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
Energy
Power ratings, efficiency, and increasing it
- 3 things to know
- Paper 12 (Physics)
Overview
Power as energy transferred each second, what a power rating means, calculations of energy, power and time, why a low-power appliance left on can cost more, calculating efficiency from energies or powers, how lubrication, streamlining, bearings, insulation and thicker cables increase efficiency, and why no device is completely efficient.
What you need to know
Answer each one before you reveal it.
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Key term
Give the two ways of calculating efficiency, from energies and from powers, and explain why efficiency can never exceed 100 per cent.
Key term
What is an appliance's power, in watts, how is power found, and what does a 2500 W kettle do compared with a 60 W lamp?
Questions from this subtopic
Each one with the model answer underneath.
- 1
What is meant by the power of an appliance?
Model answer
The energy it transfers each second, measured in watts.
- 2
State the equation linking power, energy transferred and time.
Model answer
Power = energy transferred ÷ time taken.
- 3
What does a power rating of 2 kW tell you about an appliance in use?
Model answer
It transfers 2000 J of energy every second while it is switched on.
- 4
A kettle is rated at 2500 W and a lamp at 60 W. What does this tell you about how quickly each empties the stores that supply it?
Model answer
The kettle transfers energy about forty times faster, so its stores are emptied far more quickly.
- 5
A hairdryer rated at 1000 W is used for 5 minutes. How much energy does it transfer?
Model answer
300000 J.
- 6
A microwave oven rated at 900 W transfers 108000 J of energy. For how long was it running?
Model answer
120 s.
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