Physics: Circuits, Magnetism, Waves, and Sound

Question 1

What is the difference between DC and AC circuit? How do the calculations compare?

Answer 1

DC: Direct current -> one way to go, so all christmas lights go out
AC: Alternating current -> alternate paths the current can take, ex christmas lights and see only 1 bulb out bc has own path
Calculation for both is same

Question 2

The “loop rule”: The sum of voltages around any closed loop in a circuit must equal_____

Answer 2

The “loop rule”: The sum of voltages around any closed loop in a circuit must equal zero
Elements in parallel have the same voltage

Question 3

Power equations?

Answer 3

Memorize ohm’s law and one of the P formula’s below bc can plug in ohms law (V=IR) to that P equation to get the others -> memorize PIV and plug in ohms!

plug in V =IR (I = V/R)

P = IV = I^2R = V^2/R

Question 4

Power output by a battery = ______

Power dissipated across resistors = ______

Answer 4

Power output by a battery = electrical energy

P = W/t or P = Q/t

Power dissipated across resistors = Thermal energy

Question 5

In magnets, the field points from ____ to _____

Answer 5

N to S

similarly to how electric field goes from pos to neg

Question 6

Name RHRs!

Answer 6

Find the direction of Fb, use RHR
Pointer finger = velocity
Middle finger = B
Thumb = force

For a wire, thumb is current and fingers wrap around

Question 7

Equation for force of magnetic field

Under what circumstances is Fb 0?

Answer 7

Fb=qvBsinθ

B: magnetic field, units: [B] = tesla (T)
θ: angle between v and B
B = N/A
sin 90 = 1

(remember Felectric = qE and E units was N/C)

Fb = 0 when v = 0, when sinθ = 0 (v//B), when v anti-//B -> think of RHR bc you literally can’t make your pointer finger and middle finger in opposite directions and it’s weird to put them in the same direction

Question 8

Describe the motion that the B field causes the particles to move in?
How are magnetic fields used to sort particles traveling at the same velocity by charge and mass, as in mass spectrometry?

Answer 8

B field causes particles to move in circles

Fb created Fc
qvB = mv^2/r
qB = mv/r

Question 9

Magnetic fields are created by _________
The direction of the magnetic field found using another RHR for current-generated B
Like electric fields, the magnetic field at a point in space is _____ to the magnetic field line

Current is defined by the movement of _____ charges

What is magnetic field proportional to?

Answer 9

Magnetic fields are created by moving charges

Like electric fields, the magnetic field at a point in space is tangent to the magnetic field line (circular)

Current is defined by the movement of positive charges

B ∝ I/r
directly proportional to I and inversely proportional to r

Question 10

Oscillation (may see as free standing question)
Hooke’s law equation?

KE at amplitude? PE at x=0

Answer 10

F = - kx
neg bc resistive force
and don’t forget F=ma

F -> restoring force
k -> spring constant (N/m)
**x -> displacement from equilibrium
Amplitude +/-A: greatest displacement from equilibrium during oscillation

No kinetic energy at amplitude, max PE means no KE
No PE at x=0 and max KE

Question 11

PE stored in spring as it stretches/compresses:

Spring force is conservative force (like gravity), equation:

In absence of non-conservative forces, conservation of mechanical energy applies, equation:

Answer 11

PE = 1/2kx^2 = 1/2kA^2
and KE box = 1/2mv^2
can use both of these in conservation of E equation below

Wby spring = -∆PEelastic

KE + PE = KE + PE

Question 12

Questions about simple harmonic motion are mostly free standing questions
There are 4 main formulas
What is period? Frequency?

What does a full period look like when begin at equilibrium?

Answer 12

Period, T (seconds)

Time it takes to complete one cycle
One cycle = returning to the same position and velocity
constant over time
independent of amplitude

T = 2π square root of m/k

Frequency, f (Hz)
Number of cycles that occur in one second
Units: [f] =1/s =Hz
In general: f = 1/T
For springs: f = 1/2π square root k/m (can get this by combining previous equations)

Question 13

Transverse and longitudinal waves

Propagating oscillations that transfer _____ and _____

Answer 13

Propagating oscillations that transfer energy and momentum

Question 14

What value for charge can you give S2-?

Answer 14

-2e

Question 15

What do you do when asked to find the direction the particle moves when shot through a mass spectrometer (given direction of B and Vinitial)

Answer 15

Use the RHR to find the direction of force and then can tell which direction it will move in

Question 16

a) An ion of charge +q and mass m will enter the magnetic field with what speed? Write v in terms of q,m, and V.
b) What do you do when asked to find the direction the particle moves when shot through a mass spectrometer (given direction of B and Vinitial)

Answer 16

a) The ion loses electrical potential energy in the amount qV, and as a result, gains KE = 1/2mv^2. Therefore, 1/2mv^2 = qV, so v = square root 2qV/m
b) Use the RHR to find the direction of force and then can tell which direction it will move in

Question 17

What can produce a magnetic field?

Answer 17

moving charges

Question 18

How does magnetic force affect the speed of a particle? Does the magnetic force do work?

Answer 18

Magnetic force never changes the speed of the particle (constant speed, circular motion so remember those Fc and Fb equations and that you can equate them to each other to find speed, etc)
Magnetic force does NO WORK on particle

Question 19

**How do you know how much work is done by magnetic force on a charge?

Answer 19

The work done magnetic forces is always zero! The magnetic force on an object is always perpendicular to the velocity of the object. The work done in alway calculated by multiplying the force times distance traveled times the cosine of the angle b/w them. Since the angle b/w the force the distance will always be 90º, then cos 90º = 0, and the work will always be zero!

Question 20

*How do you know how much work is done by magnetic force on a charge?

Answer 20

The work done magnetic forces is always zero! The magnetic force on an object is always perpendicular to the velocity of the object. The work done in alway calculated by multiplying the force times distance traveled times the cosine of the angle b/w them. Since the angle b/w the force the distance will always be 90º, then cos 90º = 0, and the work will always be zero!

Question 21

What is a mechanical wave? Transverse wave? Longitudinal wave? Can they be polarized?

Answer 21

Mechanical wave -> oscillations, needs medium, but the medium itself is not transported, just the energy
ex. vibrating string, sound

Transverse wave -> can be polarized, wave moves horizontally, but rope itself is moving up and down, medium oscillates perpendicular to direction of wave propagation
ex. ocean waves, wave on a string, electromagnetic waves

Longitudinal waves -> can’t be polarized, medium oscillates parallel to direction of wave propagation
ex. sound (longitudinal pressure wave)

Question 22

*MCAT loves waves, memorize these rules:
What is ONE OF THE MOST IMPORTANT equations for speed of a wave?

Two big rules for waves:
1. speed of wave in a medium depends on….
___ is constant in a medium, regardless of _____ or _______
What is the exception?
2. A wave moving from one medium to another will maintain the same______
____constant between media, ____ changes

Answer 22

Wave equation:
v = λf
THIS EQUATION AND 2 RULES TRUE FOR TRANSVERSE AND LONGITUDINAL WAVES
How fast wave travels

Two big rules for waves:
1. speed of wave in one medium depends on the type of wave and the physical properties of the medium (one medium), not by frequency
(The exception is when there is dispersion Chp. 13)
- v is constant in a medium, regardless of frequency or wavelength (according to the important equation above, changing f only changes λ, but not v)
v constant, f changes

2. A wave moving from one medium to another will maintain the same frequency (multimedia)
   f constant between media, v changes

*Rule 1 applies to different waves in one medium, while rules 2 applies to a single wave in different media

Question 23

Equation for speed of a wave in a string:

How does f, λ, and v affect amplitude? The amplitude is determined by what?

Answer 23

v = square root of tension/linear density

f, λ, and v does not affect amplitude

Amplitude determined by how much E we put into wave

Question 24

Speed of sound depends on ______

The general trend for sound waves to travel slowest in ____, faster in ____, and fastest in ______

Answer 24

Speed of sound depends on bulk modulus (resistance to compression) of the medium and its density:
v = square root of B/p
p = row (density)

Question 25

A certain rope transmits a 2 Hz transverse wave of amplitude 10 cm with a speed of 1 m/s. What would be the wavelength of a 5 Hz transverse wave of amplitude 8 cm on this rope?

Answer 25

First ignore the amplitudes (just indicates how much E wave transports but has nothing to do with wavelength, period, frequency, or wave speed). If a 2 Hz transverse wave has a speed of 1 m/s on this rope, then a transverse wave of any frequency will have a speed of 1 m/s on this rope; that’s what Big Rule 1 for waves tells us. Thus, if f = 5 Hz, and v = 1 m/s, then can use v = λf to get 0.2 m.

Question 26

Constructive and destructive interference:

What does it mean when passage says
What does “180º out of phase mean” or “pi radians out of phase”? Whole cycle or wave being 360 degrees or 2π radians, as if it were a circle?

What happens when waves aren’t exactly in phase (0º, 360º, or 2π radians) or exactly out of phase (180º or π radians)?

Answer 26

Waves that act deconstructively cancel out each other’s amplitudes

“180º out of phase mean” or “pi radians out of phase” -> waves are directly opposite each other in amplitude -> deconstructive interference bc exactly out of phase

“pi radians out of phase”? Whole cycle or wave being 360 degrees or 2π radians, as if it were a circle -> waves exactly in phase

When waves aren’t exactly in phase (0º, 360º, or 2π radians) or exactly out of phase (180º or π radians), the amplitude of the resultant wave will be somewhere between the difference and the sum of the amplitudes of the interfering waves

Question 27

How do you determine whether two waves are in phase or not?

Answer 27

One way is to look at path difference bc Wave 1 may have to travel farther distance than Wave . The path difference = d1 - d2

Question 28

How do you determine whether two waves are in phase or not?

Answer 28

One way is to look at path difference bc Wave 1 may have to travel farther distance than Wave . The path difference = d1 - d2

If the path difference = nλ and, (n = 0,1,2…), the waves will be in phase and will therefore constructively interfere
If path difference = (n+1/2)λ, the waves will be 180º out of phase and will therefore destructively interfere

If Wave 2 travels an integer number of wavelengths farther than Wave 1, the crests from each will still arrive at the same time. If Wave 2 travels λ/2, 3λ/2, 5λ/2, etc farther than Wave 1, the crest from one wave will arrive simultaneously with.a trough from the other wave

Question 29

Standing Waves
String of length L tied at both ends: displacement node at each end
What does n represent? How do you know the number of antinodes?
n = 1?
Equations for harmonic frequencies and wavelengths?

Answer 29

n = mode number or harmonic number, integer, and # of antinodes
n = 1 -> fundamental 

Harmonic frequencies
fn = nf1
fn = nv/2L
(L = length of rope)

Harmonic wavelengths
λn = 2L/n
λn = λ1/n

Question 30

Standing waves with one end open and other end closed
example: standing wave in an organ pipe: pressure at end points is atmospheric pressure

What determines which wavelengths can be trapped?

What is work and energy at open end vs closed end?

Answer 30

No work and energy at open end, sound escapes here
Closed end has max work and max energy

Endpoints determine which wavelengths can be trapped

Question 31

*What is the equation for intensity of wave (MCAT usually tests sound waves, but it can be applied to any type of wave)
What is the relationship between intensity of a spherical wave and r = distance (this relationship used pretty much whenever we hear a sound from something?
Relationship between intensity and amplitude of wave

Answer 31

Energy of a wave incident per unit area per unit time:
I = power/area
Units: [I] = W/m^2

I ∝ 1/r^2 -> MCAT loves this proportionality
I ∝ A^2 -> MCAT usually tests intensity of sound waves, but it can be applied to any type of wave
This can also be written as I ∝ E^2 (energy in a wave)
One practice problem said -> “Since the energy in a wave is related to its intensity, the light’s intensity is also reduced. I ∝ E2, so for this example, the light’s electric field magnitude is reduced bc some of the light is absorbed and as a result, the remaining energy in the light is reduced

Question 32

Intensity Level or Sound Level:
Human ears respond to intensity logarithmically (enables larger sensitivity range)
What is equation for intensity level in decibals?
What is the threshold of hearing?

Answer 32

Intensity level = sound level = decibal level = beta
Intensity of wave (sound, etc) is the energy it transmits per second (the power) per unit area -> W/m^2

β = 10log (I/I0)

I0 = threshold of hearing = 10^-12 W/m^2
this is the lowest sound human ear can perceive called I0 (I not)

[β] = dB = unitless

Question 33

To work with the β equation:
Every time we increase I by a factor of 10, what happens to β?
Every time we decrease I by a factor of 10, what happens to β?

Answer 33

Every time we increase I by a factor of 10, add 10 to β
Every time we decrease I by a factor of 10, subtract 10 from β

*every 0 in the factor, increases or decreases, gets 10dB

log math ->
log 1000 = 3
base = 10
exp = 3
10^3 = 1000

Question 34

*HIGH YIELD

Key doppler effect relationship:

Answer 34

Doppler = shift in the detected frequency of a wave due to relative motion between the detector and sound source

As detector and source get closer -> higher detected frequency (than emitted frequency)
(imagine what you would actually hear)

As detector and source get farther away -> lower detected (than emitted frequency)

These are true no matter if one thing is stationary and other is moving

Question 35

Equation for doppler:
Speed of sound in air:

According to v = λf, what how do these factors change/ stay the same when detector is moving vs when source is moving

When choosing signs for equation, what do you have to think about?

“Higher” and “lower” do not imply “increasing” or “decreasing”

Answer 35

fD = fs * (v+/- vD)/(v-/+vS)

v = 350 m/s

When detector moving -> V changes, λ stays same, so f changes

When source moving -> λ changes and v stays same so f changes
go to pg 417 for great visual

When choosing signs for equation, have to think about if fD or fS will be greater (ex. if approaching, fD is greater, choose plus for numerator to make fD>fs)

Question 36

If the velocities are constant, what else is constant

Answer 36

Detected frequency

Question 37

How does speed of light in or near the visible spectrum in a medium compare to that in vacuum?

Answer 37

The speed of light in or near the visible spectrum in a medium will always be less than or equal to the speed of light in vacuum