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
Energy stored in a charged capacitor
- 3 things to know
- Unit A2 2
Overview
Recall questions on E = ½QV and its two equivalent forms, why the factor of ½ appears, the energy as the area under a charge against potential difference graph, the fourfold rise when the voltage doubles, worked energy calculations, why only half the battery's energy is stored, and the defibrillator as a worked case.
What you need to know
Answer each one before you reveal it.
Your recallNot started
Key term
Why is the energy stored ½QV rather than QV?
Key term
A 100 μF capacitor charged to 50 V stores 0.125 J by ½CV². Check that energy using the charge stored
Questions from this subtopic
Each one with the model answer underneath.
- 1
Give the equation for the energy stored in a charged capacitor in terms of charge and potential difference.
Model answer
E = ½QV, where E is the energy in joules, Q the charge in coulombs and V the potential difference in volts.
- 2
Why is the energy stored ½QV rather than QV?
Model answer
The potential difference rises from zero to V as the capacitor charges, so the average potential difference during charging is only half the final value.
- 3
How is the energy stored found from a graph of charge against potential difference?
Model answer
It is the area under the line, which for a straight line through the origin is the area of a triangle, ½QV.
- 4
Calculate the energy stored by a 100 μF capacitor charged to 50 V.
Model answer
E = ½CV² = 0.5 × 100 × 10⁻⁶ × 50², which is 0.5 × 100 × 10⁻⁶ × 2500, giving 0.125 J.
- 5
Check that energy using the charge stored.
Model answer
Q = CV = 100 × 10⁻⁶ × 50 = 5.0 × 10⁻³ C, and ½QV = 0.5 × 5.0 × 10⁻³ × 50 = 0.125 J, which agrees.
- 6
The potential difference across a capacitor is doubled. What happens to the energy stored?
Model answer
It becomes four times as great, because energy is proportional to the square of the potential difference.
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