Flashcards · 9203 · Topic 2

Energy flashcards

21 cards on Topic 2, subtopics 3.2.1 to 3.2.3. Pick a subtopic or study them all.

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Work done: W = F × d (J, N, m), with d along the line of the force. Work done = energy transferred; 1 J = 1 N m. Holding something still does no work on it.
What happens to work done against friction?: It is transferred by heating to the surfaces and surroundings (not lost). E.g. brakes heat up; a meteor glows as air resistance heats it.
Elastic potential energy. A spring (k = 120 N/m) is stretched from 0.30 m to 0.45 m.: Ee = ½ × k × e², with e the extension (0.15 m): ½ × 120 × 0.15² = 1.35 J. Valid below the limit of proportionality.
Gravitational potential energy: Ep = m × g × h, where h is the vertical height gained (not the distance along a ramp). Use the g given in the question.
Kinetic energy: effect of doubling mass, or doubling speed: Ek = ½ × m × v². Doubling mass doubles Ek; doubling speed makes Ek four times as large; tripling speed, nine times.
An 800 kg van at 10 m/s brakes with a 4000 N force. Braking distance?: Ek = ½ × 800 × 10² = 40 000 J. Braking force × distance = Ek, so d = 40 000 ÷ 4000 = 10 m.
Power. A hoist does 15 000 J of work in 20 s:: P = E/t = W/t, in watts (1 W = 1 J/s). P = 15 000 ÷ 20 = 750 W.

Energy transfers: a released catapult, a falling object, a thrown ball going up: Catapult: elastic potential → kinetic. Falling: gravitational potential → kinetic. Thrown up: kinetic → gravitational potential.
Why does a pendulum's swing get lower over time?: Energy moves between kinetic and gravitational potential, but air resistance and friction at the pivot dissipate a little energy to the surroundings each swing. It is not destroyed.
Conservation and dissipation of energy: Energy cannot be created or destroyed, only transferred usefully, stored or dissipated. Dissipated energy is spread into the surroundings. Reduce it with lubrication, insulation and streamlining.
Efficiency. A motor takes in 150 W and gives 93 W of useful power:: Efficiency = useful output ÷ total input = 93 ÷ 150 = 0.62 (62%). Wasted power = 57 W. It has no unit and is always less than 1.
A device is 40% efficient and gives 270 J of useful energy. Input and dissipated energy?: Input = 270 ÷ 0.40 = 675 J. Dissipated = 675 − 270 = 405 J.
Sankey diagram: input arrow 40 mm, useful arrow 10 mm, scale 1 mm = 2 kJ. Efficiency?: Input 80 kJ, useful 20 kJ, so efficiency = 20 ÷ 80 = 0.25. Arrow widths are in proportion to the energy.
A 0.25 kg pendulum bob is released 0.080 m above its lowest point (g = 9.8 N/kg). Speed at the bottom?: GPE = 0.25 × 9.8 × 0.080 = 0.196 J = KE at the bottom. v² = 2 × 0.196 ÷ 0.25 = 1.568, so v = 1.25 m/s.

Renewable vs non-renewable: Renewable: replaced naturally as fast as it is used (wind, hydroelectric, tidal, wave, solar, geothermal, biofuels). Non-renewable: used up much faster than it forms, so it can run out (coal, oil, natural gas, nuclear fuel).
Factors when choosing a fuel: Ease of storage (solids and liquids easier than gases), energy content (energy released per kg) and safety (ease of ignition, toxic fumes, explosion risk).
2.0 kg of fuel (30 MJ/kg) heats water, which gains 33 MJ. Fuel efficiency?: Energy released = 2.0 × 30 = 60 MJ. Efficiency = 33 ÷ 60 = 0.55 (55%).
Environmental effects of fossil fuels and of nuclear fuel: Fossil fuels: carbon dioxide (global warming); coal can release sulfur dioxide (acid rain); mining and oil spills damage habitats. Nuclear: no CO₂ in use, but mining and radioactive waste needing long-term storage.
Wave power: advantages and drawbacks: No fuel and no CO₂ in use; good for coastal countries. Depends on sea conditions, costly to build and repair, may disturb marine life and shipping.
Solar and geothermal: one advantage and one drawback each: Solar: no fuel or emissions in use, but no output at night and less in cloud. Geothermal: steady output day and night, but needs accessible hot rock and costly drilling.
12 m² of solar panel receives 450 W/m² and is 18% efficient. Useful output?: Input = 12 × 450 = 5400 W. Output = 0.18 × 5400 = 972 W.