Key term
CCEA A-Level Physics 1210
Physical Quantities, Scalars and Vectors
Moments and Equilibrium
Linear Motion and Dynamics
Newton's Laws of Motion
Linear Momentum and Impulse
Work, Energy and Power
Current, Charge and Potential Difference
Resistance and Resistivity
Topic overviewResistance and Ohm's lawOhmic and non-ohmic behaviourResistors in series and in parallelInvestigating current, voltage and resistance in series and parallel circuitsElectrical power and energy in resistive circuitsResistivity and the equation R = ρl/AMeasuring the resistivity of a metalCurrent–voltage characteristics of a conductor, a lamp and a diodeResistance, temperature and superconductivity
Internal Resistance and Potential Dividers
Topic overviewInternal resistance and terminal potential differenceConsequences of internal resistance for external circuitsMeasuring electromotive force and internal resistanceThe potential divider and variable outputPotential dividers in lighting and heating control circuitsLoaded potential dividers and output voltage calculations
Waves
Refraction
Lenses
Superposition, Interference and Diffraction
Topic overviewSuperposition of wavesStanding waves, nodes and antinodesStanding waves on stretched stringsStanding waves in a pipe closed at one endMeasuring the speed of sound with a resonance tubeConditions for observable interferencePath difference and phase differenceYoung's double slitDiffraction at a single slitThe diffraction grating
Quantum Physics
Wave-Particle Duality
Astronomy
Practical Skills and Data Analysis
Topic overviewChoosing and using measuring instrumentsPrecision, accuracy and recording measurementsUsing the cathode ray oscilloscope to measure voltage and frequencyTables, graphs and presenting resultsLinearising a relationship and using the gradientAbsolute and percentage uncertaintyCombining uncertainties and uncertainty on a graphRandom and systematic error, reliability and refining an experiment
Deformation of Solids
Thermal Physics
Topic overviewBoyle's law, Charles's law and the pressure lawThe ideal gas equation pV = nRTThe Avogadro constant, the mole and pV = NkTInternal energy and the distribution of molecular energyThe kinetic theory equation pV = ⅓Nm⟨c²⟩Average molecular kinetic energy and temperatureAbsolute zero and the kelvin scaleSpecific heat capacity and the electrical method
Uniform Circular Motion
Simple Harmonic Motion
Topic overviewDefining simple harmonic motion and a = −ω²xDisplacement, velocity and acceleration: x = A cos ωtThe simple pendulum and T = 2π√(l/g)The loaded spiral spring and T = 2π√(m/k)SHM graphs and finding velocity from a gradientFree vibrations, forced vibrations and resonanceDamping: light, critical and over-damping
The Nucleus and Radioactive Decay
Topic overviewAlpha-particle scattering and the nuclear atomNuclear radius and nucleon numberThe density of nuclear matterAlpha, beta and gamma radiation: nature, penetration and rangeNuclear equations and changes in nucleon and proton numberThe random and exponential nature of radioactive decayActivity, the decay constant and A = −λNExponential decay calculations with A = A₀e^(−λt)Half-life and measuring it
Nuclear Energy, Fission and Fusion
Gravitational Fields
Electric Fields
Capacitors
Magnetic Fields
Charged Particles and Accelerators
Capacitors
The time constant
- 3 things to know
- Unit A2 2
Overview
Recall questions on the definition of the time constant as the time to fall to 1/e, τ = CR and the proof that the farad times the ohm gives the second, the fractions remaining after one, two and five time constants, worked calculations, the experiment to determine it, and why the voltmeter must have a very high resistance.
What you need to know
Answer each one before you reveal it.
Your recallNot started
Key term
Show that the farad multiplied by the ohm gives the second.
Key term
What fraction of the initial value remains after one time constant?
Questions from this subtopic
Each one with the model answer underneath.
- 1
Define the time constant of a capacitor–resistor circuit.
Model answer
The time constant is the time taken for the charge, potential difference or current to fall to 1/e, about 37 per cent, of its initial value.
- 2
Give the equation for the time constant and define each symbol.
Model answer
τ = CR, where τ is the time constant in seconds, C the capacitance in farads and R the resistance in ohms.
- 3
Show that the farad multiplied by the ohm gives the second.
Model answer
A farad is a coulomb per volt and an ohm is a volt per amp, so their product is coulomb per amp, and an amp is a coulomb per second, giving the second.
- 4
What fraction of the initial value remains after one time constant?
Model answer
1/e, which is 0.37 or about 37 per cent.
- 5
What fraction remains after two time constants?
Model answer
1/e², which is 0.135 or about 14 per cent.
- 6
After how many time constants is the capacitor regarded as fully discharged in practice?
Model answer
About five, by which point less than one per cent of the original charge remains.
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