Static electricity
Atoms, Electrons and Ions
Electron “cloud”.
Negatively charged
Occupies most of volume of atom
The atom
Nucleus
Contains protons and neutrons
Positively charged
99.9% of mass is concentrated in tiny nucleus,
Atoms have equal number of protons and electrons, so total charge is zero.
They are electrically neutral
The same is true of most ordinary objects (made of atoms.)
Try this for a home experiment this week – yes I’m kidding
Why does this person survive?
Reminders:
http://drmegavolt.com
Lecture 20 :
HW 8 due Today
Reading for Tuesday: 10.3
Static electricity
- Unbound (free) electrons (negatively charged)
- Positively charged ions
(ion = atom with unequal number of protons and electrons)
Particle
Charge
Mass
-e
9.11×10-31 kg
Proton
+e
1.67×10-27 kg
Neutron
0
1.67×10-27 kg
opposite charges attract
like charges repel
F
-
-
F
-
F
F
+
What if I have twice the charge as before?
e = 1.6 × 10-19 Coulombs
Coulomb (C) is the unit of charge
F
-
-
F
Twice the charges – twice the force
Three times the charge, 3x the force
Ten times the charge, 10x the force
a) Half the force as before
b) Same force as before
c) Twice the force as before
d) Four times the force
Electrostatic force between charged particles
Sorting out how charges interact
opposite charges attract
like charges repel
-
Free
electron
Sorting out how charges interact
Static electricity: What happens when charges are stationary
Electricity (and electric currents): What happens when charges
(usually electrons) are moving
F
-e
+e
+ ion
Source of electricity: Electric charges
Everything (earth, you, the table etc) made of tiny particles called atoms
Atoms are made up of 3 even tinier particles:
Electrons, neutrons and protons
The atom
Electron
atom
You can pull electrons off atoms. This leaves 2 charged particles :
-
F
-
F
F
+
opposite charges attract
like charges repel
F
-
-
F
-
F
F
+
Consider 2 ‘point’ charges, A and B. What force does charge A feel?
qA
qB
Observed behavior:
- Force depends on qA and qB : More charge, more force
What about the distance, what if they are closer?
F
-
-
F
Closer means more force!
Forceof B on A ~ qA x qB
note: if q’s are the same sign, F is positive: pushing away
if q’s are the opposite sign, F is negative: attracting together
a) Less force
b) Same force as before
c) More force
1
Putting this together: Electric Force
Sorting out how charges interact
opposite charges attract
like charges repel
opposite charges attract
like charges repel
F
-
-
F
-
F
F
+
F
-
-
F
-
F
F
+
Consider 2 ‘point’ charges, A and B. What force does charge A feel?
What about the distance, what if they are closer?
r
qA
qB
Closer means more force!
F
-
-
F
What math relationship shows this?
a) Less force
b) Same force as before
c) More force
B
proton
A
electron
-
F ~ 1/ r (is a good guess)
But it turns out to be stronger !
F ~ 1 / r2
0.001 m
What is force between them?
Calculate and write down answer.
+
𝑭𝒐𝒏 𝑩 𝒇𝒓𝒐𝒎 𝑨 = 𝒌
Observed behavior:
- Force depends on qA and qB : More charge, more force
- Force depends on distance between them (r) : less distance, more force
𝑭𝒐𝒏 𝑩 𝒇𝒓𝒐𝒎 𝑨 = 𝒌
• Describes the force between 2 point charges
• k is Coulomb constant = 8.99 x 109 N m2/C2
• qA and qB are amount of charge in coulombs (C )
• r is separation in m
1 Coulomb = 6 x 1018 electron charges!
𝒒𝑨 𝒒𝑩
𝒓𝟐
B
proton
A
electron
𝒒𝑨 𝒒𝑩
𝒓𝟐
-
0.001 m
What is force between them?
Calculate and write down answer.
+
k is Coulomb constant = 8.99 x 109 N m2/C2
e = 1.6 x 10-19C
Force between particles = -2.3 x 10-22 N
Proton and electron have charges of same magnitude (e) , opposite sign
 qA = - 1.6 x 10-19 C
qB = + 1.6 x 10-19 C
a) Force on electron points to left
𝑭𝒐𝒏 𝑩 𝒇𝒓𝒐𝒎 𝑨 = 𝒌
𝒒𝑨 𝒒𝑩
𝒓𝟐
What does the minus sign mean?
b) Force on electron points to right
Force = (8.99 x 109 N m2/C2) (-1.6 x 10-19 C) (1.6 x 10-19 C)
(1 x 10-3 m)2
= -2.3 x 10-28 N m2 = -2.3 x 10-22 N
1 x 10-6 m2
Balloon Sim
Electrostatic dust rag (think SwifferTM).
Rub it on surface, it’s very good at attracting electrons and so becomes
negatively charged.
What kind of dust will this negatively charged rag
pick up best?
http://phet.colorado.edu/en/simulation/balloons
-
+
+
-
a.
b.
c.
d.
Only dust with positive charges.
Only dust with negative charges.
The rag will pick up all dust equally.
The rag will pick up dust with positive
charges, and also neutral dust particles, just
not as well.
e. The rag will pick up dust with negative
charges, and also neutral dust particles, just
not as well.
2
Bring uncharged metalized mylar balloon up to Van de Graaff.
Discussion of Energy
Predict what will happen:
• 3 Types of energy (there are others):
1. Before it touches
2. After it touches
-
- - -
-
-
1. Before touching
a. Not affected by VdG
b. Attracted to it
c. Repelled
-
-
-
-
-
-
Kinetic - energy of motion - rock rolling down hill
Potential - ability to do work in future - rock at the top of
a hill
Thermal - energy that dissipates as heat (e.g. friction, or
smashing into a wall)
• How would this apply to charges?
2. After touching
a. Not affected by VdG
b. Attracted to it
c. Repelled
+
-
• Think of the analogy of a hill… pulling + away
from valley…
Hill Analogy for energy
Effort (push) to get ball
(at constant speed):
up the hill
Hill Analogy for energy
Potential Energy of same ball
a
a) PEa > PEb
a) harder at first
b
b) easier at first
b) PEb > PEa
c) PEa = PEb
c) takes same effort / push
Is it easier or harder to separate opposite charges
when they get further away from each other?
F = k q1 q2/r2
Mechanical work vs. Electrical
Voltage
Think, climbing up a hill
stores energy in system
Same thing, separating charges…
-
F
F
+
http://phet.colorado.edu/sims/charges-and-fields/charges-and-fields_en.html
3
Voltage has to do with conditions of
system
• Amount of charge
• Distance from that charge
• Like the electric force (related but different)
Topo- practice
Voltage is like the height of
mountain
• Think Topographical maps
Voltage tells you what a + charge will
do
• Which figure
goes with #4
+5V
-5V
+V is like top of hill
-V Is like valley
0V
Where is 0 V?
What will a +q do at 0V:
a) Be attracted to higher voltage
b) Be attracted to lower voltage
c) Not be impacted
b) Just like a ball rolls down hill
Electrostatic potential energy and voltage
Voltage is like the height of
mountain
New force: Electrostatic force between charges
New PE: Electric Potential Energy (EPE)
• Height (and gravity)
• Voltage
• Mass, m
• Charge, q
• Electrical Potential Energy • Gravitational
potential Energy
Forces and PE go in pairs - Remember gravitational force:
- Do work against gravitational force (mg) to raise an object’s GPE (mgh)
- Similarly, do work against electric force to raise an object’s EPE
EPE = q D V
Voltage (V)
- tells you EPE of any charge at that location in space
GPE = m g Dh
EPE = q DV, where q = charge of object and DV is voltage difference
Like GPE with q  m and DV  Dh
- Tells you work required to bring a unit charge from V = 0 to that location
- Determined by surrounding charges.
Closer you are to + charge the more + the voltage
- A grounded object is always at V = 0
- Usually most interested in DV: voltage difference between 2 locations
Best understood by doing practice questions!
4
Two metal plates connected by a battery.
Battery maintains a voltage difference of V between the plates
Conservation of energy and EPE
Remember conservation of energy equation:
A
-
Work Done on object = Change in Energy of object
Wext - Wfriction = DPE + DKE
-
Now we can add another PE term to this equation:
q
++
V
-
+
+
+
+
+
+
B
V
Plate A-is grounded (set to zero V)
What is the voltage of plate B?
Wext - Wfriction = DGPE + DEPE + DKE
= mgDh + qDV + D(1/2mv2)
A
q +
+
+
+
+
+
++
a)
b)
c)
d)
B
0 +V
-V
- determine from information given
Can’t
q
A
+
+
+
+
+
+
++
V
-
-
B
EPE = qDV
-
What will happen to the charge q if we let go of it (ignore gravity)?
What is the change in EPE of charge as it flies from plate B to plate A?
a) Nothing
b) It will fly over to plate A
c) Sparks will fly
d) -Something else
a)
b)
c)
d)
0
+qV
-qV
Can’t determine
-
CT 29.4b
q
A
-
++
-
V
+
+
+
+
+
+
B
Conservation of energy:
Wext - Wfriction=
DGPE + DEPE + DKE
Charged particle loses EPE as it flies from B to A.
What form has this energy turned into just before it hits plate A
a)
b)
c)
d)
-
A positive charge q is released from
position i to position f between the
charged plates.
Did the electric potential energy (PE)
increase or decrease?
Did the voltage (V) at the position of the
test charge increase or decrease?
A: PE , V
C: PE ,
V
E: None of these.
B: PE , V
D: PE , V
KE
Thermal energy
GPEPPE
©University of Colorado, Boulder (2008)
5
CT 29.4b
CT 29.4
A positive charge q is released from
position i to position f between the
charged plates.
A negative charge -q is released from position i to position f
between the charged plates of a charged capacitor.
Did the potential energy (PE)
increase or decrease?
Did the voltage (V) at the position
of the test charge increase or
decrease?
Did the electric potential energy (PE)
increase or decrease?
Did the voltage (V) at the position of the
test charge increase or decrease?
A: PE , V
C: PE ,
V
E: None of these.
B: PE , V
D: PE , V
A: PE , V
B: PE , V
C: PE , V
D: PE , V
E: None of these.
Hint:
EPE = qDV
©University of Colorado, Boulder (2008)
©University of Colorado, Boulder (2008)
Flashlights, circuits, batteries, and power
Electrostatics Summary
• Positive and negative charge: Like charges repel, opposites attract
+
+
=
• Coulombs law for point charges: F = k qA qB
r2
Force acts along line joining particles
• Given batteries, light bulbs, and wire, how can we design a light bulb circuit
a) that will burn brightest,
b) that will last longer,
c) that will be dim,
d) that will turn on and off.
• How can you control and predict current and power in light bulbs?
• All this basic circuit stuff applies to home wiring, home electronics, heaters etc.
• Thursday lecture … help save lives … physics of dangers of electrocution.
• Voltage: Determines EPE of charge at that location in space
Close to + charges voltage is more + and vice versa
Grounded object is at 0V
(Also called Electric Potential)
• EPE: = qDV
New form of potential energy
Lots of analogies to GPE (DVDh, qm)
Builds on electrostatics (like charges repel, opposite charges attract, voltage, EPE)
………but now electrons are moving…………………
………. need to start thinking like an electron!
light bulb circuit: Wiring
light bulb circuit
What will happen when hook up positive end of battery (+) to
flashlight bulb with one wire?
a. light up b. barely light up c. not light up
• Start by looking at a really simple circuit containing
- light bulb
- Battery
- Wires
• Each element made of metal containing
electrons that
are free to move
• What are the electrons doing in each
element to
ensure that the light bulb lights up?
-
+
+
Using each of these three elements can you draw a picture where the bulb lights up?
6
See this in action!
http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc
7