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Electric Force
Physics
Electric Force
https://www.youtube.com/watch?v=4S0EBxT60pw
Electric Force
GRAVITY
Created by mass
Only Pull
Reach across galaxies
(huge range).
BOTH
ELECTRIC
Follows the inverse
square law.
Created by charge
Field forces (They
can act on you even
if your not touching
them.)
Can pull or push
(attract or repel)
Can reach across atoms
(Coulomb’s equation
can only be used for
small range).
Net Charge
PROTONS
ELECTRONS
+
-
Big and not mobile,
stay in nucleus
Tiny and very
mobile, free to move
around.
NEUTRONS
0
Net Charge
NEUTRAL
++- +(Every proton has an
electron.)
NEGATIVE
+
- +- +- (More electrons than
protons.)
POSITIVE
-+ +
+
(More protons than
electrons.)
Electric Rules:
1. Likes Repel
2. Opposites Attract
3. Inverse Square
Electric Fields:
•
Electric Fields are invisible, the picture helps us
imagine what it would look like if we could see it.
•
RULE 1: Arrows go OUT of
•
RULE 2: Arrows go INTO
•
RULE 3: Arrows never cross.
+
-
particles.
particles.
More arrows show the strength
Electrostatic Induction:
electric charge
not
moving
to influence
Why does the charged balloon stick to the neural wall?
+
- +-+ -
-
+-+
+-+
Electrons Flee
+
- +-+ -
-
++++-
Wall is still neutral but
induction has happened.
Electric Demos:
What happens when I rub the balloon on my hair?
Electrons climb onto the balloon.
Why do the two balloons “run away” from each other?
They have the same charge.
Why does the balloon move towards my hand or head?
They have the opposite charges.
What happens when I touch the balloon to the electroscope?
The needle deflects.
Why does the needle deflect?
Because likes repel.
What happens when the belt in the Vandergraaf Generator turns?
Ball Fills with electrons.
Why does the paper stand up?
Electric repulsions.
Why does the metal ball “zap” you if you get too close?
Electrons are trying to escape.
Coulomb’s Law:
CHARGE (q):
Electric forces are directly proportional
to both charges involved..
Electric forces are inversely
DISTANCE (r): proportional to the distance squared.
RELATIONSHIP:
FE = Kq1 q2
r2
Means: q1 q2 top
Means: r2 bottom
Coulomb’s Law:
Equation:
Coulomb’s Constant:
Fe = Kq1 q2
r2
K=
8.99x109
Store in
calculator as
letter K.
Units
The units for charge are Coulomb (C)
A Coulomb is really big so often times we
will use microCoulombs (μC) Which
means 10-6 C.
Positives and Negatives:
A positive answer means repulsion.
A negative answer means attraction.
Patterns
Two positively charged spheres are hanging from strings in a laboratory. The
current force of electric repulsion is 18 N. What would happen if the following
changes were made to the system? (Compare each scenario with the original
scenario. And NOT the scenario above it.)
1. The first sphere is given twice as much charge.
The new force would be ________
2. The second sphere is reduced to 1/6 as much charge.
The new force would be ________
3. The charge on the first sphere triples and the second sphere doubles.
The new force would be ________
4. The separation distance is tripled.
The new force would be ________
Change Ratios:
-5 C
-3 C
2r
+2 C
5r
Electric Force = -15
N
+8 C
New Electric Force = -5.76 N
Equation:
Two metal spheres are charged in a laboratory. The first
obtains a charge of -0.1 C. The second obtains a charge of
-0.08 C. They are sitting 2 meters apart. Calculate the
strength of the electric force between them. Is it attractive
or repulsive?
ATOMS:
CHARGE OF A PROTON = CHARGE OF AN ELECTRON = CHARGE OF A NEUTRON =
1.6 x 10-19 C
- 1.6 x 10-19 C
0
Atoms:
The nucleus of Carbon contains 6 protons. What is the net charge of
a Carbon nucleus?
The distance between the nucleus and the first electron is about 5.29 x
10 -11 m. Calculate the strength of the electric force between Carbon’s
nucleus (all six protons) and the first orbiting electron.
What does the sign of our answer tell us?