Key term
OCR GCSE Physics A J249
Electricity
Topic overviewCharge, and producing static electricityTransfer of electrons, and electric fieldsCurrent as a rate of flow of chargeCharge, current and timeSeries and parallel circuits, and circuit symbolsCurrent, potential difference and resistanceResistors whose resistance changes, and investigating themLinear and non-linear circuit elements from graphsNet resistance in series and parallelCircuits for measurement, and electrical power
Energy
Topic overviewConservation of energy, and energy storesEnergy changes on heating, and calculating themEnergy of a moving body, a spring and a raised objectDissipation, and energy transfer in domestic devicesPower ratings, and calculating efficiencyReducing unwanted energy transfer, and the rate of cooling
Forces
Topic overviewMeasuring distance and time, and calculating speedConverting units, and ratesVectors and scalars in motionDistance-time and velocity-time graphsAverage speed, and the equations of uniform motionHow objects interact, and forces as vectorsNewton's first lawNewton's second lawForce and the time taken to change velocityWork done, and stored energyPower, and Newton's third lawStretching, bending and compressing; elastic and plasticForce and extension, linear and non-linearThe spring constant, and work done in stretchingGravitational fields, weight and free fallCalculations with weight and gravitational field strengthTurning forces and the moment of a forceLevers and gearsPressure in fluids, and the force on a surface
Global Challenges
Topic overviewTypical speeds and everyday accelerationsReaction time, stopping distance and the dangers of large decelerationsEstimating stopping distances and the forces in a collisionEnergy resources, and trends in their useThe national grid, and step-up and step-down transformersWhy the national grid is efficient, and power in a transformerMains electricity: a.c., d.c. and the domestic supplyLive, neutral and earth wires, and electrical dangerRed-shift and the Big BangThe formation of the Sun, and radiation from bodiesThe solar system
Magnetism and Magnetic Fields
Matter
Topic overviewHow the atomic model changed, and the structure of the atomThe size of atoms, and defining densityDensity of the states of matter, and density calculationsConservation of mass in changes of stateHeating a system, and specific heat capacityCalculations with specific heat capacity and latent heatGas molecules, temperature and pressureCompressing a gas, and changing its volumeAtmospheric pressure and why it varies with height
Radioactivity
Topic overviewProtons, neutrons and isotopesUnstable nuclei and the emissions they give outBalancing nuclear equationsElectron energy levels, and radiation from atomsHalf-life and the randomness of decayPenetration of alpha, beta and gammaContamination and irradiation, and half-life as a hazardUses of nuclear radiationNuclear fission and fusion
Waves in Matter
Topic overviewAmplitude, wavelength, frequency and periodThe wave equationTransverse and longitudinal wavesSound crossing a boundary; reflection, transmission and absorptionModelling waves with ripples and soundElectromagnetic waves as transverse, and energy transferFrequency and wavelength across the spectrumWhat our eyes detect, and light as an electromagnetic waveUses of electromagnetic wavesHazards of ultraviolet, X-rays and gamma raysRay diagrams for reflection and refractionHow colour arises
Matter
The size of atoms, and defining density
- 3 things to know
- Paper 1
Overview
The typical size of atoms and small molecules in standard form and nanometres, order-of-magnitude estimates, the symbols for much less than, much greater than and approximately, and density as mass per unit volume, its units and how it compares between objects.
What you need to know
Answer each one before you reveal it.
Your recallNot started
Numbers
Roughly how many atoms placed side by side would stretch across 1 mm?
Key term
Why can individual atoms not be seen using a light microscope?
Questions from this subtopic
Each one with the model answer underneath.
- 1
What is the typical size of an atom?
Model answer
About 1 × 10⁻¹⁰ m across.
- 2
How does the size of a small molecule compare with the size of a single atom?
Model answer
It is the same order of magnitude, around 1 × 10⁻¹⁰ m, because it is built from only a few atoms joined together.
- 3
Express the typical size of an atom in nanometres.
Model answer
About 0.1 nm, because 1 nm is 1 × 10⁻⁹ m.
- 4
Roughly how many atoms placed side by side would stretch across 1 mm?
Model answer
About ten million, since 1 mm is 1 × 10⁻³ m and each atom is about 1 × 10⁻¹⁰ m across.
- 5
Why can individual atoms not be seen using a light microscope?
Model answer
An atom is only about 1 × 10⁻¹⁰ m across, far smaller than the wavelength of visible light, so light cannot resolve it.
- 6
Why are the sizes of atoms usually written in standard form?
Model answer
Their size is about 1 × 10⁻¹⁰ m, which written out in full as a decimal would be a long and awkward number.
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