Physics II: Electricity, Magnetism, Electronics, and Modern Physics with Medical Applications

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Fields

Gravitational field: F = GMm/r²; g = GM/r²; potential V = −GM/r; escape velocity √(2GM/R).
Electrostatics: Coulomb's law F = Qq/(4πε₀r²); field E = F/q; potential V = Q/(4πε₀r); Gauss's law; capacitors C = Q/V, U = ½CV², series and parallel combinations; the RC time constant τ = RC. Medical link: the defibrillator stores energy in a capacitor and delivers a controlled shock to restore a normal heart rhythm; the cell membrane behaves as a capacitor.
Magnetic field: F = qvB sin θ; F = BIl sin θ; B = μ₀I/(2πr) for a long wire; B = μ₀nI inside a solenoid; electromagnetic induction ε = −N dΦ/dt; transformers; r.m.s. values V_rms = V₀/√2. Medical link: MRI uses strong magnetic fields and radio waves to image soft tissue without ionizing radiation; induction is used in generators and in some therapies.

Current Electricity

Current I = nAvq; Ohm's law V = IR; resistivity R = ρl/A; power P = VI = I²R; emf and internal resistance V = E − Ir.
Kirchhoff's laws; resistors in series and parallel; potentiometer, meter bridge, and Wheatstone bridge; ammeters and voltmeters.
Medical links: electrical safety (current through the body is dangerous: about 1 mA is perceptible, and currents of tens of milliamperes through the chest can cause fibrillation); earthing and fuses; the nervous system as an electrical signalling system; ECG and EEG record electrical activity of the heart and brain.

Electronics

Semiconductors; p-n junctions and diodes (rectification); transistors as switches and amplifiers; operational amplifiers (inverting gain −R_f/R_in; non-inverting 1 + R_f/R_in); logic gates; sensors (thermistors, LDRs). Medical links: amplifiers in ECG machines, pulse oximeters, digital thermometers, and patient monitors.

Matter and Radiation

Elasticity: stress, strain, Young's modulus E = stress/strain, Hooke's law; bone and tendon properties.
Quantum ideas: photon energy E = hf; photoelectric effect; de Broglie wavelength; Bohr's model E_n = −13.6 eV/n²; line spectra; lasers (surgery, eye correction).
X-rays: produced when electrons strike a metal target; used for radiography and CT scanning; absorption increases with tissue density and atomic number (bone absorbs more than soft tissue).
Radioactivity: A = λN; N = N₀e^(−λt); half-life T½ = ln 2/λ; alpha, beta, and gamma radiation and their penetrating power. Medical links: radiotherapy (gamma rays and X-rays kill cancer cells), nuclear medicine (radioactive tracers such as technetium-99m for imaging, with a half-life of about 6 hours; positron emission tomography), sterilization, and radiocarbon dating.
Radiation safety: minimize time, maximize distance, and use shielding (the ALARA principle: as low as reasonably achievable). The units are the becquerel (activity), gray (absorbed dose), and sievert (equivalent dose).
Emerging scientific knowledge: nanotechnology, medical devices, renewable energy.

Practical Skills

Measuring with instruments, planning experiments, tabulating data with uncertainties, drawing and interpreting graphs, and evaluating errors.

Common Mistakes

Confusing activity (Bq) with dose (Gy or Sv).
Forgetting to use r.m.s. values in AC power calculations.
Drawing voltmeters in series with the component.

CHAPTER 21