Electrostatics Flashcards
what is electrostatics?
Electrostatics is the study of forces between charges, as described by Coulomb’s Law. We develop the concept of an electric field surrounding charges.
mutual attraction of earth and moon
eaths attraction tugs on moon and moon tugs on the earth
how is universal law of gravitiy similar to coloums law
coloubms law uses charges not masses – force exists between 2 objects
k like g is a constant
Repulsion or attraction
positive + positive –>
neg + neg means –>
positive + neg –>
changing sign for force changes attraction
positive + positive –> repulsion
neg + neg means –> repulsuion
positive + neg –> attraction
What is Coulombs law?
Coulomb’s law describes the strength of the electrostatic force (attraction or repulsion) between two charged objects. The electrostatic force is equal to the charge of object 1 times the charge of object 2, divided by the distance between the objects squared, all times the Coulomb constant (k).
Fe is the magnitude of the electrostatic force
K is columbs constant (electrostatic const)
what is an electric field?
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles. Charged particles exert attractive forces on each other when their charges are opposite, and repulsion forces on each other when their charges are the same.
equation of electric field?
electric field is the force per q charge
Fe is the magnitude of the force felt by test charge q
Q is the source of magnitude
ratio of force per unit charge is constant
- double charge doubles the force it experiences
- note the interconversion
where does an electric field come from?
Divide coloubms law by q to get energy =
describe charges
positive charge small q is repelled by big Q positive charge
describe charges
positive charge small q is attracted by big Q neg charge
how do we represent a strong electric field?
with more lines - more density is stronger field
what happens if we go further from the nucleus?
decrease in force
As you go farther from the nucleus, electrons at higher levels have more energy, and their energy increases by a fixed, discrete amount. Electrons can jump from a lower to the next higher energy level if they absorb this amount of energy
What is electric potential energy?
electric potential energy is the work needed to move a test charge from infinity to a point in space in an electric field with a source charge
U = W = F x r in Joules
U = kQq/r in Joules
radius of charge from center
Force
all the variables that influence force influence energy
electric potential energy of two close positive charges and two close negative charges?
High positive PE
for both alike charges ( 2 pos, 2 neg)
the closer they are the less stable because they repel
as they move apart the magnitude of electric potential energy becomes greater and greater positive number
electric potential energy of opposite charges?
large Neg PE
the closer they are, the more stable because they attract
energy is increasingly negative as they are brought closer => which is more stable
why is PE neg for opposite charges?
as r increases PE is bigger in magnitude –> more neg and it drops
-25 is colder than -5 (analogy)
What is electric potential?
aka voltage
you can predict potential energy of second charge given first charge
external work needed to bring a charge from one location to another location in an electric field
What does voltage (electric potential) depend on?
bigger Q (source) has greater voltage
greater distance increases voltage
How does potential energy change as little q is futher from big q
half as much energy each energy level
desribe potential energy effect on pos big Q and neg little q
energy increases when you seperate opposite charges
only need -100 J when we are further than when closer we need -200 J ==> little q gains energy
unit of volts
J/C
how does floursence work with electric fields?
uv light high energy shines on uranium bumps which increases an electrons energy and moves it away from nucleus
high energy is unstable
electron emits photons to rid of energy
photos are in visible range =-> green
What are the 4 electrostatic equations?
medical applications of electric charge
cardioversion and defibrillation create strong electrical current through hearts conductive system to resychronize a pulse
contrast gravity and electrostatic force in terms of attraction
gravity - attraction of 2 bodies
electrosatic force - depends on charge
what is the coloumn and unit
e = 1.60 x 10^-19 C
C units
proton and electron charge in C
proton = q = +e
electron = q = -e
compare proton and electron charge and mass
same magnitude of charge but not sign
proton has greater mass than electron
describe an insulator
will not distribute charge over its surface and will not transfer that charge to another neutral object well or to another insulator
electrons are closely linked with nuclei
most nonmetals are insulators
describe a conductor
when a conductor is given a charge the charges will distribute approximately evenly upon the surface. They can transfer and transport charges and are often used in circuits or electrochemical cells.
Nuclei surrounded by a sea of free electrons that rapidly move through the material and are loosely associated with positive charges
usually metals
also ionic electrolye solutions
do neutrons contribute to charge?
nope. 1 C of electrons and 3 moles of neutrons will have a net charge only of 1 C of electrons
electrons = -e = -1.6 X 10^-19 C
when placed one meter from another, which will experience a greater acceleration: 1 C of electrons or 1 C of protons
F = ma
Force is the same for e and p
but e have much smaller mass so will experience greater acceleration
which are conductors and which are insulators?
blood, hair, copper, glass, iron, sulfuric acid, distilled water
conductors: copper, iron, sulfuric acid, blood
insulators: hair, distilled water, glass
how can you tell direction of force?
like repel
opposites attract
force points along the line connecting the center of the two charges
what happens when a pos and neg charge distance is doubled – how does force change?
distance doubles, square of distance is quadrupled, force is reduced to 1/4 of original
distance not important
What is the convention of electric field lines
how a positive charge would move in presence of a source charge ( given as pos or neg)
How do net electrical fields work?
every charge exerts its own electric field, a collection of charges will exert an electric field at a point in space that is equal to the vector sum of all electric fields
what is the electric field midpoint of two negative charges?
0 because charges are the same and they cancel out –> no field
same for 2 positive charges
what charge creates the electric field?
source charge creates electric field
electric potential energy increases when..
electric potential energy decreases when..
increase when two like charges move closer
increase when two opposite charges move apart
decrease when two like charges move apart
decrease when two opposite charges move towards eachother
what does a decrease in electrical potential energy indicate about system?
if like charges decrease electrical potential energy then..
decrease means its more stable
-4J to -7J means more negative which means more stable
if like charges decrease electrical potential energy then.. the distance increases bc they repell and are more stable that way so PE dec which is good
if opp charges with increase electrical potential energy means distance increases bc they attract and are more stable that way so PE is inc which is bad
Contrast electric potential energy in magnitude and sign
otential energy increases in magnitude if it gets further away from zero. This is equivalent to the force between the two charges becoming stronger - regardless of whether it is attractive or repulsive. PE increases in sign if it becomes more positive (and decreases if it becomes more negative). This is equivalent to the force between the two charges becoming less attractive and more repulsive.
Full newtowns first law
F = G * (Mm/r^2) ==> attractions between 2 masses
