Flashcards · J249 · Topic 7

Energy flashcards

14 cards on Topic 7, subtopics P7.1 to P7.2. Pick a subtopic or study them all.

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Conservation of energy: Energy is never created or destroyed; it is transferred from store to store. In a closed system there is no net change to the total energy.
Three ways the energy of a system can change: Heating, work done by forces, and work done by an electric current.
Energy stores: a ball thrown upwards: The kinetic store decreases and the gravitational potential store increases (plus a little thermal store if there is air resistance).
Kinetic, gravitational potential and elastic energy equations: Kinetic: E = ½mv² Gravitational potential: E = mgh Elastic: E = ½kx²
A cyclist and bike (80 kg) move at 9.0 m/s. Kinetic energy?: E = ½ × 80 × 9.0² = 3240 J
A 60 W lamp is on for 2.0 hours. Energy in kW h and in J?: 0.060 kW × 2.0 h = 0.12 kW h 60 W × 7200 s = 432 000 J
0.25 kg of water is warmed from 20 °C to 90 °C (c = 4200 J/kg °C). Energy?: ΔE = 0.25 × 4200 × 70 = 73 500 J

Dissipation: Energy spreads out and ends up stored in less useful ways, mostly in the thermal store of the surroundings. It is not destroyed.
Power rating vs efficiency: Power rating: energy transferred per second (a higher rating means stores change faster). Efficiency: the fraction of the input that is useful. A higher rating does not mean more efficient.
Efficiency equation: efficiency = useful output energy transfer ÷ input energy transfer. × 100 for a percentage. Real devices are always below 100%.
A fan takes in 640 J and wastes 160 J. Efficiency?: Useful = 480 J, so efficiency = 480 ÷ 640 = 0.75 (75%)
How does lubrication increase efficiency?: A layer of liquid separates the surfaces, so friction falls and less energy is transferred to thermal stores.
How does loft insulation or double glazing reduce energy loss?: Trapped air (or gas) is a poor conductor and cannot circulate, so conduction and convection are reduced. Insulation slows energy transfer; it never stops it.
Two features of a building’s walls that reduce its rate of cooling: Greater thickness and lower thermal conductivity.