4.1 Energy — 8463 spec coverage — Oscar Song Tutoring

4.1.1.1

Energy stores and systems

A system is one object or a group of objects; when it changes, energy is transferred between stores within the system or between the system and its surroundings.
Describe a change by naming the store that decreases, the store that increases and the transfer pathway: mechanically, electrically, by heating or by radiation.
For a complete redistribution, the increases in all stores equal the decrease in the original store because total energy is conserved.
A common error is to say that an object 'contains energy' or that energy is used up; name a store and track where the energy is transferred instead.

Tier 1 · Easy

2 marks

ORIGINAL

A wheeled toy is given a push across a level floor and gradually stops. Describe the main energy-store changes after it is released.

Tier 2 · Standard

3 marks

ORIGINAL

An electrically powered hotplate transfers $96\,\text{kJ}$ of energy. The pan's thermal energy store increases by $61\,\text{kJ}$ and the food's thermal energy store increases by $27\,\text{kJ}$ . Calculate the energy transferred to other stores and state where it is likely to be stored.

Tier 3 · Hard

4 marks

ORIGINAL

A model launcher begins with $240\,\text{J}$ in its elastic potential energy store. After the model has left the launcher, the spring is fully relaxed. The model has $150\,\text{J}$ in its kinetic energy store and $54\,\text{J}$ in its gravitational potential energy store; these and the thermal stores account for the whole system. Calculate the energy in thermal stores and describe the complete redistribution.

4.1.1.2

Changes in energy

Use $E_k=\frac{1}{2}mv^2$ for kinetic energy, $E_e=\frac{1}{2}ke^2$ for elastic potential energy below the limit of proportionality, and $E_p=mgh$ for gravitational potential energy.
Convert mass to kilograms, speed to metres per second, extension to metres and height to metres before substituting; use the given value of $g$ .
If energy is transferred between stores without dissipation, equate the decrease in one store to the increase in the other and then solve for the unknown.
A common error is to forget that speed and extension are squared, or to use the spring's total length instead of its extension.

Tier 1 · Easy

2 marks

ORIGINAL

Calculate the kinetic energy of a $2.0\,\text{kg}$ cart moving at $3.0\,\text{m/s}$ .

Tier 2 · Standard

2 marks

ORIGINAL

A $35\,\text{kg}$ climber gains $4.2\,\text{m}$ in vertical height. Calculate the increase in the climber's gravitational potential energy store. Use $g=9.8\,\text{N/kg}$ .

Tier 3 · Hard

4 marks

ORIGINAL

A spring of spring constant $320\,\text{N/m}$ is compressed by $0.15\,\text{m}$ . It launches a $0.45\,\text{kg}$ cart on a level frictionless track. Calculate the cart's launch speed.

4.1.1.3

Energy changes in systems

A temperature change transfers energy to or from a system's thermal energy store according to $\Delta E=mc\Delta\theta$ .
For a specific heat capacity investigation, measure the mass, supply a known energy using $E=Pt$ , record the temperature change and calculate $c=E/(m\Delta\theta)$ .
Specific heat capacity is the energy needed to raise the temperature of $1\,\text{kg}$ of a substance by $1\,{}^\circ\text{C}$ .
A common error is to substitute the final temperature for $\Delta\theta$ ; subtract the initial temperature and account for energy transferred to the surroundings.

Tier 1 · Easy

2 marks

ORIGINAL

State what a specific heat capacity of $900\,\text{J/(kg}\,{}^\circ\text{C)}$ means.

Tier 2 · Standard

2 marks

ORIGINAL

A $1.5\,\text{kg}$ stone block has specific heat capacity $900\,\text{J/(kg}\,{}^\circ\text{C)}$ . Calculate the change in its thermal energy store when its temperature rises by $28\,{}^\circ\text{C}$ .

Tier 3 · Hard

5 marks

ORIGINAL

A $75\,\text{W}$ heater warms a $0.30\,\text{kg}$ sample for $6.0\,\text{minutes}$ . The sample's temperature rises by $32\,{}^\circ\text{C}$ and $80\%$ of the electrical energy is transferred to its thermal energy store. Determine the sample's specific heat capacity.

4.1.1.4

Power

Power is the rate of energy transfer or the rate of doing work: $P=E/t$ and $P=W/t$ .
Use joules and seconds to obtain watts, and rearrange before substituting when energy, work or time is unknown.
Two devices can transfer the same energy but have different powers; the device taking less time has the greater power.
A common error is to treat power as an amount of energy; $1\,\text{W}$ means an energy transfer of $1\,\text{J}$ every second.

Tier 1 · Easy

2 marks

ORIGINAL

A small motor transfers $600\,\text{J}$ of energy in $20\,\text{s}$ . Calculate its power.

Tier 2 · Standard

4 marks

ORIGINAL

Winch A and winch B each do $18\,\text{kJ}$ of work. A takes $30\,\text{s}$ and B takes $45\,\text{s}$ . Calculate both powers and compare them.

Tier 3 · Hard

5 marks

ORIGINAL

A hoist raises a $250\,\text{kg}$ load through $12\,\text{m}$ in $25\,\text{s}$ . Calculate the useful power of the hoist. Use $g=9.8\,\text{N/kg}$ . A second hoist performs the same lift in $18\,\text{s}$ ; calculate its useful power.

4.1.2.1

Energy transfers in a system

Energy can be transferred usefully, stored or dissipated, but it cannot be created or destroyed; a closed system has no net change in total energy.
Reduce unwanted mechanical transfers with lubrication, and reduce transfers by heating with insulation such as thicker walls or trapped air layers.
A material with greater thermal conductivity transfers energy by conduction at a greater rate; thicker walls reduce the rate of transfer through a given material.
A common error is to call dissipated energy 'lost'; it remains in thermal stores but is spread through the surroundings and is less useful.

Tier 1 · Easy

2 marks

ORIGINAL

Explain why adding oil to the axle of a turning wheel reduces unwanted energy transfers.

Tier 2 · Standard

3 marks

ORIGINAL

Wall X is twice as thick as wall Y and both are made from the same material. Explain which wall gives the lower rate of energy transfer by conduction. Then state how replacing the material with one of lower thermal conductivity affects the rate.

Tier 3 · Hard

6 marks

ORIGINAL

Plan an investigation to compare the effectiveness of three fabrics as thermal insulators around identical beakers of hot water. Include the measurements, control variables and how the results should be used.

4.1.2.2

Efficiency

Efficiency is $\text{useful output energy transfer}/\text{total input energy transfer}$ or $\text{useful power output}/\text{total power input}$ .
Calculate the ratio using matching quantities, then multiply by $100$ only when a percentage efficiency is required.
An efficiency of $0.72$ means $72\%$ of the input is transferred usefully and $28\%$ is dissipated; at Higher Tier, improve efficiency by reducing unwanted transfers, for example with lubrication or thermal insulation.
A common error is to divide total input by useful output, producing a value greater than $1$ or $100\%$ for an ordinary device.

Tier 1 · Easy

2 marks

ORIGINAL

A device receives $90\,\text{J}$ and transfers $72\,\text{J}$ usefully. Calculate its efficiency as a decimal and as a percentage.

Tier 2 · Standard

3 marks

ORIGINAL

A pump has a total power input of $560\,\text{W}$ and a useful power output of $420\,\text{W}$ . Calculate its efficiency and its wasted power.

Tier 3 · Hard

6 marks

ORIGINAL

A motor transfers $80\%$ of its input energy to a rotating shaft. A generator then transfers $65\%$ of the shaft energy to electrical energy. Calculate the overall efficiency and the useful electrical energy produced from an initial input of $250\,\text{kJ}$ . Suggest one change that could increase the efficiency of this intended transfer and explain how it helps.

4.1.3

National and global energy resources

Renewable resources include biofuel, wind, hydroelectricity, geothermal, tides, sunlight and water waves; fossil fuels and nuclear fuel are non-renewable.
Compare resources for their uses in transport, heating and electricity generation, considering reliability, response to demand and environmental effects.
Trends in resource use can reflect cost, technology, availability, environmental targets and political, social or ethical choices, not science alone.
A common error is to call a resource renewable because it produces little carbon dioxide while operating; renewability depends on replenishment as it is used.

Tier 1 · Easy

2 marks

ORIGINAL

Classify wind, natural gas, geothermal and nuclear fuel as renewable or non-renewable energy resources.

Tier 2 · Standard

4 marks

ORIGINAL

In a region, the share of transport energy supplied by biofuel rises from $4\%$ to $11\%$ , while the share of heating energy supplied by oil falls from $38\%$ to $25\%$ . Describe both trends. Explain one environmental reason for the change and one practical reason why oil may still be used.

Tier 3 · Hard

6 marks

ORIGINAL

A country is choosing between expanding offshore wind and building a nuclear power station. Evaluate the two options for large-scale electricity supply. Your answer should consider reliability, environmental impacts and economic or social factors.