Basic Physics for Medicine

🧲 Basic Physics for Medicine

Understanding basic physics is essential for medical professionals 🩺. It underpins physiology, imaging 🖼️, ventilation, haemodynamics, and therapies like radiotherapy. This guide now includes extra facts and emojis ✨ to anchor the concepts clinically and make revision more memorable.

⚙️ Mechanics

• Newton's Laws :
  • 🚶 1st Law (Inertia): Body stays at rest/motion unless acted on ➝ e.g. immobility ➝ pressure sores.
  • 💪 2nd Law (F = m·a): CPR compressions – depth depends on applied force.
  • ↔️ 3rd Law (Action–Reaction): Chest recoil during CPR is vital for venous return.
• Laplace’s Law 🫁:
  • Tension = Pressure × Radius ➝ explains aneurysm rupture risk and alveolar collapse without surfactant.
• Work & Power ⚡:
  • Work = Force × Distance ➝ work of breathing ↑ in asthma.
  • Power = Work/time ➝ cardiac output is essentially mechanical power output of the heart ❤️.

🌊 Fluid Mechanics

• Blood Flow (Haemodynamics) :
  • 🩸 Poiseuille’s law: flow ∝ r⁴. Small change in vessel radius has huge effect on resistance.
  • 🔊 Turbulence: murmurs/bruits occur when Reynolds number is high (>2000).
  • 💧 Starling forces: govern oedema – balance between hydrostatic vs oncotic pressures.
• Respiratory Flow 🫁:
  • Resistance ∝ 1/r⁴ ➝ airway narrowing in asthma massively ↑ resistance.
  • Compliance (ΔV/ΔP): ↓ in ARDS (stiff lungs), ↑ in emphysema.

🔥 Thermodynamics

• Laws:
  • ♻️ 1st: Energy conserved ➝ relevant in metabolism.
  • 📈 2nd: Entropy ↑ ➝ heat flows hot ➝ cold.
  • ❄️ 3rd: Near absolute zero, entropy → minimum.
• Clinical Facts:
  • 🚑 Peri-op hypothermia triples wound infection risk.
  • 🧊 Heat loss mechanisms: conduction, convection, radiation, evaporation.

⚡ Electricity & Magnetism

• ECG 🫀:
  • Records cardiac depolarisation; depolarisation toward + electrode ➝ upward deflection.
  • Fact: Atrial fibrillation is the most common arrhythmia detected by ECG 📈.
• Defibrillation ⚡:
  • Delivers ~150–200 J biphasic shocks to reset rhythm.
  • Fact: Survival ↓ 10% for each minute defib delayed in VF arrest.
• Imaging 🖼️:
  • MRI: proton alignment in strong field; no ionising radiation.
  • X-rays/CT: ionising radiation ➝ need ALARA principle.

👁️ Optics

• Vision:
  • Snell’s law governs refraction. Myopia (eye too long), Hyperopia (eye too short).
  • Fact: 👓 Around 25% of UK adults wear corrective lenses for refractive error.
• Medical Instruments:
  • 🔬 Microscopes magnify small structures; limit ~0.2 µm due to light diffraction.
  • 🩻 Endoscopes use fibre optics; rely on total internal reflection.

📡 Waves & Ultrasound

• Ultrasound:
  • Uses sound ~2–15 MHz. Higher frequency = better resolution, poorer penetration.
  • Doppler shift 🎵 measures blood flow velocities (useful in DVT or valve studies).
  • Fact: Obstetric ultrasound is considered safe, but thermal/mechanical indices monitored 👶.

☢️ Radiation Physics

• Types:
  • Ionising (X-ray, γ-rays) – DNA damage potential.
  • Non-ionising (MRI, ultrasound, visible light) – less hazardous.
• Clinical Facts:
  • 📷 Chest X-ray ~0.02 mSv (equivalent to 3 days background radiation).
  • 🖥️ CT Abdomen ~10 mSv (≈3 years background).
• Therapy 🎯:
  • Radiotherapy exploits DNA damage in cancer cells.
  • Protons deliver Bragg peak – spares normal tissue more than photons.

📌 Clinical Summary

✅ Physics explains physiology: Laplace’s law in aneurysms, Poiseuille’s law in stenoses, and thermodynamics in anaesthesia warming.
✅ Electricity & Magnetism underpin ECGs, defibs, and MRI.
✅ Optics & Waves explain endoscopy, microscopy, lasers, and ultrasound.
✅ Radiation physics guides imaging safety and radiotherapy use.
✨ Mastery of these principles = safer patient care + smarter use of technology.