STP Physical Sciences 3

Question 1

Which interaction is primarily responsible for the attenuation of low-energy X-rays (below ~30 keV) in soft tissue?

Accepted Answer

Photoelectric effect

Answer

At low photon energies, the photoelectric effect dominates because its cross-section is strongly dependent on atomic number and inversely on photon energy cubed.

Question 2

A capacitor stores 0.5 J of energy with a capacitance of 100 μF. What is the voltage across it?

Accepted Answer

100 V

Answer

Using E = ½CV², V = √(2E/C) = √(2×0.5 / 100×10⁻⁶) = √10000 = 100 V.

Question 3

What does the term 'acoustic impedance' represent in ultrasound physics?

Accepted Answer

The product of tissue density and sound speed

Answer

Acoustic impedance Z = ρc, where ρ is the density of the medium and c is the speed of sound in that medium.

Question 4

In nuclear medicine, what does the effective half-life of a radiopharmaceutical account for?

Accepted Answer

Both physical decay and biological elimination

Answer

Effective half-life combines both physical half-life (radioactive decay) and biological half-life (clearance from the body): 1/T_eff = 1/T_phys + 1/T_biol.

Question 5

Which phenomenon explains why electrons do not spiral into the nucleus despite electrostatic attraction?

Accepted Answer

Wave-particle duality and quantised energy levels

Answer

Electrons occupy discrete quantised energy levels described by quantum mechanics; they cannot lose energy continuously and therefore cannot spiral inward.

(STP) Scientist Training Programme Practice Test

(STP) Scientist Training Programme Practice Test

STP Physical Sciences 3