Chapter 17 Flashcards
When a battery is connected to a capacitor why do the tow plates acquire charges of the same magnitude, will this be true if the two plates are different sizes or shapes
The two plates will have equal and opposite charges since energy and charge are conserved through conservation of energy and electric charge. When one plate loses electric it becomes positive charges and the other plate gains that same number of electrons. Charge will flow until the capacitor is full
A conducting sphere carries a charge Q and a second identical conducting sphere is neutral. The two are initially isolated, but then they are placed in contact. What can you say about the potential of each when they are in contacts, will the charge flow from one to the other, if so how much
They will have the same voltage (potential) bc they are in contacts and the surface is take to be the same potential. Since they are the same size they will each have half the original charge but the total charge must remain the same
The parallel plates of an isolated capacitor carry opposite charges Q. If the separation of the plates increases is a force required to do so, is the potential difference changed, what happens to the world done in the pulling process
A force is required to increase the separation of the plates bc you are pulling a positive plate away from a negative plate. The work done in increasing this separation goes into increasing the electric potential energy stored between the plates, the capacitance decreases and the potential voltage between the plates increases since the charge Q must remain constant (assume you can mechanically do this)
If the electric field E is uniform in region what can you infer about the electric potential V. If V is uniform in a region of space what can you infer about E… DIAGRAM
The potential voltage is either increasing or decreasing uniformly depending on the charge, it could be uniform or zero
Is the electric potential energy of two isolated unlike charges positive or negative. What’s about two like charges. What is the significance of the sign of the potential energy in each case
The electric potential energy of the unlike opposite charges I negative if we take the zero location for potential energy to be when the charges are infinitely far apart. The electric potential energy of two like charges is positive. In the case of opposite charges, work must be done to separate the charges. In the case of the same charges work must be done to separate charges. Work must be done to try to move the charges together (think of two strong magnets)
If the voltage across a fixed capacitor is doubled the amount of energy it stores doubles, halved, quadruples, unaffected, none of these explain which one it is
Quadrupled because voltage is squared in formula so double it and it is quadrupled. If you double the voltage you will have 4 times the energy stored
How does the energy stored in a capacitor change when a dielectric is inserted if the capacitor is isolated so Q doesn’t change, the capacitor remains connected to a battery so V doesn’t change, explain
When the charge is conserved and a dielectric is added to a capacitor the capacitance C will increase. To fulfill the law of conservation of charge the voltage must decrease to compensate for the increased capacitance
We have seem that the capacitance C de[pends on the size and shape of the two conductors as well as on the dielectric constant K. What then did we mean when we said that C is constant
Once the capacitance is set the capacitor will have a given value and you cannot change its rating
A +.2 mC charge is in an electric field what happens if that change is replaced by a +.4 mC charge
The electric potential stays the same but the electric potential energy doubles, the electric potential is the voltage
Two identical positive charges are placed near each other at the point halfway between the two charges
The electric field is zero adn teh potential is positive
Four identical point charges are arranged at the corners of a square the electric field E and potential V at the center of the square are
E=0 and V does not = 0
If teh field at a particular point is zero teh potential at that point must be zero true or fals
True
If it takes an amount of work W to move +q point charges from infinity to a distance d apart from each other. Then how much work should it take to move three +q point charges from. Infinity to a distance d apart from each other
W+2W=3W
A proton Q=+e and an electron Q=-q are in a constant electric field created by oppositely charged plates. You release the proton from near teh positive plate and teh electron from near the negative plate. Which feels the larger electric force
The electron bc not the same mass but the same charge
What doesn’t affect the capacitance
The charge on the plates and the energy stored in teh capacitor
A battery est a VoLTE V on a parallel plate capacitor. After the batter is disconnected the distance between teh a plates is doubled without loss of charge. Accordingly the capacitance _____ and teh voltage between teh plates _____
Decreases, increases
Which is the vector
Electric field
A +.2 mC charge is in an electric field. What happens if that charge is replaced by a -.2 mC charge
Teh electric potential stays the same but the electric potential energy changed sign
If two points are at the same potential does this mean that no net work is done in moving a test charge from one point to the other? Does this imply that no force must be exerted
The net work is zero if there is no change in position or there could be positive and negative work that cancel each other, you can apply a froce and not do any work bc you aren’t moving teh object .Yes, since there s no displacement from the original position. (You could have positive work for half the path and negative work for the second half which would result in zero net work). No, force can be applied. Force is a push or pull that may or may not change the motion of a body.
If a negative charge is initially at rest in an electric field will it move toward a reigion of higher potential or lower potential what about a positive charge how does the potential energy of teh charge change in each instance DIAGRAM
A negative charge will move from low potential (voltage) to high potential, decrease its potential energy (U) increase its Kinetic energy (KE). A positive charge will move from high potential (voltage) to low potential (voltage) decrease its potential energy and increase its kinetic energy. Total energy in any process is conserved.
State clearly the difference between electric potential and electric field, between electric potential and electric potential energy
Electric potential is electric potential/ charge and is a scalar quantity. Electric field is force/unit charge and is a vector quantity. Electric potential energy is the work done against the electric force in moving a charge from a specific location of zero potential energy to some other location. Electric potential is the electric energy per unit charge. (Energy and work are in joules (J)). Look at formulas as well
An electron is accelerated from retest by a potential difference of .2V how much greater would its final speed be if it is accelerated with four times as much voltage
The u of the electron is proportional to the voltage used to accelerate it. If the voltage is multiplied by a factor of 4, then the potential is also increased by a factor of 4. Since conservation of energy rules every situation, all of the U is converted to KE. The KE has increased by a factor of 4 also. Since the speed is proportional to the square root of KE, the speed must increase by a factor of 2.
Is there a point along the line joining to equal positive charges where the electric field is zero. Where teh electric potential is zero
Potential is high near the positive voltage is high at the positive, two negative charges between team the electric field would be zero at the midpoint because equal and opposite electric potential would be zero at infinity and at the midpoint voltage would be zero if it were a dipole
Can a particle ever move from a region of low electric potential to one of high potential and yet have its electric potential energy decrease
Yes that is what teh negative charge is doing
Can two equipotential lines cross
No because each equidash lines are perpendicular to teh electric field lines that cannot cross electric field lines are made up to show a specific magnitude and direction of a charged particle at some point, since equpotential lines ar pe erpendicular to electric field lines they cannot cross. If they are to cross you would have different potienials and voltage at the same point
The potential difference between points a and b is
The work required to move a unit charge from a to b
True or false the electric field is a vector adn potential is scalar
True
When a negative charge moves from a point of higher potential to lowere potential it
Gains potential energy
A volt is the same as
Joules/coulumbs
Doe equipotential lines begin or end
No bc they are scalar
True or false for equipotential lines there are different lines at different places in a drawing may not represne tthe same potential
False they are equipotential
Are equipotential lines perpendicular to electric field t
Yes
Is the potential on equipotential lines the same at any point
Yes
Potential energy per unit charge is
Electric potential
When a positive charge moves form a region of high potential to one of low potential
It just lose potential energy
Volt is
Potential difference
The potential 10 cm from a point charge is 60V at a distacne of 20cm the potential is
30V bc it is over the radius so less
When a charged particle moves in an opposite direction to teh indluec of teh electric field
It must gain potential energy and lose kinetic energy
The ptrocut of the charge times the potential difference through whcih it moves gives
The change in the charges potential energy
A +q moves how between two charged plates
From high V to low V decreasing potential energy and increasing kinetic energy
A -q moves how between charged plates
From low V to high V decreasing potential energy a dn increasing kinteic energy but this applies if it starts ont eh negative side of a charged plate if on the positive side then it will decrease kinetic energy and increase potential energy
A charge q experiences a displacement within an electric field from position A to B the change int ehe electric potential energy is delta Ue and teh work done by the electric field during this displacement is We then
Vb-Va=deltaUe/q
True or false if teh electric field a t a certain point is zero then teh electric potential at the same point is also zero
False
True or false if the electric potentials at a certain point is zero then the electric field at the same point is also zero
False
Tue of false The electric potential is inversely proportional to the strength of teh electric field
False
The froce on and electron located on the 0V potential line is
Directed to teh left but hte magnitude cannot be determined without knowing the distacne between the lines
Four positive charges of magnitude +q are arranged on the corners of a square, at the center C of the square the potential due to one charge alone is V0 and the electric field due to one charge alone has a magnitude E0 what is the electric potential and the magnitude of the electric field at the center of the square due to all four charges
Electric potential is 4V bc it is scalar so can add them all up bc all positive, and the electric field is zero bc all them point outwards
Consider a dipole I=with a +q ont eh left and a -q on teh right at how many points in the place of the page ar ethe electric field and the electric potential both equal to zero
0
A solid metal object is isolated fro other charges and has charge distributed on its surface teh charge distribution is not uniform so
The electric field outside the object but very close to the surface is equal to teh surface charge density at any location divided by the permityivyt of free space
Valid units for the electric field include
N/c;j/c x m; v/m
If teh electric field does negative work on a negative charge as the charge undergoes a displacement from position a to b within an electric fiel then teh electrical potential energy
Increases
Negative charges are accelerated by electric field toward points
At higher electric potential
Teh total capacitance of several capacitors in parallel is the sum of the individual capacitances for
The charge on each capacitor depends on its capacitance but eh potential difference acros is the same
What voltage would have to be applied to produce euqal magnitude charges ont eh plates of all three capacitors
It doesn’t matter bc it is the Same charge across and V is the same throughout
