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
Forces
Power, and Newton's third law
- 2 things to know
- Paper 11 (Physics)
Overview
The definition and unit of power with calculations for motors, lamps and stairs, the law describing interaction pairs, and why the two forces of a pair never cancel each other.
What you need to know
Answer each one before you reveal it.
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Key term
What is the unit of power, and what does one of it mean?
Questions from this subtopic
Each one with the model answer underneath.
- 1
Define power.
Model answer
Power is the rate at which energy is transferred, that is the energy transferred divided by the time taken.
- 2
What is the unit of power, and what does one of it mean?
Model answer
The watt, and 1 W means one joule of energy is transferred each second.
- 3
A motor transfers 6000 J of energy in 20 s. What is its power?
Model answer
6000 ÷ 20 = 300 W.
- 4
How much energy does a 60 W lamp transfer in 5 minutes?
Model answer
60 × 300 = 18000 J.
- 5
Two cranes lift identical loads through the same height, but one takes half the time. How do their powers compare?
Model answer
The faster crane has twice the power, because it transfers the same energy in half the time.
- 6
A student does 1500 J of work running up a flight of stairs in 6 s. What is her useful power output?
Model answer
1500 ÷ 6 = 250 W.
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