EK Physics Ch4 Electricity Flashcards
1 Coulombs =
6.24 X 10^18 e
e =
1.60 X 10^-19 Coulombs
coulomb’s law
- purpose is to predict what is the electrostatic force of attraction and repulsion of two charges! it states if I have two charges: so q1 and q2, distance btw two charges is r
- law states that the force/magnitude of force can be repulsion force or attractive force, telling us direction of force, but magnitude of electrostatic force is proportional to product of magnitudes of the charges
- Fe= k X [q1q2] absolute value of each/ r2
- just like newton’s law of gravitation, proportional to product of two masses and inversely proportional to distance! same thing!
- electrostatic force at close range much stronger, happening at atomic level or scale much stronger what we are used to operating at but there is a parallel* patterns in universe
insulators and conductors similarities
- usually either an insulating material or conducting material!
- similarities= both are composed of a huge number of atoms adn moelcules, these atoms and molecules are composed of a positively charged NUCLEUS
- NEGATIVELY CHARGED swarm of electrons that swarm that nucleus
- for both positively charged nuc cannot move! can wiggle from thermal vibrations cannot travel freely throughout material as long as solid, if fluid can move and migrate around, but for solid positively charged nuc is fixed
conductor vs insulators
difference
- negatively charged electrons can move- there are electrons in conductor can move about relatively freely with almost no resistance
- vs. insulators electrons cannot mvoe around freely, do not have the right energy levels or bands to move around freely, for insulators everything is basically stuck! e can jump around its own atom or share with another, but cant jump around to others
conductrs and insulators hooked up to battery…
- conductors e could start migraating down teh line
- but in an insulator electrons are sttuck
- so for electrical materials all we care about are conductors, and insulators use if dont want electrical intearction
- not totally true if set up insulator to battery, even though eelctrons cannot jump from atom to atom, can shift hte nucleus and cloud of electrons so get one side of atom more positive one side of atom more negative, set up so positive shifted from negative if get this material to do it creates overall electrical effect cna itneract with charges nearby and interact electrically even if charges cannot flow through insulator!
if add bunch of negative charges to insulator, what would happen. well since charges cannot move through insulator…
charges cannot flow through insualtor so stuck, can charge whole thing uniformally if i wanted to, spread out throughout the whole thing or bunched up on one side
charges are stuck
vs. conductor, if put extra negative charge on teh side of conductor, dont have to stay there if they dont want to, negatives repel each other like charges next to each other will not liek it, so one negative will try to get as far away from other negative as it can! cant jump off conductor that takes a lot more energy but can go to very edge, so thats what charges do for conductors, solid conducting material, extra charge on it all will reside on outside edge whether added extra negative or positive, always on otuside edge for conductor! can only add charge to outside edge for a conductor, if not on outside edge will quickly find its way to the edge becuase repel eachother!
how do you add a positive? take away a negative! if start off with amterial just as many positives or negative, like adding charge, take away negative, always resides on otuside edge of ocnductor becuase charges try to get away from each other as much as possible
what are good insulators?
wood
glass
most plasics
all can disrtibute charge and charge cannot flow through it, can stick charge on otuside edge and just stays there!
good conductors….
metals like gold, copper used becuase cheaper than gold, or any other metal silver works great
materials were charges can flow freely through them
if had two conducting rods made out of metal…
one net amount of negative cahrge, resides on otuside edge of conductor, other metal conducting rod has no charge on it, what would happen?
well charges want to get away from eachother as far as possible, so if some go onto the new rod adn some stay here can spread out even further so that is what would happen; if second rod bigger more would go onto second one becuase allows them to spread out even more
can charge something by induction!
- this means bring charged rod near other piece of metal but dont touch it! well the engatives and positives in the other rod can move want to move as far away as possible form negative charges from new rod, so net amount of negative charges mvoe to the opposite side of one rid to get away from new rod to laeave total amoutn of charge on side near rod, deficet of electrons
- e spread out
- now these positives are closer to the negatives, so these postivies in the rod are attracting negatives* and the negatives repel the other negatives, dont cancel causes the rod to be attraced to the other rod
- means if took charged rod and brought it to empty soda can and bring rod close can will start moving toward rod
how you charge by induction
- example of two rods isnt charged by induction, charged by induction is when you take rod and stick in ground or connect it to ground, palce where can gain/steal/take infinitly amount of electrons or deposit electrons infinite number and ground would not care, like metal of your car can supply lots of electrons or take them thing will not notice or care
- so now if bring metal rod with originally no net charge electrons now leave when attache dto ground, so then your rod is no longer uncharged has net amount of charge, not all going to leave still some electrons, but the rod that used to be uncharged now has a net amount of positive charge in it, charged this rod without even touching it because let negative electrons leave** if clever can cut the wire before take away thing that induced the charge, if now remove it far away, electrons negatives would have come back to rod, but cant rejoin positive becuase wire snapped, now stuck no way for these electrons to get back because you have cut the cord
- calld charged by induction quick way to do it!
so with ballon and the wall…..
- take a balloon, charge it up by rubbing it against hair steels electrons becomes negatively charged
- take it and put near wall or ceiling, if lucky sticks there- seiling is an insulating materials electrons arent being transfered but atom can reorient or poalrize! negatives in atom can shift to one side other side becomes mor epositive, causing net force btw ceiling and ballon because positives are a little closer to negatives in ballon
- negatives and positives attracting, greater force than negatives repeling other engatives in ceiling
- ballon also insulator made of rubber
- ceilign attracting ballon and ballon attracting ceiling, so ballon can stick because of insulating material’s ability to polarize and cause electric attraction, can interact with somethign electric because atom can shift and polarize!
electric field
- allows us to imagine that somehow the charge is affecting space around it in some way, creates field that whenever put another charge in that field can predict how field will impact charge
- C’s law= force btw two charges =kq1Q2/r2
- r=distance, can be d
- E= kQ2(whatever charge is creating field) / r^2 or d^2
charge is
scalar
electric field is a
vector quantity! becuase vector quantity divided by scalar quantity charged, is vector quantiy!
electric field at a point
F = qE
units= N/C makes sense becuase Force divided by charge
The units of electric field are N/C. In this sense, the electric field is the force per unit charge that will be experienced by a charged particle entering the field. This implies an important relationship between electric fields and forces:
F = qE
In this case we are relating the vectors for the electric field and the resulting force, not just their magnitudes.
electric field 3
For a variety of reasons, it is often simpler to phrase problems in electrostatics in terms of electric fields rather than electric forces. An electric force is always between two charges, whereas an electric field emanates from a single charge. We say that a given charge creates an electric field and, whenever another charge enters this field, it will experience a force. At this point, there is no formal difference between the two descriptions; it is only a matter of convenience. The electric field of a point charge is:
E = kr q2
where q is the charge creating the electric field.
how to draw an electric field
Graphically, the effects of an electric field can be represented using field lines. Field lines point in the direction in which the force on a positive charge would be directed. In the picture below, a positive charge has field lines pointing away from it:
positive charge will accelerate outward at an every slowing rate! when really close to positive charge its very strong, as get far away field becomes weaker and weaker, radially outward! goes straight out away from charged cue, called electric field lines
This indicates that another positive charge would be repelled. Of course, a negative charge would experience a force in the opposite direction. The density of field lines represents the strength of the electric field. As we’d expect, the closer you get to the positive charge, the denser the field lines become. further lines go from center of positive charge, shorter vectors get!
Imagine we have a sphere that is negatively charged. The electric field would show that …..
that an imaginary positively charged particle is pulled towards the sphere by the electric force. The electric field would always point towards the sphere, because we always use an imaginary positively charged particle to determine the electric field. As we move away from the sphere, the electric field gets weaker and weaker.
electric potential energy
Electric potential energy is the energy that is needed to move a charge against an electric field. You need more energy to move a charge further in the electric field, but also more energy to move it through a stronger electric field.
Imagine that you have a huge negatively charged plate, with a little positively charged particle stuck to it through the electric force. There’s an electric field around the plate that’s pulling all positively charged objects toward it (while pushing other negatively charged objects away).
You take the positive particle, and start to pull it off the plate, against the pull of the electric field. It’s hard work, because the electric force is pulling them together. If you let the positive particle go, it would snap back to the negative plate, pulled by the electric force. The energy that you used to move the particle away from the plate is stored in the particle as electrical potential energy. It is the potential that the particle has to move when it’s let go.
If you pulled the positive particle further away from the plate, you would have to use more energy, so the charge would have more electrical potential energy stored in it. If we doubled the charge on the plate, again, you would need more energy to move the positive particle. If we doubled the charge on the positive particle, you would need more energy to move it. You get the idea.
Imagine that instead of a negatively charged plate, our plate is positively charged. Our positive particle would be pushed away from the plate since they are both positively charged. This time, we have to put in energy to try to move the particle closer to the plate, instead of to pull it away. The closer we try to move it to the plate, the more energy we have to put in, so the more electrical potential energy the particle would have.
what is electric potential?
The electric potential, or voltage, is the difference in potential energy per unit charge between two locations in an electric field. When we talked about electric field, we chose a location and then asked what the electric force would do to an imaginary positively charged particle if we put one there. To find the electrical potential at a chosen spot, we ask how much the electrical potential energy of an imaginary positively charged particle would change if we moved it there. Just like when we talked about electric field, we don’t actually have to place a positively charged particle at our chosen spot to know how much electrical potential energy it would have.
What if our plate was positively charged?
A positively charged particle would be pushed away from the plate. This is the exact opposite of the last case. Near the plate the electrical potential is high and far from the plate the electrical potential is low.
Let’s say we have a negatively charged plate.
We know that a positively charged particle will be pulled towards it. That means we know that if we choose a spot near the plate to place our imaginary positively charged particle, it would have a little bit of electrical potential energy, and if we choose a spot further away, our imaginary positively charged particle would have more electrical potential energy. So we can say that near the negative plate the electrical potential is low, and further from the negative plate the electrical potential is high.
cell membranes- electrical potential
The membranes that surround your cells are comprised of thin layers of molecules that stick together to form a continuous, two-dimensional sheet. The sheet is held together because the molecules that form the membrane have special distributions of electric charges that allow them to stick together without dissolving in the water surrounding the cell.
Because the membrane is held together by the attraction of opposite charges, it is possible to overcome this attraction by applying a large electric potential across the membrane.
In some cells, applied electric potentials are used to open and close the cell membrane in order to allow nutrients and waste to enter and exit the cell. In nerve cells, the electric potential across the membrane can be easily changed, allowing the cells to carry messages encoded in their membrane potential.
electric potential energy vs electric potential
electrical potential energy is assocaited with a charge. its associated with a particle that has some charge, only that particle has some eenrgy. its how much total work is needed to mvoe it from one point to another, the difference in X J, associated with a particle
vs.
electric potential which is assocaited with a positioN!
can figure out electric potentil energy by multiply electric potential X charge. how much work per unit charge does it take to move any charge per unit charge from point A to point B, this is indepdent on size of particle, depends on position so would be liek 12 J divided by 2 C, = 6 J/C which is the same thing as 6 volts!
voltage
regardless of how small or big, what the difference in potential enegy would be if at two different points, so comparing points in space!
electric potential V- abstract number associated with points in space
how do electrons move through
Electrons actually move very slowly through direct current (DC) electric circuits. Remember that DC is the simple circuit you get when you connect something like a battery to a lightbulb to make a flashlight: the transfer of energy between the battery and the bulb is due to the kinetic energy of the electrons that move through the wires of the circuit.
After MgCl2 dissolves in a neutrally charged solvent, what is the net charge of the solution?
=The solution remains neutrally charged.
The law of conservation of charge states that the net charge of an isolated system remains constant.
When the ions are added to a neutrally charged solvent like water, the overall solution remains the same.
MgCl2 dissolves into one +2 cation and two -1 anions.
The only way to change the net charge of a system is to introduce a charge from elsewhere, or to remove a charge from the system. In this closed system, the resulting Mg2+ cation and Cl- anions will yield an overall neutral charge.
In the figure below, spheres q1 ,q2 , and q3 have the same positive charge magnitude. Which of the following is a possible direction for the net electric field vector on q4?
The superposition principle means that the net electric field vector is the sum of all individual electric field vector arrows made by each charge’s interaction with the charge being affected.
If q1 and q2 have the same positive charge magnitude, then their electric field vector arrows should cancel out in the horizontal direction. This will occur whether q4 has a positive or negative charge.
Since there is no other charge to influence the direction of the net electric field vector in the horizontal direction, the correct vector direction should not be angled toward the left or the right. An arrow that points straight up or straight down are possible directions for the net electric field vector.
In five seconds, a point charge moves from position A to position B as shown in the diagram below. Which one of the following two paths require the most energy?
The amount of time is not relevant to this problem.
In electrostatics, energy is determined by product of the point charge magnitude, the displacement of the point charge, and the value of the electric field, for a constant field in the direction of displacement:
W=Fd
W=qEd
When the displacement is the same for both paths of a point charge along a constant electric field, the energy expended is equivalent.
so the answer is that both paths require an equal amount of energy b/c work is independent of the path
dielectric 5
The presence of a dielectric in the gap of a parallel-plate capacitor increases the total capacitance.
The dielectric constant, k, is a property of the specific dielectric being used; it indicates how much the capacitance increases when a given dielectric is used. The higher the dielectric constant, the better a material functions as an insulator—for example, rubber has a very high dielectric constant, and so it is often used a protective coating around high voltage wires because its high k makes it a very poor conductor.
Common dielectrics include wood (k = 2), beeswax (k = 4), glass (k = 5), and plastic (k = 5). The capacitors inside consumer electronics like your computer usually use plastic as a dielectric because of its low cost and high dielectric constant value, allowing the capacitors to be made really small.
The presence of a dielectric increases the capacitance because it actually decreases the electric field inside the capacitor by a factor of the dielectric constant. When all of the atoms in a dielectric polarize, they end up creating a field that points in the opposite direction to the applied field, resulting in a smaller field electric field: look on image
The presence of a dielectric between the two plates actually decreases the electric field inside the capacitor. This should tell you a lot about the way that sticking dielectrics in capacitors messes with their properties: capacitors store energy within their electric fields, and so changing that field changes everything!
dielectic ex 1
For example, imagine that you’ve charged up your favorite parallel plate capacitor to a voltage of 6V across the plates. This 6V difference corresponds to an electric field between the plates, which is the voltage divided by the distance between the plates, d. If you have a huge capacitor with a plate separation of 1 cm, that’s an electric field of 6 V/cm. But imagine that you stick a big chunk of beeswax (k = 3) into your capacitor after it’s been fully charged and disconnected from the battery. All of the sudden, the charges on the molecules in the beeswax will shift and orient themselves in such a way that they partly cancel out the original field. Because the total amount of polarization is related to the dielectric constant, your capacitor now has an internal field of only 6/3 = 2 V/cm. Since the plate separation is the same, this corresponds to a new capacitor voltage of only 2V!
