Static Electricity
Static Electricity
Parts of the Atom cause Charge
The protons and electrons are the charged parts of an atom. Neutron are neutral
Electric Charge is an electrical property or matter that creates electric and magnetic forces
The word “electric” comes from the Greek word “elektron”, meaning “amber”. The ancient Greeks discovered static electricity by rubbing amber with fur.
Protons ‐
→ Positive ( + )
Proton
+
0 0
+
Electrons → Negative ( ‐ )
Amber
Neutrons → Neutral ( 0 )
Neutron
‐
Static electricity means electricity that “stays put”
Electron
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2
Conductors vs. Insulators
Electrical Charge of an Atom
Conductors allow charges to flow, Insulators do not allow charges to flow.
Atoms can gain or lose electrons. Electrical charges come from the imbalance of protons and electrons
Conductor
More protons than electrons → Positive ( + )
More electrons than protons
→
Negative ( ‐ )
Equal protons and electrons
→
Neutral ( 0 )
Insulator
A conductor will uniformly distribute charges.
An insulator only has rubbed areas charged.
3
Sending a Charge to Ground
4
Charging an Object
“Grounding” can remove charges from an object. This can prevent static sparks from occurring.
To Ground means charges are sent to the ground d
h
b bi
or a separate conductor that can absorb it. This is often used for safety when handling fuel, electronics, or dealing with lightning.
An object can be charged by rubbing off electrons from one object onto another.
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1
Static Electricity
Materials that gain a positive (+) electrical charge
(or tend to give up electrons)
Dry human skin
Greatest tendency to giving up electrons and becoming highly positive (+) in charge
Leather
Rabbit fur
Fur is often used to create static electricity Glass
The glass on your TV screen gets charged and collects dust Human hair
"Flyaway hair" is a good example of having a moderate positive (+) charge Nylon
Wool
Lead
A surprise that lead would collect as much static electricity as cat fur
Cat fur
Silk
Aluminum
Gives up some electrons Paper
Materials that are relatively neutral
Cotton
Steel
Best for non‐static clothes Not useful for static electricity
Materials that gain a nega ve (−) electrical charge
(Tend to attract electrons)
Attracts some electrons, but is Wood
almost neutral
Amber
Some combs are made of hard Hard rubber
rubber Copper brushes used in Nickel, Copper
Wimshurst electrostatic generator
Brass, Silver
It is surprising that these metals Gold, Platinum
attract electrons almost as much as polyester Polyester
Clothes have static cling
Clothes have static cling Packing material seems to stick Styrene (Styrofoam)
to everything You can see how Saran Wrap will Saran Wrap
stick to things Polyurethane Pull Scotch Tape off surface and Polyethylene (like Scotch Tape)
it will become charged Polypropylene
Many electrons will collect on Vinyl (PVC)
PVC surface Silicon
Greatest tendency of gathering electrons on its surface and Teflon
becoming highly nega ve (−) in charge
Charged and Neutral Objects Attract
Attraction of Charged Objects
Oppositely charged objects attract
Like charged objects repel
Determining Charged Objects
+‐
‐
+
‐
‐ ‐‐ + ‐
‐ ‐‐
‐ + ‐
‐
+ ‐
+ ‐
+‐
A charged balloon can cause wood molecules to be polarized so it can stick to a wall.
At least one is charged
Both may be charged
Both must be charged
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Charge by Conduction
Charge by Induction
When a negative rod touches a neutral doorknob, electrons move from the rod to the doorknob
When a negative rod approaches a neutral doorknob, electrons in the doorknob are repelled causing an unequal distribution of charge. The transfer of electrons to the metal doorknob gives the doorknob a net negative charge
2
Static Electricity
Charge by Induction
Pointed objects hold a greater concentration of charge.
More charge is contained in a tiny region
Sharing of Charge
Different Sizes of Spheres
Spheres of equal size will share charges equally
Spheres of equal size will share charges unequally.
Charges will spread out until there is no electric potential difference between them.
An Electroscope measures if there is a charge.
Electric Fields Near Conductors
A solid sphere has charges evenly spread out on the surface
A hollow sphere has charges entirely on the outer surface
Charges will be closest together at sharper points
3
Static Electricity
Coulomb: Unit for Charge
Coulomb’s Law – Electric Force
Electric forces can push and pull objects. This is responsible for “static cling”
The SI unit for charge is the Coulomb (C)
1 Coulomb = charge of 6.24 x 1018 electrons or protons
F K
A lightning bolt can carry 5 C to 25 C of charge.
elementary charge – the smallest increment of charge A single electron has a charge of ‐1.60 x 10‐19 C A single proton has a charge of +1.60 x 10‐19 C
q A qB
r2
F = Force → Newtons (N)
K = Coulomb's Constant = 9 x 109 N∙m2 / C2
q = charge → Coulombs (C)
r = distance between charged particles (m)
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Forces on Charged Bodies
Electric Force is Similar to Gravity
Attract
Electric Force
Gravity Force
Fe 
Fg 
k q1 q2
r2
Repel
G m1 m2
r2
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Electric Field Lines
Charged Objects Attract Water
B
a
l
l
o
o
n
Could this make a Halloween mask?
W
a
t
e
r
A stream of falling water can be moved by a charged object This happens because water molecules are polar
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4
Static Electricity
Electric Field Strength
Think of gravity acceleration, “g”
E
q
F
K 2
r
q'
EE = Electric field strength → N / C
= Electric field strength → N / C
F = Force on q’ → N
q’ = positive test charge
q = charge creating the field being tested The test charge (q’) must be very small so the effect on charge being tested (q) is negligible!!
Electric Field Lines
Electric Field Lines
+
positive charges point outward.
negative charges point inward.
The number of electric field lines drawn indicates the relative strength for each charge
Both charges are equal in strength
Like charges repel
‐
Like charges repel
Opposite charges attract
The positive charge is twice as strong
Opposite charges attract
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Static Electricity
Electric Potential Difference Electrical Potential Energy and Relative Position
V – The work done moving a positive test charge between two points in an electric field divided by the magnitude of the charge.
V 
Work on Charge (PE) Fd
q

K
Charge
q'
r
Electric Potential Difference is measured in:
Volt = 1 Joule / Coulomb
d = distance the test charge is moved (m)
F = Force on test charge (N)
q’ = test charge (C)
Electric Potential Difference
Equipotential
When the test charge is moved perpendicular to the electric field force acting on it, no work is performed.
The change in electric potential energy is zero. If this occurs at two or more positions it is at equipotential.
Move unlike charges apart
V increases
Move unlike charges closer
V decreases
Electric Potential Difference in a Uniform Field
An Electric Field Between Parallel Plates
V  Ed
E = Electric Field Strength (J/C)
d = distance the charge is moved
A uniform electric field occurs between two oppositely charge flat plates.
V = Ed
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Static Electricity
Millikan's Oil Drop Experiment
This found the charge of an electron to be 1.60 x 10‐19
Millikan’s Oil‐Drop Experiment
Oil Drop
C
Millikan sprayed oil droplets that were charged due to friction from an atomizer (spray bottle)
+
Charging the plates caused oil droplets to rise. When a specific charge suspended a droplet between the two plates, electrical force was equal to the force of gravity.
+
Millikan found that the charges of drop were all multiples of 1.60 x 10‐19 C. This meant the smallest possible quantity of charge was the charge of a single electron. –
Capacitors
Capacitors are used to store charge and can discharge in fractions of a second. Capacitors can supply an instant charge to meet the power demand of device when a battery cannot discharge fast enough
discharge fast enough. Symbol
(Insulator)
Examples include:
Powering a camera flash
Music Amplifiers (Especially for bass notes)
TV (operate cathode ray tube)
High power lasers Capacitance
Capacitance is measure in Farads (F).
1 Farad = 1 coulomb per 1 Volt
1 Farad is huge. Many capacitors are rated in microFarads (F) which is 1 millionth (10‐6) of a Farad
C
q
V
Electric Potential Difference is measured in:
V = Electric Potential Difference q = charge (Coulombs)
Larger capacitors have higher capacitance because charges can be spread further apart 7