Flashcards · 4PH1 · Topic 4

Energy resources and energy transfers flashcards

25 cards on Topic 4, subtopics 4a to 4d. Pick a subtopic or study them all.

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4a Units

Unit of energy and work done: Joule (J).
Unit of power: Watt (W). 1 W = 1 J/s.
Units in kinetic energy = ½ × mass × speed²: Mass in kilograms (kg), speed in metre/second (m/s), energy in joules (J).

4b Energy transfers

Energy stores: Kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear, magnetic, electrostatic.
Energy transfer pathways: Mechanically, electrically, by heating, by radiation. "Heat" and "light" are pathways, not stores.
Conservation and dissipation of energy: Energy cannot be created or destroyed, only transferred between stores. Dissipated energy is not destroyed: it goes to the thermal store of the surroundings, spread out and no longer useful.
Efficiency. A motor gets 500 J of electrical energy and gives 350 J of kinetic energy.: Efficiency = (useful energy output ÷ total energy output) × 100% = 350 ÷ 500 × 100 = 70%.
What does a Sankey diagram show?: Energy transfers, with each arrow's width proportional to the amount of energy, so the split between useful and wasted energy is visible.
Conduction: Vibrating particles pass energy to neighbours; in metals, free electrons carry it far faster. Mainly in solids.
Convection: Heated fluid expands, becomes less dense and rises; cooler fluid sinks to replace it. Fluids only.
Radiation: Infrared electromagnetic waves; needs no medium. Can happen anywhere, including a vacuum.
Which surfaces are the best emitters, absorbers and reflectors of radiation?: Matt black: best emitter and best absorber. Shiny silver: best reflector and poorest emitter. A good absorber is also a good emitter.
How do loft insulation, cavity walls and shiny foil behind radiators reduce energy transfer?: Insulation traps air (a poor conductor) and stops convection currents. Shiny foil reflects radiation back into the room.

4c Work and power

Work done: Work done = force × distance moved in the direction of the force. Work done = energy transferred.
Gravitational potential energy and kinetic energy equations: GPE = mass × g × height (mgh). KE = ½ × mass × speed² (½mv²).
A crane lifts 200 kg through 15 m in 10 s (g = 9.8 N/kg). Work done and power?: W = mgh = 200 × 9.8 × 15 = 29 400 J. P = W/t = 29 400 ÷ 10 = 2940 W.
Power: The rate of transfer of energy, or the rate of doing work: power = work done ÷ time taken.
An object falls with no air resistance. How are its GPE and KE linked?: By conservation of energy, the GPE lost equals the KE gained (the work done by gravity).
A car doubles its speed. What happens to its kinetic energy?: It quadruples, because KE depends on v².

4d Energy resources and electricity generation

Renewable and non-renewable energy resources: Renewable: solar, wind, hydroelectric, tidal, wave, geothermal, biomass. Non-renewable: coal, oil, gas, nuclear. (Paper 2 only)
Energy transfers in a fossil-fuel power station: Chemical energy in the fuel → thermal energy (burning heats water to steam) → kinetic energy of the turbine → electrical energy from the generator. (Paper 2 only)
Wind and hydroelectric generation: Moving air or falling water turns a turbine directly, which drives a generator: kinetic energy → electrical energy. (Paper 2 only)
Solar cells vs solar heating systems: Solar cells transfer light directly to electrical energy. Solar heating systems use the Sun's radiation to heat water. (Paper 2 only)
Nuclear power: one advantage and two disadvantages: High output with no CO₂ during generation (non-renewable but not a fossil fuel). It produces long-lived radioactive waste and has high decommissioning costs. (Paper 2 only)
Factors for evaluating an energy resource: Reliability, environmental impact, cost (setup and running) and power output. E.g. wind is clean but unreliable; gas is reliable but emits CO₂. (Paper 2 only)