What does OCR Module 6 Particles and medical physics cover?

It covers capacitors with capacitance, series and parallel combinations, energy stored and exponential charge and discharge, electric fields with Coulomb's law, field strength and potential, magnetic fields and the motor effect with the force on currents and moving charges, electromagnetic induction with Faraday's and Lenz's laws and transformers, nuclear and particle physics with alpha scattering, the standard model, beta decay and binding energy, radioactive decay with the decay constant, activity and half-life, and medical imaging with X-rays, the gamma camera, PET and ultrasound.

Which equations recur most across Particles and medical physics?

Capacitance $C = \frac{Q}{V}$, energy $E = \frac{1}{2}CV^2$ and discharge $V = V_0 e^{-t/RC}$ with $\tau = RC$, Coulomb's law $F = \frac{1}{4\pi\varepsilon_0}\frac{Q_1 Q_2}{r^2}$ with field strength $E = \frac{V}{d}$, the motor force $F = BIL\sin\theta$ and the force on a charge $F = BQv$ with $r = \frac{mv}{BQ}$, flux $\Phi = BA\cos\theta$ and Faraday's law $\varepsilon = \frac{\Delta(N\Phi)}{\Delta t}$ with the transformer ratio $\frac{V_s}{V_p} = \frac{N_s}{N_p}$, the nuclear radius $R = R_0 A^{1/3}$, mass-energy $E = mc^2$ with binding energy $\Delta m\, c^2$, the decay law $N = N_0 e^{-\lambda t}$ with $A = \lambda N$ and $T_{1/2} = \frac{\ln 2}{\lambda}$, and the attenuation law $I = I_0 e^{-\mu x}$.

What is the most common mistake in this module?

Swapping the series and parallel rules for capacitors with those for resistors (capacitors add in parallel), and omitting the factor of one half in the capacitor energy. Other frequent errors are mishandling exponentials (the time constant is $RC$ and the value falls to 37 per cent after one constant), confusing mass defect with binding energy, mixing up half-life and decay constant, and forgetting that an ultrasound echo travels there and back, so the depth is half the round trip.

How is nuclear physics examined in OCR Physics A?

Expect the nuclear radius equation $R = R_0 A^{1/3}$ and the idea of constant nuclear density, the standard model with quark compositions and conservation laws, balanced beta-decay equations including the (anti)neutrino, and mass-energy calculations with binding energy per nucleon to explain why fission and fusion release energy. Radioactivity is tested with the decay constant, activity $A = \lambda N$, the exponential decay law and half-life, often in a dating or medical context.

How should I revise Particles and medical physics?

Drill the capacitor routines (charge, energy, series and parallel, exponential discharge), then the field work for electric and magnetic fields and the comparison with gravity, then induction and transformers. Secure the nuclear ideas: the standard model, beta decay, binding energy and the exponential decay law, taking care with units and logarithms. Finally learn the medical imaging methods and their physics. This module is assessed in Paper 2 alongside Module 4.

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OCR A-Level Physics A Particles and medical physics: capacitors, fields, nuclear physics and imaging

A deep-dive OCR A-Level Physics A guide to Module 6, Particles and medical physics. Covers capacitors with exponential discharge, electric fields and Coulomb's law, magnetic fields and the motor effect, electromagnetic induction and transformers, nuclear and particle physics with the standard model and binding energy, radioactive decay, and medical imaging with X-rays, PET and ultrasound.

Generated by Claude Opus 4.818 min readH556 Module 6Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this module actually demands
Capacitors and fields
The nucleus and radioactivity
Medical imaging
How this module is examined
Check your knowledge

What this module actually demands

Particles and medical physics brings together the physics of fields, the nucleus and modern technology. It develops capacitors and their exponential behaviour, the electric and magnetic fields that steer charges, electromagnetic induction and the transformer, the structure of the nucleus and the standard model, radioactive decay, and the physics behind medical imaging. The examiners reward fluent exponential calculation, careful unit work, and precise statements of nuclear and conservation principles. With Module 4 it makes up the Exploring physics paper.

This guide walks through the topics in order and sets out the exam patterns OCR repeats. Each topic has a matching dot-point page with practice; this overview ties them together.

Capacitors and fields

Capacitors defines capacitance, combines capacitors in series and parallel, finds the energy stored with $E = \frac{1}{2}CV^2$ , and analyses exponential charge and discharge with the time constant $\tau = RC$ . Electric fields applies Coulomb's law, defines field strength for radial and uniform fields, treats electric potential, and compares electric with gravitational fields. Magnetic fields and the motor effect defines flux density, finds the force on a current $F = BIL\sin\theta$ and on a moving charge $F = BQv$ , and analyses the circular motion of charges. Electromagnetic induction defines flux and flux linkage, applies Faraday's and Lenz's laws, finds the emf in a moving conductor, and explains transformers.

The nucleus and radioactivity

Nuclear and particle physics interprets alpha scattering and the nuclear radius $R = R_0 A^{1/3}$ , describes the strong force, classifies particles in the standard model, writes beta-decay equations, and uses binding energy to explain fission and fusion. Radioactive decay describes decay as random and spontaneous, defines the decay constant and activity, uses the exponential decay law, relates half-life to the decay constant, and applies dating.

Medical imaging

Medical imaging covers the production and exponential attenuation of X-rays with the half-value thickness, the gamma camera and PET with positron annihilation, and ultrasound with acoustic impedance, the intensity reflection coefficient and the Doppler effect for blood flow.

How this module is examined

A typical OCR profile for Particles and medical physics:

Calculations. Capacitor charge, energy, combinations and discharge, Coulomb and field-strength problems, motor and moving-charge forces and radii, induced emf and transformer values, nuclear radius and binding energy, decay constant, activity and half-life, and X-ray attenuation and ultrasound reflection.
Graph questions. Capacitor discharge curves and log-linear plots, binding energy per nucleon against nucleon number, and decay curves.
Explanation and definition. Lenz's law and energy conservation, the standard model and conservation laws, the random and spontaneous nature of decay, and the use of coupling gel in ultrasound.
Extended answers. Comparing electric and gravitational fields, explaining how transformers and the grid work, and describing and contrasting the medical imaging methods.

Check your knowledge

A mix of recall and calculation questions covering the module. Attempt them under timed conditions, then check against the solutions.

A $100\ \mu\text{F}$ capacitor is charged to $20\ \text{V}$ . Find the energy stored. (2 marks)
A $220\ \mu\text{F}$ capacitor discharges through a $5.0\ \text{k}\Omega$ resistor. Find the time constant. (2 marks)
State the equation for the force on a charge moving through a magnetic field. (1 mark)
State Faraday's law of electromagnetic induction. (2 marks)
Give the quark compositions of the proton and the neutron. (2 marks)
A source has a decay constant of $0.050\ \text{s}^{-1}$ . Find its half-life ( $\ln 2 = 0.693$ ). (2 marks)

Solutions

Step 1: Find the energy stored in the capacitor

The energy stored in a capacitor is given by $E = \frac{1}{2}CV^2$ , where the factor of one half arises because the voltage builds up from zero as charge accumulates. Substitute $C = 100 \times 10^{-6}\ \text{F}$ and $V = 20\ \text{V}$ .

E = \frac{1}{2}CV^2 = \frac{1}{2}(100 \times 10^{-6})(20)^2 = \frac{1}{2}(100 \times 10^{-6})(400) = 2.0 \times 10^{-2}\ \text{J}

Step 2: Find the time constant for the RC discharge

The time constant $\tau = RC$ sets the timescale on which the capacitor discharges: after one time constant the voltage falls to about 37 percent of its initial value. Multiply the resistance and capacitance, taking care with units ( $\text{k}\Omega \times \mu\text{F} = \text{s}$ ).

\tau = RC = (5.0 \times 10^{3})(220 \times 10^{-6}) = 1.1\ \text{s}

Step 3: State the force on a moving charge in a magnetic field

The magnetic force on a moving charge depends on the component of velocity perpendicular to the field, giving a maximum when the velocity is at right angles to the field. The general expression is:

F = BQv\sin\theta \quad \text{(maximum } F = BQv \text{ when } \theta = 90°\text{)}

Step 4: State Faraday's law of electromagnetic induction

Faraday's law quantifies the induced emf in terms of how quickly the flux linkage through a circuit changes. A changing magnetic flux is the cause, and the magnitude of the induced emf equals the rate of that change.

\varepsilon = \frac{\Delta(N\Phi)}{\Delta t}

Step 5: Give the quark compositions of the proton and neutron

The proton and neutron are both baryons, each made of three quarks. The proton carries charge $+1e$ from two up quarks ( $+\frac{2}{3}e$ each) and one down quark ( $-\frac{1}{3}e$ ); the neutron is neutral with one up and two down quarks.

Proton: $uud$ . Neutron: $udd$ .

Step 6: Calculate the half-life from the decay constant

The half-life and decay constant are linked by $T_{1/2} = \frac{\ln 2}{\lambda}$ , because it takes $\ln 2$ time constants for the activity to halve. Dividing $\ln 2 = 0.693$ by $\lambda = 0.050\ \text{s}^{-1}$ gives the result.

T_{1/2} = \frac{\ln 2}{\lambda} = \frac{0.693}{0.050} = 14\ \text{s}

Sources & how we know this

OCR AS and A Level Physics A (H156, H556) specification — OCR (2015)