Efficiency & Energy Resources Flashcards

All 28 cards in this deck

What does it mean to say energy is dissipated?

It is transferred to less useful stores, usually the thermal store of the surroundings, and spreads out so it cannot be used.

What makes a mechanical process wasteful?

Friction between moving parts causes a temperature rise, so energy is dissipated by heating the surroundings instead of being transferred usefully.

Which energy store is wasted energy usually transferred to?

The thermal store of the surroundings.
e.g. a filament lamp heats the air around it as well as giving out light.

True or false? Wasted energy is destroyed.

False. Energy is never destroyed — it is dissipated to less useful stores, mainly heating the surroundings.

True or false? In every system change, some energy is dissipated so that it is stored in less useful ways.

True. No real energy transfer is completely useful.

What is thermal conductivity?

A measure of how quickly energy is transferred by conduction through a material.

What effect do thicker walls have on the rate of cooling of a building?

They reduce the rate of cooling — energy is transferred through them more slowly.

What effect does a high thermal conductivity of the walls have on the rate of cooling of a building?

The building cools more quickly, because energy is conducted through the walls faster.

True or false? Trapped air makes a good thermal insulator.

True. Air has a low thermal conductivity, which is why cavity wall insulation and double glazing work.

What are the steps to investigate which of two materials is the better insulator?
e.g. comparing sand and sawdust around a can of hot water

1. Put the same volume of hot water, at the same starting temperature, into identical cans surrounded by the same thickness of each material.
2. Use a thermometer and stop clock to record the water temperature at equal time intervals.
3. The material giving the smaller temperature drop (sand or sawdust) is the better insulator.

What is the equation for efficiency in terms of energy?

$\text{efficiency} = \dfrac{\text{useful energy transferred by the device}}{\text{total energy supplied to the device}}$

What is the equation for efficiency in terms of power?

$\text{efficiency} = \dfrac{\text{useful power output}}{\text{total power input}}$

What are the steps to calculate the efficiency of a device as a percentage?
e.g. a motor is supplied with $200\ \text{J}$ and usefully transfers $50\ \text{J}$

1. Write the equation: efficiency = useful energy transferred ÷ total energy supplied
2. Substitute: $50 \div 200 = 0.25$
3. Multiply by 100: efficiency = $25\%$

What are the steps to find the total energy supplied to a device from its efficiency?
e.g. a device of efficiency $0.5$ usefully transfers $20\ \text{J}$

1. Substitute into the equation: $0.5 = \dfrac{20}{\text{total}}$
2. Rearrange: total = useful energy ÷ efficiency
3. Evaluate: $20 \div 0.5 = 40\ \text{J}$

What does an efficiency of $0.4$ tell you about a device?

$40\%$ of the energy supplied to it is transferred usefully; the other $60\%$ is wasted.

True or false? A device can have an efficiency greater than $1$.

False. Useful energy transferred can never be more than the total energy supplied.

Why is the efficiency of a real engine always less than $100\%$?

The useful output energy is less than the input energy, because some energy is dissipated to less useful stores (heating the surroundings).

What effect does lubricating moving parts with oil have?

It reduces friction, so less energy is dissipated by heating — more useful energy output for the same input.

True or false? Oiling a bicycle chain increases the total energy supplied to the bicycle.

False. It reduces the energy wasted, so more of the same input becomes useful output (or less input is needed).

In general, how is the efficiency of a device increased?

By reducing unwanted energy transfers, e.g. lubricating moving parts, insulating, streamlining, or making parts lighter.

How does thermal insulation in the walls and loft make a house more efficient?

The insulation has a low thermal conductivity, so energy is transferred out more slowly and less energy is needed for heating.

Which unwanted energy transfer is reduced by streamlining a vehicle?

Energy wasted against air resistance (drag).

Name the non-renewable energy resources.

Fossil fuels (coal, oil, natural gas) and nuclear fuel.

Name five renewable energy resources.

Wind, hydro-electricity, the tides, the Sun (solar) and bio-fuel.

What is a renewable energy resource?

One that is replenished as it is used, so it will not run out.

True or false? Bio-fuel is a non-renewable energy resource.

False. Bio-fuel is renewable — the crops or animal waste it comes from can be replaced.

Give one advantage of fossil fuels compared with renewable resources.

They are reliable — they can supply large amounts of energy on demand, unlike wind or solar which depend on the weather.

Describe the trend in the use of energy resources for electricity in recent decades.

Use of renewables is increasing and use of non-renewables is decreasing, because fossil fuels will run out and release carbon dioxide.