Current applications of physics
This domain asks you to apply physics to contemporary technologies and natural systems, especially medical imaging, energy conversion, communications and astronomy. The exam tests whether you can link a device’s physical principle to its measurable output and limitations, rather than quoting a name.
Medical imaging and sensing
- Ultrasound: a piezoelectric transducer emits and detects pulses; depth is found from d = vt/2; acoustic impedance Z = ρc controls reflection at boundaries; coupling gel reduces reflection at the skin.
- X-rays: attenuation follows I = I₀e^(−μx); half-value thickness x½ = ln2/μ; contrast depends on differing μ in bone and tissue.
- PET: a positron-emitting tracer annihilates with an electron, producing two 0.511 MeV gamma photons emitted almost back-to-back; coincidence detection locates the event.
- MRI: proton nuclear magnetic resonance in a strong magnetic field; radio-frequency pulses at the Larmor frequency and relaxation times provide contrast without ionising radiation.
Energy and materials applications
- Solar cells: photons with energy above the band gap create electron–hole pairs; below the band gap they are not absorbed; excess photon energy becomes heat.
- Wind power: available power scales with swept area and the cube of wind speed, so small speed changes matter greatly.
Worked example
An ultrasound pulse of frequency 5.0 MHz travels in soft tissue at 1540 m s⁻¹. An echo returns 52 µs after transmission. Find the reflector depth and the wavelength.
The time is for the round trip, so d = vt/2 = (1540)(52 × 10⁻⁶)/2 = 0.040 m = 4.0 cm. Wavelength: λ = v/f = 1540/(5.0 × 10⁶) = 3.1 × 10⁻⁴ m = 0.31 mm. Axial resolution is roughly λ/2 ≈ 0.15 mm. Doubling the depth doubles the echo time but does not change λ; the factor 2 arises because the pulse travels to the reflector and back.
Traps that cost marks
- Forgetting the factor 2 in pulse–echo timing.
- Using 343 m s⁻¹ for sound in tissue instead of about 1540 m s⁻¹.
- Saying PET produces one 1.022 MeV photon rather than two 0.511 MeV photons.
- Claiming MRI uses ionising radiation; it uses strong magnetic fields and RF pulses.
- Treating half-value thickness as independent of photon energy; μ depends on energy and material.