Energy Resources, Efficiency, and Energy Transfers Flashcards

Flashcards reviewing key concepts of energy resources, efficiency calculations, unwanted energy transfers, and conservation of energy.

What is the Law of Conservation of Energy?

Energy can be stored, transferred between stores, and dissipated, but it can never be created or destroyed. The total energy of a closed system has no net change.

What is a closed system in physics?

A system (a collection of objects) that can be treated completely on its own, without any energy being exchanged to or from the surroundings.

What are the four main ways energy can be transferred between stores?

1) Mechanically (a force acting on an object doing work), 2) Electrically (a charge doing work), 3) By heating (energy transferred from a hotter object to a colder object), and 4) By radiation (energy transferred by waves).

How is energy transferred when a ball rolls up a slope?

Energy is transferred mechanically from the kinetic energy store of the ball to its gravitational potential energy store as it does work against gravity.

How is energy transferred when a kettle boils water?

Energy is transferred electrically from the mains to the heating element of the kettle, and then by heating to the thermal energy store of the water.

How is energy transferred when a car slows down without braking?

Energy in the kinetic energy store of the car is transferred mechanically due to friction between the tyres and road, and then by heating to the thermal energy stores of the car and road.

What is the formula for calculating the efficiency of an energy transfer?

$\text{efficiency} = \frac{\text{useful energy transferred by device}}{\text{total energy supplied to device}}$ (To express as a percentage, multiply by $100$).

If a toaster receives $216\,000\,J$ of energy from the mains and transfers $84\,000\,J$ to the bread's thermal energy store, what is its efficiency?

$\text{efficiency} = \frac{84\,000\,J}{216\,000\,J} \approx 0.39$ (or $39\%$ to 2 significant figures).

An electrical device wastes $420\,J$ of energy when given an input energy of $500\,J$. What is its efficiency as a percentage?

Useful energy = $500\,J - 420\,J = 80\,J$. Efficiency = $\frac{80\,J}{500\,J} \times 100 = 16\%$.

Why can no device have an efficiency equal to or greater than $1$ (or $100\%$)?

Because in every energy transfer, some energy is always dissipated or wasted, often to the thermal energy stores of the surroundings.

What feature of arrows on a Sankey diagram represents the amount of energy transferred?

The thickness (or width) of the arrows represents the amount of energy transferred; the length has nothing to do with it.

What are two main methods used to reduce unwanted energy transfers and increase efficiency?

Lubrication (to reduce friction) and thermal insulation (to reduce the rate of energy transfer by heating).

How do lubricants reduce friction between moving objects?

Lubricants are usually liquids (like oil) that flow easily between touching objects and coat them, reducing the frictional force and preventing energy from being mechanically dissipated as heat.

What is thermal conductivity, and how does it differ between metals and gases?

Thermal conductivity describes how well a material transfers energy by conduction. Metals have a high thermal conductivity, whereas gases (like air) have a low thermal conductivity.

How do cavity walls with an air gap reduce energy transfer in a building?

The air gap between the inner and outer walls reduces energy transferred by conduction because air has a very low thermal conductivity.

What are non-renewable energy resources, and what are the main examples?

Energy resources that will run out one day. The three main fossil fuels are coal, oil, and natural gas, along with nuclear fuel (uranium and plutonium).

What are the main advantages of fossil fuels and nuclear energy?

They are reliable, there is plenty of fuel in stock to meet current demand, power plants can respond quickly to changes in demand, extraction costs are low, and fossil fuel power plants are relatively cheap to build and run.

What environmental problems are caused by burning coal and oil?

Burning them releases carbon dioxide ($CO_2$), which contributes to the greenhouse effect and global warming, and sulfur dioxide ($SO_2$), which causes acid rain.

What are the specific disadvantages associated with nuclear power?

Nuclear power plants are costly to build and safely decommission, nuclear waste is dangerous and difficult to dispose of, and there is a risk of major catastrophes (such as the Fukushima disaster in Japan).

What are bio-fuels, and why are they described as carbon neutral under ideal conditions?

Bio-fuels are renewable resources made from plant products or animal dung. They are considered carbon neutral if plants are grown or animals are raised at the exact same rate that bio-fuels are being burned.

What are the main disadvantages of bio-fuels?

They cannot respond to immediate energy demands, refining costs are very high, growing crops for bio-fuels can take up space and water needed for food crops, and clearing forests leads to habitat loss and the release of methane and $CO_2$.

How do wind turbines generate electricity, and what are their limitations?

Wind rotates the turbine blades, which turn a generator inside to produce electricity. Limitations: initial costs are high, lots of turbines are needed to match a coal plant, they spoil the view, are noisy, and only work when it is windy.

How do solar cells work, and what are their primary advantages and disadvantages?

Solar cells use energy transferred by light to create an electric current. Advantages: no pollution in use, no running costs, ideal for remote places (e.g., Australian outback). Disadvantages: high initial costs, cannot increase production for extra demand, cannot generate power at night.

What is the setup and main operational advantage of hydro-electricity as shown in the energy generation diagram?

Rainwater is caught behind a big dam built across a flooded valley and released through turbines connected to a generator. Its major advantage is being able to immediately respond to increased electricity demand by letting out more water.

What are tidal barrages, and when do they fail to produce electricity?

Tidal barrages are big dams built across river estuaries with turbines in them that generate electricity as the tide fills and empties the estuary. They do not work when the water level is the same on both sides of the barrage.

What energy transfer process occurs when a motor lifts a load?

The motor transfers energy usefully from its kinetic energy store to the kinetic and gravitational potential energy stores of the load, while wasting energy mechanically to the thermal stores of its moving parts and electrically to its circuits.