Physics: Electrostatics and Circuit Elements Flashcards
Circuit - rarely tested!! And if do it’s not scary
Charge of elementary particle (e)?
Charge is quantized what does this mean?
Avogadro’s number?
1.6*10^-19 C BUT USE:
2 * 10^-19 C
q = n(+/- e)
n -> 0,1,2,etc
6 * 10^23
Equation for force b/w 2 charged particles
What is value of k?
F = k∣q1q2∣/r^2
Do VECTOR sum, can’t just add
absolute value of q1q2
k = 9 * 10^9 (N*m^2)/C^2
Attractive forces are___positive/negative
Attractive forces are negative! And repulsive charges are positive
We have an electric field whether or not another charge is present
How is the direction of the arrows/electric field determined
Determined by pretending that there is pos text charge put in field
So electric fields point away from pos source charges and toward neg source charged
Electric field equation?
How does F related to E equation?
E = k∣Q∣/r^2 literally the same as F if take the test charge away
F = qE -> q is the additional charge thrown in there that feels the F of the source charge and if plug in equation for E you get the normal big force equation
In a dipole, arrows point from pos/neg to pos/neg
Pos to neg -> just imagine where test charge would go
Describe where the electric field is with conductors
In conductors with neg charge, all the e- rush to the outside of object bc e- want to be as far from each other as possible so no electric field inside the conductor
no matter if conductor is hollow or solid sphere
*If an object moves “with nature” (i.e. in the direction that gravity points), then potential energy increases/decreases. If an object moves “against nature,” then potential energy increases/decreases
Using the ΔPE equation, since we want it to be neg/pos, negative charges naturally move toward regions of higher/lower potential and vice versa for positive charges
We want negative PE (thus natural movement is positive work)
If an object moves “with nature” (i.e. in the direction that gravity points), then potential energy decreases (neg). If an object moves “against nature,” then potential energy increases (pos)
The sign of W is determined by if you move in the same direction as the force applied
Using the ΔPE equation, since we want it to be neg, negative charges naturally move toward regions of higher potential and vice versa for positive charges
Equation for electric potential
and units
Meter stick!
V = (kQ)/r (almost same as E equation except r instead of r^2)
scalar value
J/C called a volt (V)
How does electric potential compare to electric field?
Is the electric field the same at every point a distance r from Q?
electric potential -> scalar, equipotential lines
since scalar can simply add components up (don’t need to add vectors like for force and electric field)
electric field -> vector, direction matters
The electric field is not the same at every point bc of direction differences, but magnitude is same -> unlike electric potential equipotential lines
Change in electrical potential energy
and what do you plug into ΔV?
ΔPE = qΔV
for V can plug in (kQ)/r
the change in potential energy of a charge q that moves between two points whose potential difference is ΔV is just given by the product ΔVq it can also be expressed as qV where V is voltage
A charge experiences no change in potential energy when its initial and final positions are at the same potential
*All charged particles naturally move to positions of higher/lower potential energy
To accomplish this, looking at the ΔPE equation, positively charged particles naturally tend toward higher/lower potential and negatively charged particles tend toward higher/lower potential
*All charged particles naturally move to positions of lower potential energy
To accomplish this, looking at the ΔPE equation, positively charged particles naturally tend toward lower potential and negatively charged particles tend toward higher potential
Work done by electric field? What about ΔKE?
W = -ΔPE (we have seen this equation before for W = -ΔPEgravity) KE = -ΔPE (we have seen this equation before!)
1eV = ____J
1.610^-19J but can round to 210^-19J (same value as e)
Equation for current and units
What created a current?
When question asked for intensity, what is it asking for?
The “quit” formula
I = ΔQ/Δt makes total sense bc its the movement of charge over time
Unit: C/s or A
even a small fraction of a current can KILL you (not voltage)
Since we know negative charges naturally drift to regions of higher electric potential, e- would be induced to drift to the right if the right end of the wire were maintained at a higher potential than the left end
intensity of light (resistor) is in reference to power it dissipates
Equation for current
Even a small amount of current to kill you
I = Q/t
If there is a voltage, that means there is a ____. What creates a current?
What is electromotive force (emf)?
potential difference
Need a potential difference/voltage for e- want to move somewhere, bc as discussed in a previous flashcard, e- move toward higher potential
emf is not really a force, its the voltage that drives movement of e- (current)
If there is a voltage, that means there is a ____. What creates a current?
What is electromotive force (emf)?
potential difference
Need a potential difference/voltage for e- want to move somewhere, bc as discussed in a previous flashcard, e- move toward higher potential
emf is not really a force, its the voltage and the same as V that drives movement of e- (current) -> can be denoted V or E (a fancy-looking e)
Ohm’s law? unit for R? I? V?
V=IR
I -> A
R -> ohm
V -> V
Equation for resistance
The "replay" formula R = (row*L)/A row -> resistivity of material L -> length A -> cross sectional area
How to get Req in parallel and in series
Req in series just add it up
Req in parallel -> 1/Req = 1/R1 + 1/R2 +…
How do the battery ends relate to each other (what do long and short ends mean)? In what direction does currently actually flow, and then what is the real convention?
Long end is the positive terminal and denotes higher potential
The short end it the neg end and denotes the lower potential
e- will drift to higher potential so from neg to pos terminal so the direction of current should be this way BUT the direction of current is taken to be the direction that positive charge carriers would flow, even though the actual charge carriers that do flow might be negatively charged.
Actual direction of e- -> neg to pos
Convention -> pos to neg
What is kept the same for series and parallel circuits?
“Working backward” technique
Series -> current same
Parallel -> voltage same
technique:
going back to series combination -> bring back I
going back to parallel -> bring V
What does it mean to dissipate heat? Rate of dissipating heat equation?
When current passes through a resistor it dissipates heat. The rate at which it dissipates heat energy is the power dissipated by the resistor
P=I^2R
(or could just get this equation by plugging in V=IR into P=IV
The power dissipated by all the resistors is equal to the power supplied by the battery
The total power dissipated by the resistors and absorbed by other voltage sources (i.e. the total power used by the circuit) is equal to the power supplied to the circuit by the highest-voltage power source
energy equation that relates to power? How do you solve for energy dissipated?
solve for P then plug into
energy = P*t
What is the intensity of light proportional to?
The intensity of light is directly related to the power the resistor dissipates
Equation for charge on a capacitor and units?
Q=CV so C = Q/V and unit is C/V or F (farads)
C -> capacitance
2 Equations for charge on a capacitor and units?
Q=CV so C = Q/V and unit is C/V or F (farads)
C -> capacitance
C = e(A/d)
DONT NEED TO KNOW e CONSTANT
2 Equations for charge on a capacitor and units?
Q=CV so C = Q/V and unit is C/V or F (farads)
C -> capacitance
C = e(A/d)
DONT NEED TO KNOW e CONSTANT (permittivity of free space)
Formula for E between capacitor plates
E is the same at ANY point b/w the plates
Units for E
“Ed’s” formula
V=Ed
Units V/m or N/C (from equation F=qE)
2 Equations for charge on a capacitor and units?
Think “QVC shopping”
Q=CV so C = Q/V and unit is C/V or F (farads)
C -> capacitance
*V IS VOLTAGE DIFFERENCE B/W TWO PLATES OF THE CAPACITOR, NOT NECESSARILY THE VOLTAGE OF A BATTERY IN THE CIRCUIT
C = e(A/d)
DONT NEED TO KNOW e CONSTANT (permittivity of free space)
Formula for E between capacitor plates
E is the same at ANY point b/w the plates
Units for E
Capacitor has 2 main uses
“Ed’s” formula
V=Ed
Units V/m or N/C (from equation F=qE)
Now have 2 methods for calculating electric field Point charges -> E = kQ/r^2 Capacitor -> E = V/d (Ed's formula switched around)
Capacitor
a) create uniform E
b) store potential energy
Equations for electrical potential energy stored in a capacitor?
Teacher mentioned using certain equations for graph, PE, and work
PE = (1/2)QV
then can plug in Q = CV for rest of the equations and get
PE = (1/2)QV = (1/2)CV^2 = Q^2/2C
first equation for area under graph = PE, second for PE, third for work
What is a dielectric and what does it do? equation?
What is K for air and vacuum? (but we don’t really care bc usually in air)
Slab of insulating material that polarizes in an electric field -> polarization created an induced electric field within the dielectric that opposes that external electric field -> if Enet drops, so does V, which means that if the capacitor is connected to a battery, the battery will have a greater voltage than the capacitor and so can move more charge onto it
Cwith dielectric
= K * Cwithout electric
= Ke0(A/d)
That bottom equation is literally plugging in C equation we already know into this equation
K for air and vacuum is 1
Capacitor is series and parallel
Opposite of what you do with resistors so ex for parallel you do C = C1 + C2 + …
How to calculate net E field when dielectric is put in there
E - Einduced by dielectric
E induced will be opposite of the normal E bc the dielectric will polarize in there and the pos ends attracted to neg side of capacitor, etc so it will produce its own electric field in the opposite direction of OG electric field
extra If capacitor charged by a battery and then disconnected from it, what happens to Q, V, PE, C when disconnected? ehh read bottom of 336 for more, can finish section on dielectrics 338 -340
When not connected to battery Q the same
How does E compare to V?
E is vector so direction matters (need vector sum with pos and neg values)! but V scalar to always just add things up without a vector sum, literally just add it up
How does E compare to V?
Once you have a CHANGE in potential, you have voltage
E is vector so direction matters (need vector sum with pos and neg values)! but V scalar to always just add things up without a vector sum, literally just add it up
*Sources of voltage (battery) are/are not sources of charge/current
Sources of voltage (battery) ARE NOT not sources of charge/current
How do Q and V respond depends on the particular situation:
Disconnect battery, then insert dielectric
Insert dielectric while still connected to the battery
Disconnect battery, then insert dielectric -> Q = CV Q same C increases so... V decreases (battery supplies V)
Insert dielectric while still connected to the battery Q = CV Q increases C increases V same (battery supplies V)
