Efficiency & Energy Resources Flashcards
All 24 cards in this deck
What makes a mechanical process wasteful?
It causes a rise in temperature, dissipating energy in heating the surroundings.
In an electric motor lifting a load, which energy transfer is useful and which is wasted?
Useful: energy transferred to the kinetic (and gravitational) store of the load. Wasted: energy dissipated to the thermal store of the surroundings by friction, plus sound.
Why is the efficiency of an engine always less than 1 (100%)?
The useful output energy is less than the input energy, because some energy is transferred to less useful forms — dissipated to the surroundings as thermal energy.
Give a reason, other than friction, why energy is wasted in a pulley system.
There is additional mass in the system (the bottom pulley and the rope have weight), or the rope stretches.
How does lubrication reduce unwanted energy transfer in a machine?
Oil between the moving surfaces reduces friction, so less energy is dissipated to the thermal store of the surroundings.
How do the thickness and the thermal conductivity of a building's walls affect its rate of cooling?
Thicker walls cool the building more slowly; walls of lower thermal conductivity also cool it more slowly.
Hot water cans are wrapped in two different materials. How do you tell which material is the better insulator?
The better insulator is around the can whose water temperature falls the least in the same time (it cools more slowly).
What is the equation for efficiency?
efficiency = useful energy transferred by the device ÷ total energy supplied to the device
How is efficiency worked out from power values instead of energy values?
efficiency = useful power output ÷ total power input
How can the efficiency of a device be increased?
Reduce the energy that is wasted (dissipated to the surroundings), so a greater fraction of the input energy is transferred usefully.
How does streamlining a vehicle increase its efficiency?
It reduces air resistance, so less energy is dissipated as thermal energy and more of the input energy becomes useful output.
Name the non-renewable energy resources available for use on Earth.
Fossil fuels (coal, oil and natural gas) and nuclear fuel.
What is a renewable energy resource?
One that is replenished as it is used, so it will not run out; e.g. wind, hydro-electricity, the tides, the Sun and bio-fuel.
True or false? A wind farm can generate electricity reliably whenever it is needed.
False. Wind turbines only generate when the wind blows, whereas a fossil-fuel or nuclear power station can generate on demand.
Give one reason why renewable sources provide an increasing fraction of the electricity supply in many countries.
Renewables will not run out and do not release carbon dioxide, while non-renewables are running out and cause pollution.
Why do insulating materials such as loft insulation and cavity wall insulation contain trapped air?
Trapped air is a poor thermal conductor and cannot form convection currents, so energy is transferred out much more slowly.
True or false? Insulating a hot water tank stops energy being transferred to the surroundings completely.
False. Insulation only reduces the rate of energy transfer; the water still cools, just more slowly.
What does the thermal conductivity of a material describe?
How readily energy is conducted through the material: the higher it is, the faster energy passes through. e.g. copper is high, polystyrene foam is low.
Why does burning fossil fuels contribute to global warming?
Burning them produces carbon dioxide, a greenhouse gas that traps more heat in the atmosphere.
What is the main environmental advantage of nuclear power stations?
They release no carbon dioxide while generating electricity.
What is the main environmental disadvantage of nuclear power?
Its spent fuel is radioactive waste that stays hazardous for a very long time and must be stored securely.
What are bio-fuels made from?
Plant material or animal waste, e.g. crops, wood or manure.
Why is hydro-electric power useful when demand for electricity suddenly rises?
Extra stored water can be sent through the turbines at once, so its output can be raised very quickly.
Why has UK electricity use been falling since the start of the 21st century?
Devices have become more energy-efficient, and households make more effort to save energy.