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
Electricity
Resistors whose resistance changes, and investigating them
- 3 things to know
- Paper 1
Overview
What constant resistance means, why a filament lamp and a heated wire resist more, how diodes, thermistors and light dependent resistors behave, and circuits for investigating each of them.
What you need to know
Answer each one before you reveal it.
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Key term
Explain, in terms of the particles in a metal, why heating a wire raises its resistance.
Key term
Why does a metal wire keep a steady resistance when only a small current is passed through it?
Questions from this subtopic
Each one with the model answer underneath.
- 1
What is meant by a component having a constant resistance?
Model answer
Its resistance stays the same however much current passes through it, provided its temperature does not change.
- 2
Why does the resistance of a filament lamp increase as the current through it increases?
Model answer
The larger current heats the filament, so its particles vibrate more and get in the way of the moving electrons more often.
- 3
Explain, in terms of the particles in a metal, why heating a wire raises its resistance.
Model answer
The metal ions vibrate further and faster, so the electrons collide with them more often and find it harder to move through the wire.
- 4
Why does a metal wire keep a steady resistance when only a small current is passed through it?
Model answer
A small current barely warms the wire, so its temperature and therefore its resistance stay almost unchanged.
- 5
A diode is connected the other way round in a circuit. What happens to its resistance?
Model answer
It becomes extremely high, so almost no current passes.
- 6
How does the resistance of a thermistor change as it gets hotter?
Model answer
Its resistance falls as its temperature rises.
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