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
Option A: Alternating Currents
- 8 subtopics
- 24 things to know
- A2 Unit 4 Section B
- Alternating Currents, one of four A2 unit 4 options
What Option A: Alternating Currents covers
Option A: Alternating Currents is one of four options on A2 Unit 4 of WJEC Physics, Fields and Options, and learners answer on one of the four. 108 recall questions across 8 subtopics.
Where it is assessed
A2 Unit 4 Section B
Fields and Options, one option of four answered.
Check the WJEC specification ↗The 8 subtopics
Each one has its own page.
- Alternating current and rms valuesRecall questions on Alternating current and rms values.Open →
- Rms values, mean power and UK mainsConverting between peak and rms current and voltage, mean power in a resistor from rms and peak values, and the frequency, rms and peak voltage of the UK mains.Open →
- Induced emf and the oscilloscopeRecall questions on Induced emf and the oscilloscope.Open →
- Reactance, impedance and resonanceRecall questions on Reactance, impedance and resonance.Open →
- Flux linkage in a rotating coilRecall questions on Flux linkage in a rotating coil.Open →
- Mean power and energy in a.c. circuitsRecall questions on Mean power and energy in a.c. circuits.Open →
- Phasor addition in series a.c. circuitsRecall questions on Phasor addition in series a.c. circuits.Open →
- Resonance curves and the Q factorRecall questions on Resonance curves and the Q factor.Open →
What you need to know
Answer each one before you reveal it.
Your recallNot started
Key term
What is the impedance of a series RCL circuit?
Key term
State the equation for the emf induced in a flat coil rotating in a uniform magnetic field.
Key term
Why can the angle between the coil normal and the field be written as ωt for a coil spun at a steady rate?
Key term
State the phase relationship between the flux linkage of a rotating coil and the emf it induces.
Key term
Why is a rotating-coil generator fitted with slip rings rather than a split-ring commutator?
Key term
Explain why a sinusoidal alternating current has a mean value of zero over a whole cycle yet still heats a resistor.
Key term
Explain why the mean power dissipated in a pure inductor over a whole cycle is zero.
Numbers
A heating element of resistance 100 Ω is connected to a 230 V rms supply. Calculate the mean power and the peak instantaneous power.
Numbers
A series RC circuit has 8.0 V across the resistor and 6.0 V across the capacitor. Calculate the supply potential difference and the phase angle.
Numbers
A series RL circuit has R = 40 Ω and L = 0.15 H and is driven at 50 Hz. Calculate its impedance.
Key term
Describe the shape of a graph of rms current against frequency for a series RCL circuit driven by a constant rms potential difference.
Key term
Why does a radio receiver need a tuned circuit with a high Q factor?
Key term
What is an alternating current (AC)?
Key term
How does direct current (DC) differ from alternating current (AC) in terms of direction of flow?
Key term
State the equation relating rms current to peak current.
Numbers
Calculate the rms current when peak current = 7.5 A.
Key term
State Faraday's law of electromagnetic induction.
Key term
State the equation that describes how an alternating voltage varies with time.
Key term
State the equation relating rms voltage to peak voltage.
Key term
What does the peak value of the induced emf in a rotating coil depend on?
Key term
State the phase relationship between current and potential difference for an inductor in a sinusoidal a.c. circuit.
Key term
In a series circuit containing a resistor and an inductor, state the direction of each potential difference phasor relative to the current.
Key term
Does the Q factor of a circuit have a unit?
Take Option A: Alternating Currents further
The same content, four other ways.
Commonly confused
Pairs that lose marks when they are blurred together.
Reactance vs Impedance
Reactance: the opposition of a capacitor or an inductor alone to alternating current. Impedance: the combined opposition of the resistor and the capacitor or inductor together.
Root mean square voltage vs Peak voltage
Root mean square voltage: the steady value that would deliver the same power. Peak voltage: the greatest instantaneous value the alternating supply reaches.
Questions from this topic
Each one with the model answer underneath.
- 1
What is an alternating current (AC)?
Model answer
An electric current that periodically reverses direction, in contrast to direct current (DC) which flows in one constant direction.
- 2
How does direct current (DC) differ from alternating current (AC) in terms of direction of flow?
Model answer
DC flows in one constant direction, whereas AC periodically reverses direction.
- 3
State the equation that describes how an alternating voltage varies with time.
Model answer
V = V₀ sin(ωt), where V₀ is the peak voltage and ω is the angular frequency.
- 4
In the equation V = V₀ sin(ωt), what does V₀ represent?
Model answer
The peak voltage (maximum voltage reached during each cycle).
- 5
In the equation I = I₀ sin(ωt), what does I₀ represent?
Model answer
The peak current (maximum current reached during each cycle).
- 6
State the equation relating angular frequency ω to frequency f.
Model answer
ω = 2πf.
What is inside Levo
Six ways to work the same material.
Option A: Alternating Currents revision questions
- What does Option A: Alternating Currents cover in WJEC A-Level Physics?
- 8 subtopics: alternating current and rms values, rms values, mean power and uk mains, induced emf and the oscilloscope, reactance, impedance and resonance, flux linkage in a rotating coil, mean power and energy in a.c. circuits, phasor addition in series a.c. circuits and resonance curves and the q factor. Each one has its own page with questions and model answers.
- How is Option A: Alternating Currents organised?
- By the WJEC 1420QS specification, subtopic by subtopic, so it matches the order you are taught it in.
Every question in Option A: Alternating Currents, 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.