Electrostatics
Which spheres experience the
greatest attraction?
Properties of “Charge”
• Reflects relative number of electrons in a
substance
• Conserved
• Units of Coulombs (C)
• An electron has a charge of
1.6 x 10-19C of charge
Significant Charge Amounts
• When we rub balloons on rabbit hair in the
lab, we’re generating 10’s of mC
Coulomb's Law
• Coulomb’s Law gives us a way to
calculate the force between two charged
objects
• FE = kq1q2 /d2
• **Note: q1 and q2 represent charge
magnitudes (we’ll talk about direction later)
• k is a constant = 8.99 x 109 N m2/C2
Similarities to Gravity
• Recall how we calculated the gravitational
force between objects:
• FG = Gm1m2/d2
• G is a constant = 6.67 x 10-11 N m2/kg2
• Look familiar?
Different Constants
• G = 6.67E-11, k = 8.99e9
• What does this tell us about the difference
between gravitational forces and
electrostatic forces?
Examine the configuration below.
Which charge would exert the
greatest force on the -2 charge?
• 1. +8
• 3. +6
2. +4
4. +20
Which list below ranks the charges
in order of increasing force on the
-2 charge?
•
•
•
•
•
1.
2.
3.
4.
5.
A, B, D, C
A, C, B, D
D, C, B, A
C, A, B, D
D, B, C, A
Which arrow represents the direction of the net
force on the -2 charge?
• Calculate the electrostatic force on a +6mC
charge by a -5mC charge, located 2m to its
left
• F = .067 N, to the left
Balloon Demo
• Calculate the electrostatic force between a
proton in the nucleus of the atom (q =
+1.60e-19C) and an electron (q = -1.60e19C) located in an outer energy level (d =
3e-11m)
• Calculate the electron’s acceleration
Levitation
• I once heard a person ask, couldn’t you
make a person float using charges?
• Perhaps
• Imagine a person (m = 70kg) gathered 10mC of charge by rubbing herself with
rabbit fur
• What charge would we need to lift her off
the ground?
What is this?
“Scaling Force”
•
•
•
•
Recall Newton’s second law:
The ratio of Force/mass is acceleration
What is the analog for charged particles?
Force/charge =
• We call this ratio the electric field
Electric Fields
• Like gravity, the electrostatic force is a
non-contact force
• To conceptually deal with this, we talk
about electric fields
• This is a region of space surrounding a
charged particle that “carries” the
electrostatic force
• An electric field tells us the direction of the
electrostatic force
• It also gives us a sense of the force
magnitude
Drawing the Field
• Place a positive “test” charge near a
charge, or charge configuration
• Determine the direction of the net force
acting on that positive charge
• Draw an arrow in that direction (arrow
length represents force magnitude)
• Move the charge to another place and
repeat
• Field lines start/end at infinity
(somewhat hard to draw…)
• Negative charges?
• With both of these configurations, what
happens to field strength as I get further
and further from the charge?
• Can you envision a scenario in which field
strength remains constant?
Field Strength
• Imagine two, positive charges of charge q,
separated by distance d. What is E for
one of those two?
• E = k|q|/d2
• Units of N/C
• As they are vectors, fields add together as
vectors do
Draw the Field for this charge
configuration:
Find the field strength at point p
(q = 5.0 mC)
Is there a point at which E = 0?
Other configurations
• http://phet.colorado.edu/new/simulations/si
ms.php?sim=Charges_and_Fields
Levitation
• Imagine a human being of mass 75kg
• If she were to somehow acquire an excess
charge of 20mC by vigorously rubbing
rabbit fur on a dry day, what field strength
would be necessary to levitate her above
the ground?
• Which way would the field point?
Calculate the acceleration of
an electron (m = 9E-31kg, q =
1.6E-19C), placed in a field of
strength 3E-7 N/C