physics final rdhsbjkdhgb

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[…] are more likely to be transferred when the plastic rod is rubbed

[…] are more likely to be transferred when the plastic rod is rubbed

Electrons (valence electrons - on outer layer)

Identical objects rubber with identical materials […] each other

repel

Electric interactions are [the same as/different from] magnetic interactions

different from

Methods of charging

• friction

• conduction

• induction

• grounding

Friction

two neutral objects acquire opposite charge by rubbing against each other

Conduction

charged object directly transfers some excess charge to another object

• objects touch → charges move

• one object is initially charged

When charges are transferring between objects they [can/cannot] occur in any amount

cannot

• electric charge only comes in multiples of e (charge of an electron). charge is quantized, meaning it only occurs in particular amounts

• e = -1.6 × 10^-19 C

is 4.0 × 10^-19 C a possible electric charge?

no

• divide value by e

• 2.5 charge transferred, not possible

is -0.00072 C a possible electric charge?

yes

• 4.5×10^15 charge transferred, possible

Conductor

• electrons are not bound and can move freely

• can transfer electrons

• excess charge resides on outer surface

Non-Conductors

• electrons bound to protons and cannot move freely

  • can only move around proton

• cannot transfer electrons

• weaker attraction and repulsion than conductors

Induction

• a charged object is brought near a neutral object

• neutral object becomes charges

Grounding

path for excess charge to flow to ground

Water is a […]

conductor

• polar molecule: has positive and negative end

• humidity: tiny water molecules in air → act as conductor → distracting electrons by surrounding charged object

• people are ~60% water, making them conductors

k =

9×10^9 Nm²/C²

Coulomb’s Law: F_q =

k q₁q₂/r²

Imagine 2 charged balls are hanging from threads. Predict what will happen tot he angle between the strings and the vertical.

Increasing the charge on the balls

increases the angle

Imagine 2 charged balls are hanging from threads. Predict what will happen tot he angle between the strings and the vertical.

Increasing the mass of the balls

decreases the angle

Imagine 2 charged balls are hanging from threads. Predict what will happen tot he angle between the strings and the vertical.

Increasing the string length

decreases the angle

source charge

object that created change in its surrounding space

electric field

change in surrounding space created by source charge; are vectors

test charges

object that interacts with electric field

• assume they’re positive and do not have fields

force and field

field allows force to be exerted as distance

E =

F_q /q, q - charge in field

k q₂/r² , q₂ - source charge (only for a point charge; one source charge and one test charge)

Electric field (E-field) lines

• represents electric field

• number of lines is proportional to strength of object’s charge

• can never cross or touch other lines

• when there is a positively charged object and a negatively charged object, lines start on positively charged object

• positive charge: arrows point away from charge

• negative charge: arrows point towards charge

uniform electric field

occurs when E-field vectors and E-field lines in a given area are parallel to each other

faraday cage

• external electric field causes electric charges in cage (hollow metal box) to be distributed

  • electrons move from one part of box to another

  • part where electrons left is now positive

• distribution of charges cancel the field’s effect in the cage’s interior

  • inside, fields cancel out

Electrostatic equilibrium

• state reached by charged conductors

• when excess charge is placed on a conductor, charges spread out until they reach the lowest energy state possible

• there is no net movement of charge

For objects in electrostatic equilibrium…

1. excess charge migrates to and remains on the outer surface of conductor

2. there is no electric field inside the conductor (neutral)

3. the direction of the electric field is perpendicular to the surface of conductor

4. there is a greater surface charge density at location of higher curvature

Why are you more likely to get shocked when touching a metal object with your finger as opposed to your entire hand?

the finger is smaller and more curved, therefore there is a greater surface charge density

As you increase the potential on an irregularly shaped conductor, a bluish-purple glow called the cornea forms around a sharp end sooner than a smoother end. Why?

As the sharper ends, more curvature → higher charge density

A person rubs a neutral comb through their hair and the comb becomes negatively charged. Why?

The hair loses electrons to the comb.

Consider four charges, A, B, C, and D, which exist in a region of space. Charge A attracts B, but B repels C. Charge C repels D, and D is positively charged. What is the sign of charge A?

A is negatively charged

Two neutral spheres A and B are near each other. A negatively charged rod is brought near one of the spheres as shown. The far right side of sphere B is

negative

An electroscope is given a positive charge, causing its foil leaves to separate. When an object is brought near the top plate of the electroscope, the foils separate even further. We could conclude

that the object is positively charged.

According to Coulomb’s law, if the electric force between two charges is positive, the force between the charges is [attractive/repulsive]

The force between the charges is repulsive

What methods of charging result in two oppositely charged objects?

Friction and induction

• friction: 2 neutral objects acquire opposite charges via rubbing (one gains electrons, other loses electrons)

• induction: a charged object is brought near neutral objects, resulting in the separation of charges in the neutral objects

Identify the type of charging: you touch a charged rod to an electroscope

conduction

• charged object transfers excess charge to neutral object

Identify the type of charging: you connect a charged object to a much large conductor to neutralize the object

grounding

• connecting charged object to larger conductor, which can add or accept excess electrons

Identify the type of charging: you bring a charged object near a neutral soda can. the soda can is attracted to the charged object.

induction

pitch

perception of the frequency of a sound wave

loudness

determined by amplitude of sound wave

open end pipes

1. ends of harmonics are antinodes

2. nodes = n

3. antinode = n+1

4. wavelength = 2L/n

5. frequency = nv/2L

closed end pipes

• begins with node

• nodes = antinodes = n

• wavelength = 4L/n

  • n is odd

• frequency = nv/4L

possible harmonics

• open end pipes - any number

• closed end pipes - odd numbers

fundamental frequency

n =1 (first harmonic)

relationship with overtones: fn = f1 * n

doppler effect

Frequency of sound (pitch) heard by the observer is higher when the observer and source get closer. When observer and source are farther, frequency gets lower

beat frequency

number of "beats" heard per second

fb = f2-f1

beat

two waves completely in phase

standing wave

• stationary

• remains in constant position

wave sources

• amplitude

• frequency

• type of waves (transversal or longitudinal)

• DO NOT affect speed

wave medium

• density

• tension

• affect speed

traverse wave

disturbance and coil move perpendicular to each other; moves side to side

longitudinal wave

disturbance and coils move parallel to each other

constructive interference

when two pulses meet and they have an amplitude in the same direction → bigger wave

destructive interference

when two pulses meet and they have amplitudes in opposite directions → smaller wave

node (N)

location of minimum/no vibration

antinode (A)

location of max vibration

The SI units for electrical PE are…

joules

What are the accepted symbols for electric potential energy?

U

PE

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

decrease their separation

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

decrease their separation

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

increase their separation

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

increase their separation

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

K$\frac{q1q2}{r}$

Electric potential energy is a (scalar/vector) quantity

scalar

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

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

kinetic energy

Units for electric potential

volts

Is electric potential a scalar or vector

scalar

volts in terms of joules/coulombs

1 V = 1 J/C

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

positive

Electric fields point towards locations of ___ potential.

lower

Which way do positively charged particles freely move towards?

direction of lower potential

Which way do negatively charged particles freely move towards?

direction of higher potential

 

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

closer to

What are equipotential surfaces?

locations of the same potential in an electric field

Equipotential surfaces are ___ to electric field vectors

perpendicular

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

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.

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

decreases

How does the capacitance of a capacitor increase?

• increase area (make plates bigger)

• decrease r (decrease separation of plates)

• use more polar dielectric

The places of capacitors are (insulators/conductors)

conductors

Dielectric is a (insulator/conductor)

insulator

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

How would you increased the energy stored in a capacitor?

charge it with a higher voltage battery

Units for capacitance

farads (F)

Equation for finding sum of capacitors in series

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

Equation for finding sum of capacitors in parallel

Ceq = C1 + C2 + … Cn

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

test charges seek (higher/lower) energy states

lower

(think of a ball rolling down an incline)

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

infinitely far away

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

positive

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

negative

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

increase

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

When is the equation ∆Ue = -qEdcosθ used?

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