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 exponential decay equations
- 3 things to know
- Unit A2 2
Overview
Recall questions on the three decay equations for charge, potential difference and current with every symbol, what exponential decay means, worked calculations of the voltage after a stated time and of the time to reach a stated voltage, showing that equal intervals give equal fractional falls, and straightening the curve by plotting ln V against t.
What you need to know
Answer each one before you reveal it.
Your recallNot started
Key term
Give the equation for the potential difference across a discharging capacitor
Key term
Give the equation for the current during the discharge of a capacitor
Questions from this subtopic
Each one with the model answer underneath.
- 1
Give the equation for the charge remaining on a discharging capacitor and define each symbol.
Model answer
Q = Q₀exp(−t/CR), where Q is the charge at time t, Q₀ the initial charge, t the time in seconds, C the capacitance in farads and R the resistance in ohms.
- 2
Give the corresponding equation for potential difference.
Model answer
V = V₀exp(−t/CR), where V₀ is the initial potential difference.
- 3
Give the corresponding equation for current.
Model answer
I = I₀exp(−t/CR), where I₀ is the initial current.
- 4
What is meant by exponential decay?
Model answer
A decay in which the quantity falls by the same fraction in every equal interval of time, so its rate of fall is proportional to how much is left.
- 5
A 100 μF capacitor charged to 12 V discharges through 10 kΩ. Calculate the potential difference after 1.0 s.
Model answer
CR is 1.0 s, so V = 12 × e^(−1.0), which is 12 × 0.368, giving 4.4 V.
- 6
Calculate the potential difference in that circuit after 2.0 s.
Model answer
V = 12 × e^(−2.0) = 12 × 0.135, which is 1.6 V.
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