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Hint

1

The SI units for electrical PE are…

joules

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2

What are the accepted symbols for electric potential energy?

U

PE

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3

How would you** increase the electrical potential energy** of a pair of charged particles of the **SAME sign**?

decrease their separation

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4

How would you **decrease the electrical potential energy** of a pair of** OPPOSITELY charged** particles?

decrease their separation

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5

How would you **decrease the electrical potential energy** of a pair of charged particles of the **SAME sign**?

increase their separation

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6

How would you **increase the electrical potential energy** of a pair of** OPPOSITELY charged** particles?

increase their separation

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7

Equation for computing the electrical potential energy stored in a pair of charged particles

K$\frac{q1q2}{r}$

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8

Electric potential energy is a (scalar/vector) quantity

scalar

It may increase or decrease, but it has no compass direction.

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9

When charged particles are released and can move freely, their electric potential energy will be converted to…

kinetic energy

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10

Units for electric potential

volts

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11

Is electric potential a scalar or vector

scalar

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12

volts in terms of joules/coulombs

1 V = 1 J/C

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13

Electric fields point in the direction that ___ charges freely move when released.

positive

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14

Electric fields point towards locations of ___ potential.

lower

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15

Which way do positively charged particles freely move towards?

direction of lower potential

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16

Which way do negatively charged particles freely move towards?

direction of higher potential

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17

Locations of higher electric potential are _____ positively charged bodies than locations of lower potential

closer to

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18

What are **equipotential surface**s?

locations of the same potential in an electric field

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19

Equipotential surfaces are ___ to electric field vectors

perpendicular

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20

If a location in an electric field has a **potential of 10 V**, it means that **1 coulomb of charge** would ________ at this location

possess 10 J of potential energy

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21

Is potential constant in an uniform electric field?

no

potential changes as you move from one position to another. an electric field is a potential gradient.

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22

If a charged particle moves freely in an electric field, its PE…

decreases

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23

How does the capacitance of a capacitor increase?

increase area (make plates bigger)

decrease r (decrease separation of plates)

use more polar dielectric

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24

The places of capacitors are (insulators/conductors)

conductors

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25

Dielectric is a (insulator/conductor)

insulator

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26

What do capacitors do?

provide a quick burst of energy in automated external defibrillators

provide a quick burst of current in a computer keyboard

maintain a uniform electric field in air purifiers

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27

How would you increased the energy stored in a capacitor?

charge it with a higher voltage battery

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28

Units for capacitance

farads (F)

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29

Equation for finding sum of capacitors in **series**

$\frac{1}{Ceq}$ = $\frac{1}{C1}$ +$\frac{1}{C2}$ + … $\frac{1}{Cn}$

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30

Equation for finding sum of capacitors in **parallel**

Ceq = C1 + C2 + … Cn

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31

Characteristics of equipotential surfaces/lines/contours

perpendicular to electric field lines

areas where electric potential is the same, regardless of object’s charge

concentric spheres around a single source charge

equipotential contours can never meet or cross

contours are closer = more electric potential energy

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32

test charges seek (higher/lower) energy states

lower

(think of a ball rolling down an incline)

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33

The electric potential energy of interaction of two charged objects is defined to be zero when the **distance **between them is ____

infinitely far away

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34

If two objects have the same sign charges, their electric potential energy is (positive/negative)

positive

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35

If two objects have opposite charges, their electric potential energy is (positive/negative)

negative

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36

If two oppositely charged objects move closer together, their electric potential bar will (increase/decrease) in size

increase

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37

if the Ue between two equal charges quadruples, what happens to the distance between the particles?

distance decreases by a factor of 4 → 1/4

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38

When is the equation ∆Ue = -qEdcosθ used?

when a charge, q, moves freely in an electric field

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39

Electric potential definition

the electric potential at a point is defined as the amount of electric potential energy a positive test charge would have if placed there, the energy per unit charge

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40

Equation for electric potential, V

V = Uq/q

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41

How to calculate electric potential of multiple point charges?

add the V values together

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42

Compare electric potential and electric field

electric field is force per charge

electric potential is energy per charge

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43

Potential difference (∆V)

voltage, the difference between the electric potential a the final location minus the electric potential at the initial location

∆V = Vf-Vi

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44

What does a negative ∆V signify?

decrease in V (high → low)

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45

What does a positive ∆V signify?

increase in V (low → high)

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46

Describe the force on a charged particle placed anywhere in an uniform field

the force is constant everywhere

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47

What is the equation ∆V = -Edcosθ used for?

the charge of 1 Coulomb of charge experiences when it is displaced a distance “d“ in a uniform electric field. θ is the angle between the E-field and the displacement of charge.

The minus sign indicates E pointing in the direction of decreasing potential

at 0°, E and d are in the same direction

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48

Will the electric field inside the dielectric be zero?

No.

(The electric charges do not flow through the dielectric as they do in a conductor (dielectrics are insulators). The electric field can be reduced, but cannot be minimized to zero. In addition, the dielectric constant k cannot be zero since it is a measure of a material’s ability to store electric energy in an electric field.)

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49

How is k, the dielectric constant, related to E?

as k increases, E (net E-field) weakens

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50

What is capacitance?

how much charge accumulates on 1 plate per volt applied across the plates

C = Q/V

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51

___ plates are the most common type of capacitor

parallel

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52

what is ε

8.85 × 10^-12 F/m

permittivity of free space

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53

for parallel capacitors, what occurs when the dielectric is inserted into the capacitor?

capacitance increases

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54

What happens to the charge on a parallel plate capacitor is the potential difference doubles?

capacitance stays the same, Q doubles

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55

Do uncharged parallel plates still have a capacitance?

yes

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56

Why is Ue for a capacitor referred to as stored energy?

energy is stored in the electric field between the capacitor’s plates. as the capacitor is being charged, the electric field builds up.

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57

Which quantities (Q, C, or Ue) change when using a higher voltage power supply to charge a capacitor?

Q, Ue

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58

A parallel plate capacitor is charged and then disconnected from a battery. how does the stored energy change when the plate separation is doubled?

Q and V stay the same. Energy doubles

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59

why is it dangerous to touch the terminals of a high-voltage capacitance even after the potential difference has been removed?

because capacitors still hold charge

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60

What does discharging do?

make the capacitor safe to handle after potential difference has been removed.

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61

Why do two metal plates near each other not become charged until connected to a source of potential difference?

Both plates start neutral. If there is no potential difference, there is no electric force to set charges into uniform motion. By providing voltage, there is a potential difference, allowing for motion.

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62

What does the area under the curve of a current vs. time graph represent?

total charge placed on capacitor

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63

How are capacitors charged?

electrons flow from negative terminal of battery to empty capacitor plate → repel electrons from other plate → repelled electrons flow back to battery

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64

Describe a graph that depicts **charge vs. time **as a capacitor is being charged

Starts at zero, increases, slows down until it reaches potential difference (looks like a log. graph)

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65

Describe a graph that depicts **potential (v) vs. time **as a capacitor is being charged

Starts at zero, increases, slows down until it reaches potential difference (looks like a log. function)

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66

Describe a graph that depicts **current vs. time **as a capacitor is being charged

really high, decreases as potential get closer to battery’s potential (looks like exponential decay function)

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67

Describe a graph that depicts **current vs. time **as a capacitor is being discharged

really high → decreases to zero point when it is fully discharged so there is no more current (looks like exponential decay function)

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68

Describe a graph that depicts **charge vs. time **as a capacitor is being discharged

rate of decrease proportional to current (starts high → decreases)

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69

Describe a graph that depicts **potential (v) vs. time **as a capacitor is being discharged

rate of decrease proportional to current (starts high → decreases)

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70

Would capacitors connected in parallel or in series have a greater time constant (time it takes to discharge a capacitor)?

capacitors in parallel

(t = RC, R = resistance. you want a higher C)

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71

Characteristics of Q and V for capacitors in series

Q is equal

V is split

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72

Characteristics of Q and V for capacitors in parallel

V’s are equal

Q is split

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73

What could you change in a circuit to increase the charge stored in a capacitor?

increase potential difference

remove all the other capacitors

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74

What happens to the resistor over time when the switch is closed?

the current starts high → slowly becomes lover → zero

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75

What happens to the potential difference across a resistor when the switch is open and after the switch is closed?

voltage starts high → plummets quickly → zero

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76

What happens to the potential difference across a capacitor when the switch is open and after the switch is closed?

voltage starts low → rises gradually

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77

What does a capacitor act as when fully charged?

an open switch

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78

What is steady state?

everything in equilibrium, capacitor is fully charged

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79

What factors affect the amount of time required for the circuit to reach steady state?

capacitance, resistance

increasing capacitance → amount of charge increases → takes longer to reach steady state

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80

What happens qualitatively if the switch was opened after the switch had reached steady state?

nothing happens because it is in equilibrium

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81

What quantities at steady state would be different if ∆V across the battery was increased?

charge, current, time needed to reach steady state, ∆V

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82

What quantities at steady state would be different if the resistance of the resistor was increased?

current, time (increases)

(∆V does not change, therefore Q does not change)

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83

Current takes the path of ___ resistance

least

(will typically go through capacitors first → capacitors are fully charged → go through resistors)

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84

How to calculate current

I = ∆V/R

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