Key term
OCR GCSE Combined Science Physics J250
Electricity
Topic overviewCharge, and producing static electricityTransfer of electrons in charging and dischargingCurrent 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, efficiency, and increasing itReducing 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 lawResolving forces, and free body diagramsResultant forces, and balanced forcesNewton's second law, and inertiaMomentum and its conservationWork done, and stored energyPower, and Newton's third lawCircular motion and changing velocityStretching, 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 strength
Global Challenges
Topic overviewTypical speeds and everyday accelerationsReaction time, stopping distance and the dangers of large decelerationsStopping distance, acceleration and collision force calculationsEnergy 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 danger
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 pressure
Radioactivity
Waves in Matter
Topic overviewAmplitude, wavelength, frequency and periodThe wave equationTransverse and longitudinal wavesModelling 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 raysProducing and receiving radio wavesAbsorption, transmission, refraction and reflectionInvestigating reflection and refraction of light
Radioactivity
Half-life and the randomness of decay
- 3 things to know
- Paper 12 (Physics)
Overview
Half-life in terms of nuclei and count rate, why decay is random yet a sample has a reliable half-life, finding a half-life from count rates and graphs, the net decline in activity as a ratio after several half-lives, activity calculations, and modelling decay with dice.
What you need to know
Answer each one before you reveal it.
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Key term
What is meant by saying that radioactive decay is random?
Key term
If individual decays cannot be predicted, why does a sample still have a reliable half-life?
Questions from this subtopic
Each one with the model answer underneath.
- 1
What is meant by the half-life of a radioactive source?
Model answer
It is the average time taken for half of the unstable nuclei in a sample to decay.
- 2
How can half-life be described in terms of the reading taken on a detector?
Model answer
It is the time taken for the measured count rate from the source to fall to half its starting value.
- 3
Why is it impossible to say which nucleus in a sample will decay next?
Model answer
Because radioactive decay is a random process, so no nucleus can be picked out in advance.
- 4
What is meant by saying that radioactive decay is random?
Model answer
Every undecayed nucleus has the same chance of decaying in the next moment, and nothing can be done to make a particular one decay.
- 5
Two identical unstable nuclei sit side by side. Will they decay at the same moment?
Model answer
No, one may decay almost at once and the other may last for a very long time.
- 6
If individual decays cannot be predicted, why does a sample still have a reliable half-life?
Model answer
Because a sample holds an enormous number of nuclei, so the average behaviour of all of them is very predictable.
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