electrons from one are transferred to the
other
• one object becomes negatively charged and one object becomes
positively charged.
Ex: Rubbing a balloon on your hair.
• The balloon gains electrons from your hair• the balloon becomes negatively charged,
your hair becomes positively charged
• Opposite charges attract!
Methods of Charging
INDUCTION
Grounded: when an object is connected to the earth by a wire, pipe, finger, etc so
that electrons can transfer to or from the earth.
Why connect to the Earth?
The earth is so large that we say it can ACCEPT or DONATE an unlimited amount
of electrons.
Steps:
1. Start with a negatively charged rod and a neutral
metal sphere.
2. A negatively charged rod brought near a neutral
metal sphere, the negative charges are repelled and
redistribute themselves.
**Part of the sphere closest to the rod has more
POSITIVE charges.
3. Ground the sphere by connecting to the earth, the
electrons will flow into the earth leaving a NET
POSITIVE CHARGE on the sphere.
4. Pull the rod away and the positive charges
redistribute themselves.
INDUCTION
*** NO CONTACT WITH THE CHARGED ROD!
Methods of Charging
POLARIZATION
Polarization: the redistribution of charge within a group of particles
that produces an apparent charge on the surface of an object.
When Polarized- net charge = 0
* No charge has actually been transferred.
* Charge is only REDISTRIBUTED.
Ex: A polar molecule- one side of the molecule is more positive, one side is more
negative. I.e. Water Molecule
++
+
+
+
H2O
Coulomb’s Law
Electric Force- the force between any two charges (or charged objects)
Attractive between opposite charges
Repulsive between like charges
kq1q2
Fe  2
d
Where:
k = 9 x 109 Nm2/C2 Coulomb’s Constant
q1 & q2 = two charges
d = distance between charges (always in METERS)
Relationship between Fe and q1q2 is DIRECT.
• Double one charge and Fe will be doubled.
• Double both charges (x4) and Fe will be quadrupled.
Relationship between Fe and distance is an Inverse Square Law.
• Double the distance and Fe will be 1/4 as large.
• Triple the distance and Fe will be 1/9 as large.
Electric Force
Fe always acts along a straight line between two charged objects.
Always draw the force ON the object you’re talking about.
q1
Fe is a FIELD FORCE.
Def: A force that acts on objects at a distance;
* NO CONTACT.
Another Example:
Gravitational Force (Fg)
Gm1m2
Fg 
2
d
kq1q2
Fe  2
d
+
F2,1
-
Differences:
a. Fg is only attractive; Fe can be
attractive or repulsive
b. Fg is a relatively weak force;
Fe is very strong.
q2
Superposition- Net Electric Force
If there are more than 2 charges, the net electric force on any charge is found by
getting the vector sum of all electric forces.
**Draw all forces directly on the charge that you are finding the net force on.
1-Dimension: forces in same direction are ADDED
forces in the opposite direction are SUBTRACTED- (right - left)
*if you draw it correctly, do not use negatives in coulomb’s law formula!*
Fc,b
+
+
A
B
Net force on Charge C:
-
Fc,a
Fc,a + Fc,b
C
Fc,b
+
-
A
B
-
Fc,a
C
Fc,b - Fc,a
Superposition- Net Electric Force
2-Dimensions:
Add up all the x’s, add up all the y’s and get the RESULTANT.
Fb,c
B
Net force on Charge B:
C
-
Fnet  x 2  y 2
-
Fb,a
+
A
2
Fnet  Fb,c  Fb,a


y 
  tan  
x 
1
2
Electric Fields
Def: a region in space around any charged object in which another charged object will
experience an electric force.
Fe
E
q0
Units: N/C
q0 is a small positive test charge placed
near a second object with a charge
Fe is the magnitude of the electric force
E is a VECTOR quantity- depends on direction
** the direction of the E field shows the direction of the electric force that would be
exerted on a small positive test charge.
+ +
+
+ +
-
E
Near a Positive Object- the test charge would
be repelled so the field is away from a
positive.
+
Near a Negative Object- the test charge would
be attracted so the field is toward a negative.
+
-
-
E
E Fields
The direction of the Electric Field depends on the charge of the object producing the
field.
The magnitude of the electric field (strength) depends on: charge and distance
* The greater the charge of the object, the stronger the field will be around that object.
* The greater the distance from the object, the weaker the field.
kq
E 2
d
q is the charge of the object producing the field
If q is + the field is always AWAY FROM THE OBJECT.
If q is (-) the field is always TOWARDS THE OBJECT.
E fields exist anywhere near a charged object whether there is a test charge or not.
Electric Field Lines
Imaginary lines that represent the size and direction of an electric field.
-The # of lines is proportional to the size (strength) of the field.
Rules for Drawing E Field Lines:
1. Lines begin on positives and end on negatives.
2. The number of lines leaving a + charge or
approaching a (-) charge must be proportional to
the magnitude of the charge.
3. No two field lines from the same field can
intersect each other.
E Field Lines
Electric Dipole: two point charges of equal magnitude but opposite charge
Because size of
charges is the
same, the # of lines
leaving the +
charge is equal to
the # of lines
ending on the (-)
charge.
The closer the lines,
stronger the
strength of the
field.
E fields between 2 like charges won’t go from one to
the other. Between the charges, lines will repel.
E Field Lines
Between Charges of Unequal Magnitude
+2 Charge & -1 Charge
* the # of lines leaving the +2
charge is twice as much as
the # ending on the -1 charge.
* Half of the lines that leave
the +2 charge will end on the 1 charge. The other half will
terminate at infinity.
Electric Field Inside a Conductor
When a positive charges totaling Q are placed on a sphere, they all go to the
outside and distribute themselves in such a way to get as far from each other
as possible.
Inside the sphere (r < R), the electric field is zero.
Outside the sphere (r > R), the electric field behaves as if the sphere is a
point charge centered at the center of the sphere, that is,
kq
E 2
d