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
Gravitational Fields
The field strength of a point mass
- 3 things to know
- Unit A2 2
Overview
Recall questions on g = GM/r² and its derivation from Newton's law, why the test mass cancels, worked calculations of the Earth's surface field strength and of the value at one Earth radius above it, finding a planet's mass from g and r, the shape of the g against r graph, and plotting g against 1/r² for a straight line.
What you need to know
Answer each one before you reveal it.
Your recallNot started
Evaluation
Give the equation for the gravitational field strength due to a point mass and define each symbol.
Key term
Derive the equation g = GM/r² for the field strength of a point mass from Newton's law of universal gravitation
Questions from this subtopic
Each one with the model answer underneath.
- 1
Give the equation for the gravitational field strength due to a point mass and define each symbol.
Model answer
g = GM/r², where g is the field strength in N kg⁻¹, G the gravitational constant, M the mass producing the field in kilograms and r the distance from its centre in metres.
- 2
Derive that equation from Newton's law of universal gravitation.
Model answer
The force on a test mass m is F = GMm/r², and field strength is F/m, so dividing by m gives g = GM/r².
- 3
Calculate the field strength at the Earth's surface, taking M as 6.0 × 10²⁴ kg and r as 6.4 × 10⁶ m.
Model answer
g = 6.67 × 10⁻¹¹ × 6.0 × 10²⁴ ÷ (6.4 × 10⁶)², which is 4.00 × 10¹⁴ ÷ 4.10 × 10¹³, giving 9.8 N kg⁻¹.
- 4
Calculate the field strength at a height of one Earth radius above the surface.
Model answer
The distance from the centre is doubled, so by the inverse square law the field strength falls to a quarter, giving 2.5 N kg⁻¹.
- 5
How is the mass of a planet found from its surface field strength and radius?
Model answer
Rearranging g = GM/r² gives M = gr²/G.
- 6
A planet has a surface field strength of 3.7 N kg⁻¹ and a radius of 3.4 × 10⁶ m. Calculate its mass.
Model answer
M = gr²/G = 3.7 × (3.4 × 10⁶)² ÷ 6.67 × 10⁻¹¹, which is 4.28 × 10¹³ ÷ 6.67 × 10⁻¹¹, giving 6.4 × 10²³ kg.
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