Key term
WJEC A-Level Physics 1420QS
Basic Physics
Kinematics
Dynamics
Energy Concepts
Solids Under Stress
Using Radiation to Investigate Stars
Particles and Nuclear Structure
Topic overviewQuarks, leptons and hadronsThe four interactions and conservation lawsThe first generation of quarks and leptonsAntiparticles and annihilationBaryons, antibaryons and mesonsDeducing quark compositions from chargeBeta decay and quark flavour changeTesting a reaction against the conservation rules
Conduction of Electricity
Resistance
Topic overviewCalculating resistance from resistivityPotential difference, Ohm's law and powerResistivity, temperature and superconductivityThe volt: energy transferred per unit chargeCurrent–voltage characteristics of non-ohmic componentsMeasuring resistance and resistivity in the laboratoryThe heating effect of a current and choosing a power equationThermistors and how a semiconductor's resistance falls with temperatureConductance, conductivity and comparing materials
D.C. Circuits
Topic overviewKirchhoff's laws, and resistors in series and parallelPotential dividers, emf and internal resistanceCells, resistivity, power and energyWorking out currents and potential differences in resistor networksSensor circuits with thermistors and LDRsInternal resistance, power transfer and short-circuit currentAmmeters, voltmeters and their effect on the circuit
The Nature of Waves
Wave Properties
Topic overviewEnergy transfer and the wave equationTransverse and longitudinal waves, path difference and intensityDiffraction and superpositionTwo-source interference and the diffraction gratingStationary wavesHow much a wave spreads at a gap or an obstacleCoherence and the conditions for observable interferenceYoung's double slit and λ = ay/DMeasuring wavelength with a diffraction gratingMeasuring the speed of sound with stationary waves
Refraction of Light
Photons
Topic overviewPhoton energy and the electronvoltThe photoelectric effectStopping potential and the Planck constantThe photoelectric effect and photon energiesLine spectra and energy levelsDemonstrating the photoelectric effectThe electromagnetic spectrum and typical photon energiesElectron diffraction and the de Broglie relationshipRadiation pressure
Lasers
Circular Motion
Vibrations
Topic overviewSimple harmonic motion and its equationsOscillating systems and energyDamping and resonanceThe simple pendulumThe mass–spring oscillatorThe displacement and velocity equationsGraphs of displacement, velocity and accelerationThe energy interchange in simple harmonic motionFree and forced oscillationsLight, heavy and critical damping
Kinetic Theory
Thermal Physics
Topic overviewInternal energy and heatWork, the first law and specific heat capacityThermal equilibrium, temperature and absolute zeroThe internal energy of an ideal monatomic gasWork done by a gas at constant pressureThe p–V diagram and work as an areaSign conventions and special cases of the first lawMeasuring specific heat capacity
Nuclear Decay
Nuclear Energy
Capacitance
Topic overviewCapacitance and the parallel plate capacitorEnergy, combinations and RC circuitsCharging at constant current and measuring capacitanceDielectrics and relative permittivityThe uniform field between capacitor platesEnergy stored in a capacitor and the Q–V graphExponential discharge, the time constant and log graphsCapacitors in series and parallel: charge and potential difference sharing
Electrostatic and Gravitational Fields of Force
Topic overviewElectric and gravitational fields comparedField lines, equipotentials and combining fieldsElectric potential and the work done on a chargeGravitational potential and potential energyAdding potentials and fields from several sourcesEquipotential surfacesComparing the two fields point by point
Orbits and the Wider Universe
Topic overviewKepler's laws and orbital motionThe Doppler effect, dark matter and the expanding universeNewton's law of gravitation applied to orbitsDeriving Kepler's third lawFinding the mass of a central objectGalactic rotation curves and dark matterThe centre of mass of a two-body systemRadial velocity and the Doppler shift of spectral linesHubble's law and the age of the universe
Magnetic Fields
Topic overviewMagnetic fields and field patternsMagnetic flux density and the force on a conductorThe force on a moving charge and its circular pathThe Hall effect and the Hall voltageMagnetic fields due to a current in a wire and in a solenoidForces between current-carrying conductorsCharged particle beams, velocity selectors and accelerators
Electromagnetic Induction
Option A: Alternating CurrentsOption A
Option B: Medical PhysicsOption B
Topic overviewMedical physicsUltrasound imagingRadiation and living tissueX-ray production and beam controlAttenuation and half-value thicknessRadiography, fluoroscopy and CT scanningAcoustic impedance and ultrasound reflectionMagnetic resonance imagingAbsorbed dose, equivalent dose and effective doseRadionuclide tracers, the gamma camera and PET
Option C: The Physics of SportsOption C
Topic overviewCentre of gravity, stability and momentsRestitution, moment of inertia and angular momentumRotational energy, projectiles and the flight of a ballMoments in the muscles and jointsApplying the impulse equation in sportMoment of inertia and angular kinematicsTorque, angular momentum and its conservationRotational energy and energy conservation in sport
Option D: Energy and the EnvironmentOption D
Topic overviewFactors affecting the rate at which the temperature of the Earth risesThe Earth's thermal equilibrium, Wien's law and Stefan's lawRenewable and non-renewable sourcesSolar energy and photovoltaic cellsWind, tidal and hydroelectric powerNuclear fission and fusion as energy sourcesU-values and rates of heat lossFuel cells and how they workFuel cells: benefits, limitations and the hydrogen economyThermal conduction, conductivity and insulation
Mathematical Requirements (Appendix C)
Topic overviewSignificant figures and arithmetic meansUncertainty and how uncertainties combineAlgebraic symbols and proportionalityRearranging and solving equationsSmall angle approximationsTangents and instantaneous rates of changeAreas under curves and numerical methodsReading a tangent on a motion graphStraight-line graphs and y = mx + cWhat a gradient and an intercept representLinearising a relationship to find a constantLog plots of exponential decayChoosing what to plot to find a constantLogarithmic scales and orders of magnitudeWorking with base-ten logarithmsThe shapes of common graphsInverse and root relationships on a graphAreas, volumes and PythagorasGeometry applied to physical situationsSubstituting into physical equationsTrigonometry and right-angled trianglesResolving a force into componentsRadians, arc length and force diagramsRandom processes and translating between representationsMean values and periodic functionsSymbols for approximation and comparison
Capacitance
- 8 subtopics
- 24 things to know
- A2 Unit 4 Section A
What Capacitance covers
Capacitance is examined on A2 Unit 4 of WJEC Physics, Fields and Options. 114 recall questions across 8 subtopics.
The 8 subtopics
Each one has its own page.
- Capacitance and the parallel plate capacitorRecall questions on Capacitance and the parallel plate capacitor.Open →
- Energy, combinations and RC circuitsRecall questions on Energy, combinations and RC circuits.Open →
- Charging at constant current and measuring capacitanceRecall questions on Charging at constant current and measuring capacitance.Open →
- Dielectrics and relative permittivityRecall questions on Dielectrics and relative permittivity.Open →
- The uniform field between capacitor platesRecall questions on The uniform field between capacitor plates.Open →
- Energy stored in a capacitor and the Q–V graphRecall questions on Energy stored in a capacitor and the Q–V graph.Open →
- Exponential discharge, the time constant and log graphsRecall questions on Exponential discharge, the time constant and log graphs.Open →
- Capacitors in series and parallel: charge and potential difference sharingRecall questions on charge and potential difference sharing.Open →
What you need to know
Answer each one before you reveal it.
Your recallNot started
Key term
How do capacitances combine in series and in parallel?
Key term
State the equation for the capacitance of a parallel plate capacitor filled with a dielectric, and define every symbol with its unit.
Key term
Explain in terms of molecules why a dielectric increases the capacitance.
Key term
Why is a thin dielectric layer desirable in a capacitor, and what limits how thin it can be?
Evaluation
State two equivalent units for electric field strength and show that they are the same.
Key term
State the equation for the force on a charge in a uniform electric field, defining every symbol with its unit.
Key term
Show that calculating the work done on a charge crossing the gap as QV agrees with calculating it as force times distance.
Key term
Why can a capacitor deliver a very large power even though the energy it stores is small?
Key term
State the equation for the potential difference across a capacitor discharging through a resistor, defining every symbol with its unit.
Key term
Explain why the discharge current is always proportional to the charge still on the capacitor.
Key term
Why do capacitors connected in series all carry the same charge?
Key term
Describe the construction of a simple parallel plate capacitor.
Key term
State the equation for the energy stored in a charged capacitor in terms of charge and potential difference.
Key term
Why does the charging current fall if the series resistance is left unaltered?
Numbers
A capacitor is charged at a constant current of 50 μA and the potential difference across it rises at 0.25 V s⁻¹. Calculate its capacitance.
Key term
What happens to the energy stored in a capacitor if the potential difference across it is doubled?
Key term
Why is the combined capacitance of capacitors in series always smaller than the smallest one?
Key term
How does a capacitor store energy?
Key term
Why is the stored energy ½QV rather than QV?
Numbers
A capacitor charged at a steady current of 20 μA for 30 s reaches a potential difference of 4.0 V. Calculate its capacitance.
Numbers
A 100 μF capacitor charged to 20 V is connected across an identical uncharged capacitor. Calculate the final potential difference.
Key term
State the equation for the current during the discharge of a capacitor through a resistor.
Numbers
Calculate the combined capacitance of three 6.0 μF capacitors connected in series.
Take Capacitance further
The same content, four other ways.
Commonly confused
Pairs that lose marks when they are blurred together.
Capacitance vs The time constant
Capacitance: the charge stored on one plate per volt of potential difference. The time constant: the product RC, which sets how quickly stored charge decays through a resistor.
A dielectric vs The permittivity of free space
A dielectric: an insulator between the plates that raises the stored charge at a given voltage. The permittivity of free space: the constant relating plate area and separation to capacitance in a vacuum.
Questions from this topic
Each one with the model answer underneath.
- 1
Describe the construction of a simple parallel plate capacitor.
Model answer
A pair of equal parallel metal plates separated by a vacuum or by air.
- 2
How does a capacitor store energy?
Model answer
Work is done moving charge from one plate to the other against the field between them, so the energy is stored in the electric field between the oppositely charged plates.
- 3
What is the net charge on a charged capacitor?
Model answer
Zero, because the two plates carry equal and opposite charges.
- 4
State the definition of capacitance.
Model answer
C = Q/V, the charge on one plate divided by the potential difference between the plates.
- 5
What is the SI unit of capacitance, and how is it defined?
Model answer
The farad (F); a capacitor has a capacitance of one farad when one coulomb of charge produces a potential difference of one volt.
- 6
Why are practical capacitances usually quoted in microfarads or picofarads?
Model answer
The farad is a very large unit, so everyday capacitors have values many orders of magnitude smaller.
What is inside Levo
Six ways to work the same material.
Capacitance revision questions
- What does Capacitance cover in WJEC A-Level Physics?
- 8 subtopics: capacitance and the parallel plate capacitor, energy, combinations and rc circuits, charging at constant current and measuring capacitance, dielectrics and relative permittivity, the uniform field between capacitor plates, energy stored in a capacitor and the q–v graph, exponential discharge, the time constant and log graphs and capacitors in series and parallel: charge and potential difference sharing. Each one has its own page with questions and model answers.
- How is Capacitance organised?
- By the WJEC 1420QS specification, subtopic by subtopic, so it matches the order you are taught it in.
Every question in Capacitance, on a schedule.
Attempt a question from memory, write the correct version, then attempt it again. Levo records the jump between your first attempt and your last, and brings each question back on the day you are due to forget it.
Or £46 a year, about £3.83 a month. Cancel anytime.