Transcript Slide 1

Electrostatics
Section 1: Intro to Static Electricity
• Electrostatics- Physics that deals with the
attractions and repulsions of electrical
charges not dependent on their motion.
(Electricity at rest)
• Electrical forces arise from charged
particles in the atoms.
• What are the charged particles called?
- ____________
+ ____________
• Electrical forces arise from charged
particles in the atoms.
• Proton (+)
• Electron (-)
• Charge- The fundamental electrical
property to which mutual attractions or
repulsions between electrons or protons is
attributed.
• Neutral atoms contain equal numbers
of positive protons and negative
electrons. (net 0 charge)
• Only the electrons move to create
unbalanced charges.
• When atoms lose electrons they become
positively charged ions.
11p
12 n
Becomes
11p
12 n
Neutral Sodium (Na)
Positive Sodium Ion (Na+)
11 protons(+) and 11 electrons(-)
11 protons(+) and 10 electrons(-)
• When atoms gain electrons they become
negatively charged ions.
17 p
18 n
Becomes
17 p
18 n
Neutral Chlorine (Cl)
Negative Chlorine Ion (Cl-)
17 protons(+) and 17 electrons(-)
17 protons(+) and 18 electrons(-)
• Electrical charges are conserved!
– When one atom becomes a positive ion
another one/few must have accepted those
electrons and become equally negative
• Ex sodium is +1 because it gave its extra electron
to chlorine
11p
12 n
17 p
18 n
Becomes Positive Sodium Ion (Na+)
Becomes Negative Chlorine Ion (Cl-)
11 protons(+) and 10 electrons(-)
17 protons(+) and 18 electrons(-)
• Interaction between charges
– Like charges repel and opposite charges
attract.
• Static Electricity - Electricity at rest
• Electric charges that can be confined to an
object
I hate static
electricity
Some materials have a
greater affinity for electrons
• Greater affinity for e- : stick to
electrons more and tend to gain electrons
becoming negative
• Less affinity for e- : don’t hold electrons
as tight and are more likely to loose
electrons become positive
• Friction can cause charge
separation
• Electrons are stripped from
one material and added to
the other when rubbed
together
Charging by Friction
• A wool cloth does not have much affinity
for electrons.
• Becomes Positive
• PVC becomes negative
Activity 1
List some examples of charging by friction
When have you noticed static electricity or attraction of one
object to another
• Static cling from dryer (cotton socks w/
nylon pants)
• Balloon rubbed in hair
• Walking on carpet
Attraction for electrons
(Hold electrons tightly)
Most likely to gain electrons and
become negative
(Hold electrons loosely)
Most likely to loose electrons and
become positive)
PVC
Rubber
Cotton
Paper
Silk
Fur
Wool
Nylon
Hair
Acetate
Glass
Question Set 1
1. A girl pulls a wool cap off her head. What charge will
be produced:
a) on her hair?
b) on her cap?
2. Which will produce the most static cling with a
cotton t-shirt in a dryer. Wool socks or a nylon
nightgown?
3. Can there be static cling if only cotton items are
placed in a dryer?
4. Predict the charges on the underlined objects:
a) A rubber rod rubbed with fur
b) A glass test tube rubbed with silk
c) A PVC pipe rubbed with nylon
Question Set 1
1. A girl pulls a wool cap off her head. What charge will
be produced:
a) on her hair?
positive
b) on her cap?
negative
2. Which will produce the most static cling with a
cotton t-shirt in a dryer. Wool socks or a nylon
nightgown?
Nylon
3. Can there be static cling if only cotton items are
placed in a dryer?
No
4. Predict the charges on the underlined objects:
a) A rubber rod rubbed with fur
negative
b) A glass test tube rubbed with silk
positive
c) A PVC pipe rubbed with nylon
negative
Conductor:
• Material through which electrons move freely
• Examples (gold, silver, copper, and aluminum)
• The general rule is that good thermo conductors
are good electric conductors
• Metals tend to share electrons in electron
clouds
• electrons are free to move around making
them better conductors.
Electrical
Insulator
Electrical
Conductor
Insulator:
• Material through which electrons do
not freely move
• Examples: rubber, paper, plastic, air
Grounding
• Removing a static charge by
producing a path to the ground
• Electrons move from a negatively
charged objects to the ground until
the object is neutral
• Electrons move from ground to
neutralize positively charged
objects
• The earth both accepts and gives
electrons while remaining overall
neutral
Grounding wand
for Van De Graaff
generator
Grounding
It’s easy to ground conductors since
electrons transfer readily
It’s hard to ground insulators since charges
don’t move away easily
Section 2: Charging Objects
Three Ways of putting a charge on an object
1. Friction
2. Induction
3. Conduction
1. Charging by Friction
• Charging by rubbing objects that have
different affinities for electrons together
Induction (charging without contact)
1. Bring a charged object (rod) close to a
neutral one (ball) without contact
+ +
- + +
- + +
- -
+
+
+
-
Induction (charging without contact)
1. Bring a charged object (rod) close to a
neutral one (ball) without contact
2. The electrons in the ball will be repelled
leaving a positive side
- +- -- +- -+-- ++ +
- - -- -
+
+
+
-
Induction (charging without contact)
1. Bring a charged object (rod) close to a
neutral one (ball) without contact
2. The electrons in the ball will be repelled
leaving a positive side
- +- - + +
- -+- + +
+ + +
---
3. The now positive sided ball with be
attracted to the negative rod
Induction (charging without contact)
• Induction is only a temporary change without
contact therefore electrons are not transferred
• The charge induced is opposite
• Take away the rod and a neutral charge will
return
- +- -- +- -+-- ++ +
- - -- -
+
+
+
-
Conduction (charging with contact)
• Conduction is a more permanent change with
contact; electrons are transferred and then
isolated.
• Charge conducted is the same
+ +
- + +
- + +
- -
+- - -+-- -+-- -
• After conduction the ball and rod will repel each
other
Make this table in your notes
Conduction
Induction
Contact?
Contact
No Contact
Permanent?
Permanent
Temporary
Charge vs.
Charging Device
Same
Opposite
Section 3: Coulomb’s Law
Electric Charge
•
•
•
Symbol is Q or q
The MKS unit is the coulomb (C)
1 C = the charge on 6.25 x 1018 electrons
Extra info to help you with problems
• 1 electron = 1.60 x 10-19 C
• A coulomb is a huge charge. Static charge is
usually stated in µC which is 1x10-6 C.
Magnitude of force
3 factors affecting the magnitude of the force
between two charged objects:
1. Charge on the objects
2. Distance between objects
3. Material separating objects
Coulomb’s Law
•
•
•
•
•
•
F: electrical force
Q1: charge 1
Q2: charge 2
d: distance between charges
k: constant depending on materials separating objects
For air, k = 8.99 x 109 N·m2/C2
When using this equation:
• A positive force (F) signifies repulsion
– Both charges (Qs) must be positive or both
negative
• A negative force (F) signifies attraction
– One charge (Q1 or Q2) must be positive and
the other negative
Example 1
a. What is the electrostatic force between
two objects, +13 μC and -22 μC which are
0.055m apart (μC = x 10-6 C)
b. Is it an attraction or a repulsion?
Example 1
a. What is the electrostatic force between
two objects, +13 μC and -22 μC which are
0.055m apart (μC = x 10-6 C)
b. Is it an attraction or a repulsion?
Attraction (Q1 and Q2 are opposite signs)
el
• Coulomb’s Law is similar to Newton’s Law of Gravity
Similarities:
– They both are used to calculate a field force
– Both forces have an inverse square relationship to distance
– They are both related by a constant
Differences:
– Force of gravity is always attractive
– Electrostatic force can be either attractive or repulsive
– Gravities constant is very small since gravity is a very weak force
– Fg relates force created by a masses, Fel relates force created by
charges
el
• Both electric and gravitational forces are
field forces because objects do not have to
touch to be subjected to the force.
Section 4: Electrical Fields
• Electrical Field (E): an area of electrical
influence around a charged object.
• Variable E
• Unit: newton per coulomb (N/C)
Drawing electrical fields• Arrows point away from the positive and
toward the negative
• In the direction a positive charge would
travel in the field
• Spacing of lines show field strength
Drawing electrical fields• Arrows point away from the positive and
toward the negative
• In the direction a positive charge would
travel in the field
• Spacing of lines show field strength
This is what it would be seen if you
used iron filings to see the field
Common point charge examples
Electric field between two parallel plates
• All charge lies on the surface of a
conductor
• Electrical field inside a conductor is zero
E = 0 inside
conductor
True or false: A cars tires protect
you from being struck by lightning
False: Electrical shielding does
• The metal outside the car gives the car a
path to the ground
Shielding
• Here is more proof of shielding
Section 5: Voltage
I.
• Work must be done on a
positive charge to move it
away from a negative
sphere.
• The electric PE of the charge
will increase
• When the charge is
released, it will move closer
to the negative sphere. Its
electric PE will decrease and
work can be done by the
charge.
II.
• Work is required to push the
small + charge against the
electric field around the +
sphere.
• Since work is done on the
small charge, its PE
increases.
• The closer it gets, the more it
is repelled by the field and the
more work is required.
III.
• Once the little charge is placed on
the sphere, the charge on the
sphere increases, and the field
around it becomes stronger.
• Moving the next + charge toward
the sphere will take even more work
and give the small charge more PE.
Potential Difference (Electric Potential)
• Potential difference (or electrical potential) is
work done as a single charge is moved in an
electric field.
• Unit is the volt
1V = 1J/1C
• Potential difference is measured in volts and
commonly called voltage (V).
• Which positive charge has more potential
energy?
• Which positive charge has more potential
energy?
Its closer and therefore
has a greater repulsion
Activity 2
• Which rock has more potential energy?
• Which rock has a greater potential
difference?
A
B
• Both rocks have the same potential
difference (potential energy per charge)
but
• Rock B has more charges and therefore
more potential energy (PE)
A
B
• Electric potential is not the same as
electrical potential energy. Electric
potential is electric potential energy per
charge.
Potential Energy
Electric Potential
(PE of one charge)
(PE Total)
Section 6: Electric Storage and Discharge
Electrical Energy Storage
• Capacitor- simple device used to store electrical
energy. The simplest form is a pair of
conducting plates separated by a small distance.
• The plates hole equal and opposite charges
• The electrical energy in a capacitor comes from
the work done to charge it.
These capacitors consist of
thin metal foils rolled up into
a cylinder
Electric Discharge
• Discharge occurs when the electric field
around a conductor becomes so strong.
The air is ionized helping the charge make
a break for the ground.
Arc Discharge
• Arc- a rapid discharge producing heat,
light, and sound.
Lightning
1
2
Storm clouds cause a separation of
charges trough updrafts and downdrafts.
The bottoms of clouds become negative.
3
The negative bottom of the clouds induces
the top of the ground to become positive
Moist air is ionized by the strong
electrical field creating a conducting
path. Lightning is the arc formed.
Facts about lightning
• Charges are separated in storm clouds; +
top and – bottom
• Ground under the cloud becomes
positively charged by induction
• V = millions of volts; causes arc discharge
with tremendous energy.
Corona Discharge
• Corona- a slow discharge of static
electricity from a pointed conductor
Lightning Rods• Prevent strike by allowing
induced charge to leak off
building in a corona
discharge
• Provide a path to the
ground in case of a strike