Physics Exam Review: Waves, Sound, Electricity & Magnetism

Elastic & Inelastic Collisions

• Elastic collision
  • Both momentum and kinetic energy are conserved.
  • In problem-solving you must write two conservation equations:
  • $m<em>1 v</em>{1,\text{before}} + m<em>2 v</em>{2,\text{before}} = m<em>1 v</em>{1,\text{after}} + m<em>2 v</em>{2,\text{after}}$
  • $\frac12 m<em>1 v</em>{1,\text{before}}^2 + \frac12 m<em>2 v</em>{2,\text{before}}^2 = \frac12 m<em>1 v</em>{1,\text{after}}^2 + \frac12 m<em>2 v</em>{2,\text{after}}^2$
• Inelastic collision (mentioned implicitly by contrast)
  • Momentum is still conserved but kinetic energy is not; part of it converts into deformation, heat, or sound.

Waves: Core Vocabulary & Equations

• Wavelength ((\lambda))
  • Distance from crest → crest (transverse) or compression → compression (longitudinal).
• Frequency ((f))
  • Number of waves passing a point per unit time ((\text{s}^{-1}) or Hz).
• Period ((T))
  • Time for one complete wave to pass.
  • Reciprocal relationship with frequency:
    $f = \frac{1}{T} \qquad \text{or} \qquad T = \frac{1}{f}$
• Wave speed ((v))
  • Distance a wave travels per unit time: $v = f \lambda$

Types of Waves & Particle Motion

• Transverse
  • Particle vibration ⟂ energy propagation (up–down while wave goes forward).
• Longitudinal
  • Particle vibration ∥ energy propagation (back-and-forth compressions).
  • Sound travels by this mechanism.

Energy Transformations in Medical Ultrasound (4-step chain)

1. Electrical Energy → Mechanical Vibrations
   • Electric pulse drives piezoelectric crystal.
2. Mechanical Vibrations → Sound (Ultrasonic) Waves
   • Crystal oscillates; tissue is the medium.
3. Reflected Ultrasound (Mechanical) → Mechanical Deformation of Crystal
   • Returning echo compresses the same crystal.
4. Mechanical → Electrical Signals → Image
   • Crystal’s inverse piezoelectric effect generates voltage → processed into a 2-D image.

Wave–Matter Interactions

• Reflection – wave bounces off a boundary; angle in = angle out.
  • Shown in diagrams with incident and reflected rays.
• Refraction – wave changes direction/speed when entering a medium with different properties; illustrated with bent ray.

Speed of Sound in Matter

• Determined chiefly by:
  • Elasticity of the medium (higher → faster).
  • Density (higher → slower, if elasticity is constant).
• Environmental modifiers likely tested:
  • Temperature ↑ → molecules move faster → speed ↑.
  • Humidity ↑ → effective density ↓ (water vapor < dry air) → speed ↑.

Two-Way (Echo) Travel Calculations

• Typical setup: pulse travels to an object and back.
• Total distance traveled = 2 × one-way distance.
• Key steps (show your work):
  1. Determine total travel time.
  2. Apply $v = \frac{d<em>{\text{total}}}{t</em>{\text{total}}}$.
  3. Solve for one-way distance: $d<em>{\text{object}} = \frac{v t</em>{\text{total}}}{2}$.
• Expect a problem identical in structure to quiz & study guide.

Doppler Effect (replaces “Resonance” in syllabus)

• Concept: Apparent frequency shift due to relative motion between source and observer.
• Approaching → perceived frequency ↑ (wavelength compressed).
• Receding → perceived frequency ↓.
• Practical applications:
  • Police radar guns.
  • Medical Doppler ultrasound (blood-flow velocity).
  • Weather radar (storm tracking).

Electricity & Magnetism Essentials

• Static Electricity Source = Polarization
  • Separation of positive/negative charges in an object without overall net charge transfer.
• Fields
  • Electric Field ((\vec E)): Region of force surrounding a charge; direction = force on + test charge.
  • Magnetic Field ((\vec B)): Region of force surrounding magnetic poles/current; visualized by field lines from North → South.
• Predicting Charge Movement
  • Use field-line diagrams: like charges repel, opposites attract; draw vectors showing direction of force on test charges.
• Right-Hand Rule (straight conductor version)
  • Thumb: direction of current (I).
  • Fingers curl: direction of magnetic field (B) encircling wire.
  • Must be able to describe verbally and use in sketches.

Study/Exam Reminders

• Echo-problem workflow and full work-showing are mandatory.
• Be prepared to label reflection vs. refraction on provided figures.
• Memorize $f = \frac{1}{T}$ and $v = f \lambda$; apply with proper units (Hz, m, s).
• Review temperature & humidity effects on sound speed.
• Doppler effect conceptual and application questions likely.
• Static electricity → polarization, field line drawings, and right-hand rule explanations will be tested.