Numbers
OCR A-Level Physics A H556
Practical Skills in the Written Papers
The Practical Endorsement
Topic overviewAnalogue and digital measuring instrumentsImproving accuracy with repeat timing, fiducial markers and plumb linesTiming, calipers and micrometersConstructing and testing circuitsSignal generators and oscilloscopesGenerating and measuring wavesData loggers, modelling software and ionising radiation
Physical Quantities and Units
Making Measurements and Analysing Data
Nature of Quantities
Motion
Forces in Action
Work, Energy and Power
Materials
Newton’s Laws of Motion and Momentum
Charge and Current
Energy, Power and Resistance
Electrical Circuits
Waves
Topic overviewProgressive waves and their propertiesThe wave equationReflection, refraction and diffractionWave graphs and intensityThe electromagnetic spectrumPolarisationRefraction and refractive indexCritical angle and total internal reflectionThe principle of superpositionInterference, coherence and path differenceTwo-source interference, Young's double-slit experiment and the diffraction gratingForming stationary wavesStationary waves on strings and in air columnsMeasuring the speed of soundNode spacing, amplitude and phase in stationary wavesHarmonics and the fundamental mode
Quantum Physics
Thermal Physics
Topic overviewThermal equilibrium and absolute temperatureThe kinetic model of solids, liquids and gasesBrownian motion and internal energySpecific heat capacityMeasuring specific heat capacitySpecific latent heatMeasuring specific latent heatMoles, the Avogadro constant and kinetic theoryGas laws, the ideal gas equation and r.m.s. speedMaxwell-Boltzmann distribution, Boltzmann constant and internal energy
Oscillations
Gravitational Fields
Astrophysics and Cosmology
Topic overviewThe solar system and the birth of a starThe death of a low-mass starSupernovae, neutron stars and black holesThe Hertzsprung-Russell diagramEnergy levels and emission spectraAbsorption spectra and spectral linesPhoton energy and the diffraction gratingWien's law and Stefan's lawAstronomical distances, parallax and the cosmological principleDoppler shift and Hubble's lawThe expanding universe and the Big BangEvidence for the Big Bang and dark energy
Capacitors
Topic overviewCapacitance and charging a capacitorCapacitors in seriesCapacitors in parallel and combined circuitsEnergy stored by a capacitorCharging and discharging through a resistorInvestigating capacitor dischargeThe time constantIterative modelling, exponential decay and the constant-ratio property
Electric Fields
Electromagnetism
Nuclear and Particle Physics
Topic overviewAlpha-particle scattering and the nuclear modelProton number, nucleon number and isotopesThe strong nuclear force and nuclear radiusParticles, antiparticles and hadronsLeptons and the quark modelQuark composition and chargesBeta decay in the quark modelRadioactive decay and the three radiationsNuclear decay equationsActivity, decay constant and half-lifeExponential decay and radioactive datingMass-energy equivalenceAnnihilation, mass defect and binding energyBinding energy per nucleon and induced fissionThe fission reactor and nuclear wasteNuclear fusion and balancing nuclear equations
Capacitors
The time constant
- 2 things to know
- Papers 1, 2 and 3
Overview
Recall questions on the time constant CR with calculations, the fractions reached after one time constant during charging and discharging, the exponential discharge equation, and finding CR from a ln(x) against t graph.
What you need to know
Answer each one before you reveal it.
Your recallNot started
Numbers
State the exponential equation for capacitor discharge and what x₀ represents, and the fraction of its final value the charge or voltage reaches after one time constant of charging.
Questions from this subtopic
Each one with the model answer underneath.
- 1
State the equation for the time constant of a capacitor-resistor circuit.
Model answer
τ = CR, where C is capacitance and R is resistance.
- 2
What physical quantities are multiplied together to give the time constant of an RC circuit?
Model answer
Capacitance and resistance.
- 3
Calculate the time constant when capacitance = 0.0086 F, resistance = 94000 Ω.
Model answer
Time constant = 0.0086 × 94000 = 808 s.
- 4
During charging, what fraction of its final value does the charge or voltage reach after one time constant?
Model answer
It rises to (1 − 1/e), approximately 63%, of its final value.
- 5
State the general equation for the exponential decrease of a quantity x during capacitor discharge.
Model answer
x = x₀e^(−t/CR), where x₀ is the initial value, t is time, C is capacitance and R is resistance.
- 6
How can the time constant CR be determined from a ln(x) against t graph?
Model answer
By calculating the negative reciprocal of the gradient.
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