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
Global Challenges
Why the national grid is efficient, and power in a transformer
- 3 things to know
- Paper 12 (Physics)
Overview
Why power = potential difference × current means a higher potential difference needs a smaller current, how the energy wasted in cables depends on the square of that current, why thick cables and one shared network help, and equal power in and out of an ideal transformer, with calculations.
What you need to know
Answer each one before you reveal it.
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Key term
What happens to the energy that is wasted in transmission cables?
Key term
How does the energy wasted in a cable depend on the current in it?
Questions from this subtopic
Each one with the model answer underneath.
- 1
Why does transferring power at a high potential difference make the grid efficient?
Model answer
For a given power the current in the cables is much smaller, and a smaller current wastes far less energy.
- 2
What happens to the energy that is wasted in transmission cables?
Model answer
The cables have resistance, so some energy is transferred to their thermal store and dissipated to the surroundings.
- 3
How does the energy wasted in a cable depend on the current in it?
Model answer
The power wasted rises with the square of the current, so halving the current wastes only a quarter as much.
- 4
What would happen if electrical power were sent across the country at the low potential difference used in homes?
Model answer
The current would be enormous, the cables would get very hot and most of the energy would be wasted before it arrived.
- 5
Besides using a high potential difference, how is energy loss in transmission cables kept low?
Model answer
The cables are thick and made of a good conductor, which gives them a low resistance.
- 6
Roughly how much of the energy put into the grid reaches consumers?
Model answer
Most of it, as only a few per cent is lost as heating in the cables.
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