Physics: Electrostatics and Circuit Elements

Question 1

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?

Answer 1

1.6*10^-19 C BUT USE:
2 * 10^-19 C

q = n(+/- e)
n -> 0,1,2,etc

6 * 10^23

Question 2

Equation for force b/w 2 charged particles

What is value of k?

Answer 2

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

Question 3

Attractive forces are___positive/negative

Answer 3

Attractive forces are negative! And repulsive charges are positive

Question 4

We have an electric field whether or not another charge is present
How is the direction of the arrows/electric field determined

Answer 4

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

Question 5

Electric field equation?

How does F related to E equation?

Answer 5

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

Question 6

In a dipole, arrows point from pos/neg to pos/neg

Answer 6

Pos to neg -> just imagine where test charge would go

Question 7

Describe where the electric field is with conductors

Answer 7

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

Question 8

*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

Answer 8

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

Question 9

Equation for electric potential

and units

Answer 9

Meter stick!
V = (kQ)/r (almost same as E equation except r instead of r^2)
scalar value

J/C called a volt (V)

Question 10

How does electric potential compare to electric field?

Is the electric field the same at every point a distance r from Q?

Answer 10

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

Question 11

Change in electrical potential energy

and what do you plug into ΔV?

Answer 11

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

Question 12

*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

Answer 12

*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

Question 13

Work done by electric field? What about ΔKE?

Answer 13

W = -ΔPE (we have seen this equation before for 
W = -ΔPEgravity)  
KE = -ΔPE (we have seen this equation before!)

Question 14

1eV = ____J

Answer 14

1.610^-19J but can round to 210^-19J (same value as e)

Question 15

Equation for current and units

What created a current?

When question asked for intensity, what is it asking for?

Answer 15

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

Question 16

Equation for current

Even a small amount of current to kill you

Answer 16

I = Q/t

Question 17

If there is a voltage, that means there is a ____. What creates a current?
What is electromotive force (emf)?

Answer 17

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)

Question 18

If there is a voltage, that means there is a ____. What creates a current?
What is electromotive force (emf)?

Answer 18

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)

Question 19

Ohm’s law? unit for R? I? V?

Answer 19

V=IR
I -> A
R -> ohm
V -> V

Question 20

Equation for resistance

Answer 20

The "replay" formula 
R = (row*L)/A
row -> resistivity of material 
L -> length
A -> cross sectional area

Question 21

How to get Req in parallel and in series

Answer 21

Req in series just add it up

Req in parallel -> 1/Req = 1/R1 + 1/R2 +…

Question 22

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?

Answer 22

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

Question 23

What is kept the same for series and parallel circuits?

“Working backward” technique

Answer 23

Series -> current same
Parallel -> voltage same

technique:
going back to series combination -> bring back I
going back to parallel -> bring V

Question 24

What does it mean to dissipate heat? Rate of dissipating heat equation?

Answer 24

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

Question 25

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?

Answer 25

solve for P then plug into

energy = P*t

Question 26

What is the intensity of light proportional to?

Answer 26

The intensity of light is directly related to the power the resistor dissipates

Question 27

Equation for charge on a capacitor and units?

Answer 27

Q=CV so C = Q/V and unit is C/V or F (farads)

C -> capacitance

Question 28

2 Equations for charge on a capacitor and units?

Answer 28

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

Question 29

2 Equations for charge on a capacitor and units?

Answer 29

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)

Question 30

Formula for E between capacitor plates
E is the same at ANY point b/w the plates
Units for E

Answer 30

“Ed’s” formula
V=Ed
Units V/m or N/C (from equation F=qE)

Question 31

2 Equations for charge on a capacitor and units?

Answer 31

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)

Question 32

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

Answer 32

“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

Question 33

Equations for electrical potential energy stored in a capacitor?
Teacher mentioned using certain equations for graph, PE, and work

Answer 33

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

Question 34

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)

Answer 34

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

Question 35

Capacitor is series and parallel

Answer 35

Opposite of what you do with resistors so ex for parallel you do C = C1 + C2 + …

Question 36

How to calculate net E field when dielectric is put in there

Answer 36

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

Question 37

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

Answer 37

When not connected to battery Q the same

Question 38

How does E compare to V?

Answer 38

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

Question 39

How does E compare to V?

Once you have a CHANGE in potential, you have voltage

Answer 39

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

Question 40

*Sources of voltage (battery) are/are not sources of charge/current

Answer 40

Sources of voltage (battery) ARE NOT not sources of charge/current

Question 41

How do Q and V respond depends on the particular situation:
Disconnect battery, then insert dielectric
Insert dielectric while still connected to the battery

Answer 41

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)