Energy stores and systems — AQA GCSE Combined Science
Test yourself on Energy stores and systems with AQA GCSE practice questions.
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Energy stores and systems explained
A system is simply whatever you choose to study: a single object, or a group of objects that you treat as one unit.
Read the full explanation
Choosing the system sets the boundary for your energy account. Anything inside the boundary is part of the system; everything else is the surroundings. For example, a falling ball can be the system, or the ball plus the air it falls through can be the system. A boiling kettle can be the system, or the kettle plus the water inside it. Once the boundary is chosen, you can list the energy stores inside it and track transfers between them or to the surroundings. The same physical situation can be modelled with different systems, and the choice affects which stores you count as internal and which changes you describe as transfers across the boundary.
There are changes in the way energy is stored when a system changes.
When a system changes, energy is transferred between stores rather than being used up. A system is an object or group of objects. In a closed system, no energy transfers in or out, so the total energy remains exactly the same, though the way it is stored changes. For example, a falling book in a vacuum transfers energy from its gravitational potential store to its kinetic store. In an open system, energy can be transferred to the surroundings. A battery-powered lamp transfers energy from a chemical store to a thermal store in the filament and to the surroundings by heating and radiation. To describe a change, name the decreasing store, the increasing store, and the pathway (mechanical work, electrical work, heating or radiation).
Students should be able to describe all the changes involved in the way energy is stored when a system changes, for common situations. For example:
A system is an object or group of objects; when it changes, energy is transferred between stores rather than created or destroyed. To describe a change fully, name the store that decreases, the store that increases, and the pathway of transfer, such as mechanical work, electrical work, heating or radiation. For a falling ball, the gravitational potential store of the ball–Earth system decreases while its kinetic store increases, transferred mechanically by gravity doing work. For a battery-powered torch, the chemical store of the battery decreases while the thermal store of the surroundings increases, transferred electrically then by heating and radiation. Always state the system boundary and avoid saying energy is 'used up' or 'lost'.
an object projected upwards
When an object is projected upwards, the system is the object and the Earth. At the point of release, the object has a kinetic store because it is moving. As it rises, it slows, so the kinetic store decreases while the gravitational potential store of the object–Earth system increases. The transfer is mechanical work done against gravity. At the highest point, the kinetic store is momentarily zero and the gravitational potential store is maximum. As it falls back, the reverse happens: gravitational potential store decreases and kinetic store increases. If air resistance is ignored, the energy transferred equals the gain in gravitational potential store; with air resistance, some energy is dissipated to the thermal store of the surroundings.
a moving object hitting an obstacle
When a moving object strikes an obstacle, its kinetic energy store decreases as it slows or stops. Energy is transferred mechanically by the contact force, doing work on the obstacle and on the object. Some energy raises the thermal energy store of both bodies because friction and deformation generate heating; some may transfer by sound to the surroundings. The total energy is conserved, so the decrease in kinetic energy equals the work done plus the energy dissipated. For example, a car braking to a stop transfers energy from its kinetic store to thermal stores in the brakes and road, with a little sound. Students should describe the stores involved, the transfer pathway and the conservation principle, using equations such as Ek = ½mv² where needed.
an object accelerated by a constant force
A constant applied force doing work transfers energy to an object's kinetic store, and potentially its gravitational potential store if it rises. The work done equals force times distance moved along the line of action of the force, W = Fs. If friction is present, some energy is dissipated to the thermal store of the surroundings. Conversely, the work done by the resultant force equals only the change in the kinetic store. If the resultant force is constant, acceleration is constant by Newton's second law, F = ma, so speed increases uniformly. For example, a constant push accelerates a trolley along a bench, transferring energy from the pusher's chemical store to the trolley's kinetic store, with some dissipated by friction.
a vehicle slowing down
When a vehicle slows down, its store of kinetic energy decreases because its speed falls. The energy is not lost: it is transferred by mechanical work done against resistive forces, mainly friction from the brakes and air resistance. Brake friction transfers energy to the thermal energy store of the brake discs and pads, while air resistance transfers energy to the thermal energy store of the surrounding air. The vehicle's gravitational potential energy store may also change if it moves up or down a slope. Students should describe the energy transfer pathway and identify the stores that decrease and increase, using the conservation of energy principle.
bringing water to a boil in an electric kettle.
In an electric kettle, energy is transferred from the chemical store of the power station fuel via the national grid. Electrical work is done on the kettle's heating element by the current, transferring energy to the thermal store of the element. Energy is then transferred by heating to the thermal store of the water. As the water heats, its temperature rises until it reaches boiling point, undergoing a change of state from liquid to gas. During boiling, energy is transferred to the water's thermal store without a change in temperature; this energy is the latent heat of vaporisation. Some energy is also dissipated by heating to the surroundings, reducing efficiency.
Throughout this section on Energy students should be able to calculate the changes in energy involved when a system is changed by:
This overarching statement sets the quantitative expectation for the whole Energy section: whenever a system changes, you should be able to work out the energy transferred, using the correct relationship for the mechanism involved. A system is an object or group of objects; a change means energy moves between stores or is transferred by heating, by forces doing work, or by electrical work. For example, lifting a 2.0 kg mass by 3.0 m transfers ΔE = m × g × h = 2.0 × 9.8 × 3.0 = 58.8 J to the gravitational potential store. Identify the store that decreases and the one that increases, choose the equation linking the given quantities, substitute with units, then calculate. Energy is conserved, so the total input equals the useful output plus any energy dissipated to the surroundings.
heating
Heating is one way a system can be changed: energy is transferred from a hotter region to a cooler region, raising the temperature of the cooler material or changing its state. The energy transferred for a temperature change depends on mass, specific heat capacity and temperature change, using ΔE = m × c × Δθ. For example, heating 0.50 kg of water (c = 4200 J/kg°C) through 20 °C transfers ΔE = 0.50 × 4200 × 20 = 42 000 J. During a change of state, temperature stays constant while energy breaks or forms bonds. Here, use the specific latent heat equation E = m × L. Energy is conserved: energy gained by the cooler object equals energy lost by the hotter one, ignoring dissipation.
work done by forces
When a force moves an object, energy is transferred mechanically and we say work is done. The work done equals the force multiplied by the distance moved along the line of the force: W = F s. If a 20 N force pushes a box 3 m in the direction of the force, the work done is 20 × 3 = 60 J. One joule is one newton metre. If the force acts at an angle to the motion, only the component along the motion does work, so the full force does not transfer energy. Work done is a measure of energy transferred, so it is measured in joules, the same unit as energy. A force at right angles to motion, such as the normal contact force on a sliding box, does no work because there is no movement along the force.
work done when a current flows
When charge flows through a component, work is done. Energy is transferred from the chemical store of a battery by the electrical working pathway to other stores, such as the thermal store of a resistor. The work done (energy transferred) can be calculated using E = Q × V, where Q is charge flow and V is potential difference. For example, a 12 V supply moving 10 C of charge transfers 12 × 10 = 120 J of energy. It is vital to distinguish between energy transfer pathways: electrical work transfers energy to the resistor's thermal store, while heating transfers energy from the hot resistor to the thermal store of the surroundings.
use calculations to show on a common scale how the overall energy in a system is redistributed when the system is changed.
Energy is conserved overall, but within a system it can move between stores when the system changes. To show redistribution on a common scale, list every store before and after the change, calculate each energy value in joules, then compare them using the same axis or bar scale. For example, a ball held 2 m above the ground has gravitational potential energy E = mgh = 0.5 × 9.8 × 2 = 9.8 J. When released, air resistance is negligible, so at the ground the store is kinetic: 9.8 J. A bar chart with both stores on one 0–10 J scale shows the gravitational bar falling to zero as the kinetic bar rises to 9.8 J. If friction transfers 2 J to the thermal store of the surroundings, the kinetic bar reaches only 7.8 J and a thermal bar of 2 J appears, keeping the total at 9.8 J.
Your focus
- Define a system as an object or a group of objects.
- Identify the boundary between a chosen system and its surroundings.
- Apply the idea of a system to a simple physical situation.
Show all 39 objectives
- Describe how energy stores change when a system changes.
- Identify the pathway by which energy is transferred between stores.
- Apply conservation of energy to closed systems where total energy remains constant.
- Identify the energy stores that change when a named system changes.
- State the pathway by which energy is transferred between stores.
- Describe complete energy changes for common situations using conservation of energy.
- Describe how kinetic and gravitational potential stores change as an object moves upwards and downwards.
- Identify the highest point as the position where kinetic store is zero and gravitational potential store is maximum.
- Explain the effect of air resistance on the energy transferred during the motion.
- Describe the energy transfers when a moving object hits an obstacle, naming stores and pathways.
- Explain how conservation of energy applies to the impact, including thermal and sound transfers.
- Calculate kinetic energy using Ek = ½mv² and relate it to energy transferred on impact.
- Describe how a constant force transfers energy to an object's kinetic energy store.
- Calculate work done using W = Fs and relate it to energy transfers.
- Explain how a constant resultant force produces constant acceleration and how friction affects the energy transfer.
- Identify the kinetic energy store of a moving vehicle and explain how it changes when the vehicle slows down.
- Describe the energy transfer pathways from the vehicle's kinetic energy store to thermal energy stores of brakes and surroundings.
- Apply the conservation of energy to account for the decrease in kinetic energy when a vehicle slows down.
- Describe the energy transfers that occur when an electric kettle brings water to a boil.
- Explain why the temperature of water remains constant during boiling despite continued energy transfer.
- Evaluate the efficiency of an electric kettle by considering energy transferred to the surroundings.
- Identify the energy stores that change when a described system is altered.
- Select and apply the correct equation to calculate an energy change in joules.
- Explain, using conservation of energy, why the total energy before and after a change is the same.
- Describe how heating transfers energy from a hotter region to a cooler region.
- Calculate the energy transferred using ΔE = m × c × Δθ or E = m × L as appropriate.
- Interpret a temperature-time graph to identify changes of state and temperature changes.
- State the condition needed for a force to do work on an object.
- Apply W = F s to calculate work done, force or distance, including rearrangement.
- Explain why a force at right angles to motion does no work and relate work done to energy transferred.
- State that work is done when charge flows in a circuit.
- Apply E = Q × V to calculate work done, charge flow or potential difference.
- Distinguish between electrical work and heating as distinct energy transfer pathways.
- Identify the energy stores present before and after a described change in a system.
- Calculate the energy in each relevant store using the appropriate equation and correct units.
- Present before-and-after energy values on a common scale and use them to explain how energy has been redistributed.
Energy stores and systems exam tips
Marking Points
- Defines a system as a single object or a group of objects treated as one unit for analysis.
- Explains that choosing a system sets a boundary between the system and its surroundings.
- States that energy stores inside the system can be listed and tracked as they change.
- Uses a concrete example, such as a ball, a kettle plus water, or a battery plus lamp, to show the boundary.
- Recognises that the same situation can be modelled using different systems, changing which stores are internal.
- States that when a system changes, energy is transferred between stores rather than created or destroyed.
- Identifies the store that decreases and the store that increases during a described change.
- Names the transfer pathway, such as mechanical work, electrical work, heating or radiation.
- Applies conservation of energy to a closed system, stating that no energy transfers in or out, so the total energy remains constant.
- Uses a concrete example, such as lifting a book or a falling ball, to describe the store changes.
- Identifies the system and the stores that change, naming both the store that decreases and the store that increases.
- States the transfer pathway: mechanically, electrically, by heating or by radiation.
- Recognises that total energy is conserved, so a decrease in one store equals the increase in others plus any energy dissipated to the surroundings.
- Describes changes for common situations such as a falling object, a moving vehicle braking, a kettle heating water, a battery-powered device, a stretched spring or an object projected upwards.
- Uses correct terminology: 'energy is transferred' rather than 'energy is used up' or 'lost'.
- Links each store to a physical quantity where useful, for example kinetic store to speed and mass, gravitational potential store to height and mass, thermal store to temperature.
- Names the object–Earth system and identifies the kinetic store and gravitational potential store as the stores that change.
- States that as the object rises, kinetic store decreases and gravitational potential store increases, transferred mechanically by gravity doing work.
- Recognises that at the highest point the kinetic store is zero and the gravitational potential store is maximum.
- Describes the reverse transfer on the way down, with gravitational potential store decreasing and kinetic store increasing.
- Explains that with air resistance some energy is dissipated to the thermal store of the surroundings, so the energy returned as kinetic is less than the initial kinetic store.
- Uses conservation of energy to relate the decrease in one store to the increase in the other when dissipation is negligible.
- Identify the moving object's kinetic energy store as the initial store that decreases.
- State that the contact force between object and obstacle does mechanical work, transferring energy.
- Recognise that friction and deformation transfer energy to thermal energy stores of the object, obstacle and surroundings.
- Note that some energy may be transferred by sound to the surroundings.
- Apply conservation of energy: total energy before impact equals total energy after, accounting for all stores.
- Use Ek = ½mv² to calculate kinetic energy where mass and speed are given.
- Identify the constant force doing mechanical work on the object.
- State that work done by an applied force transfers energy to the object's kinetic energy store and potentially its gravitational potential store.
- Use W = Fs to calculate work done when force and distance are known.
- Recognise that a constant resultant force produces constant acceleration via F = ma, and its work equals the change in kinetic energy.
- Account for any energy dissipated by friction or air resistance if present.
- The kinetic energy store of the vehicle decreases because the vehicle's speed decreases.
- Energy is transferred from the kinetic energy store of the vehicle to the thermal energy store of the brakes and surroundings.
- Mechanical work is done by the brakes against friction, and by air resistance against the motion of the vehicle.
- The total energy is conserved; the decrease in kinetic energy equals the increase in thermal energy stores, assuming no change in gravitational potential energy.
- If the vehicle moves along a slope, gravitational potential energy store changes must also be considered.
- Electrical work is done on the kettle's heating element by the current, increasing its thermal store.
- Energy is transferred by heating from the element to the thermal energy store of the water.
- The temperature of the water rises until it reaches 100 °C at standard atmospheric pressure.
- At boiling point, further energy transfer causes a change of state from liquid to gas without a change in temperature.
- Energy is also transferred by heating to the surroundings, so the kettle is not 100% efficient.
- The total energy transferred from the electrical supply equals the increase in thermal energy of the water plus energy transferred to the surroundings.
- State the system and identify which energy store decreases and which increases before calculating.
- Select the correct relationship for the change described, such as ΔE = m × g × h for a change in height or E = ½ × m × v² for a change in speed.
- Substitute values with consistent SI units, converting grams to kilograms, centimetres to metres and minutes to seconds where needed.
- Calculate the energy change and give the unit joule (J), or kilojoule (kJ) where the value is large.
- Apply conservation of energy: energy transferred in equals energy transferred usefully plus energy dissipated, so totals balance.
- Show working clearly so that a correct method can be followed even if an arithmetic slip occurs.
- Recognise heating as a pathway that transfers energy from a hotter object to a cooler one, not as a substance that flows.
- Use ΔE = m × c × Δθ for a temperature change, identifying mass, specific heat capacity and temperature change correctly.
- Use the specific latent heat equation E = m × L for a change of state, where temperature remains constant during melting, boiling or condensing.
- Convert units before substituting: grams to kilograms and, where needed, minutes to seconds for power calculations.
- Interpret a heating graph: sloping sections show temperature change, flat sections show a change of state.
- Apply conservation of energy so that energy lost by the hotter object equals energy gained by the cooler object in an insulated system.
- State that work is done when a force causes an object to move through a distance.
- Recall and apply the relationship work done = force × distance moved along the line of the force, W = F s.
- Use the correct units: force in newtons (N), distance in metres (m) and work done in joules (J).
- Recognise that work done equals energy transferred, so a 60 J transfer means 60 J of work done.
- Explain that a force perpendicular to the direction of motion does no work because distance moved along the force is zero.
- Calculate a missing quantity by rearranging W = F s when the work done and one other value are given.
- State that work is done when charge flows through a circuit component.
- Calculate work done using the equation E = Q × V, identifying energy, charge flow and potential difference.
- Use the correct units: potential difference in volts (V), charge in coulombs (C) and work done in joules (J).
- Distinguish between pathways: electrical work transfers energy to a component's thermal store, whereas heating transfers energy to the surroundings.
- Recognise that the work done by the current equals the energy transferred from the power supply.
- Identifies all relevant energy stores in the system before and after the change, such as gravitational potential, kinetic, elastic potential, thermal and chemical stores.
- Calculates each store's energy in joules using the correct equation, for example E = mgh for gravitational potential energy or E = ½mv² for kinetic energy.
- Uses one common scale, such as a single axis or a consistent bar-chart scale, so that before-and-after values can be compared directly.
- Shows that the total energy is unchanged when stores are added, demonstrating conservation of energy across the change.
- Explains any difference between the initial and final mechanical energy as transfer to a thermal store by heating, for example due to friction or air resistance.
Examiner Tips
- 💡Underline the object or group of objects named in the question before listing energy stores.
- 💡State the boundary explicitly, for example 'taking the ball as the system'.
- 💡If a question allows more than one system, choose the simplest one that answers the question.
- 💡Use the pattern 'energy is transferred from the ... store to the ... store by ...' in your answer.
- 💡If the question specifies a closed system, state that the total energy before the change equals the total energy after.
- 💡Use the structure 'the ... store of the ... decreases, and the ... store of the ... increases, transferred by ...' for each situation.
- 💡Name the system explicitly, such as 'the ball–Earth system' or 'the water in the kettle', so the examiner can see which stores you mean.
- 💡If a question says 'describe all the changes', include the pathway and the final store as well as the initial store, and mention dissipation where it occurs.
- 💡Describe the motion in stages: release, rising, highest point, falling, and state the stores at each stage.
- 💡Use the phrase 'transferred mechanically by gravity doing work' to show the pathway clearly.
- 💡If air resistance is mentioned, add a sentence about dissipation to the thermal store of the surroundings rather than saying energy is lost.
- 💡Name the stores and the transfer pathway explicitly rather than writing only 'energy is transferred'.
- 💡Link each energy transfer to a mechanism such as friction, deformation or a contact force.
- 💡Check numerical answers by confirming the decrease in kinetic energy matches the sum of other energy transfers.
- 💡Write the work equation W = Fs before substituting values to make the method clear.
- 💡State the direction of motion and force so the sign of work done is unambiguous.
- 💡When friction is mentioned, calculate the useful work and the dissipated energy separately.
- 💡Name the store that decreases and the store that increases, and state the transfer pathway, for example 'by mechanical work done by friction'.
- 💡Use the conservation of energy to justify why the total energy remains constant even though the vehicle slows down.
- 💡If a diagram or data is given, refer to it explicitly, such as brake temperature rising or stopping distance increasing.
- 💡Clearly distinguish the pathways: electrical work to the element, then heating to the water.
- 💡Use the correct terms 'thermal energy store' and 'change of state' rather than vague words like 'heat' or 'boiling'.
- 💡If asked about efficiency, calculate useful energy output divided by total energy input, and explain where the rest goes.
- 💡Underline the quantities given in the question and the quantity required, then choose the equation that links exactly those quantities.
- 💡Write the equation, substitute numbers with units, then calculate; this earns method credit and makes errors easy to spot.
- 💡Check the size of your answer: lifting a book transfers tens of joules, while heating a kettle transfers hundreds of thousands of joules.
- 💡Give the unit with every numerical answer and round sensibly, usually to two or three significant figures.
- 💡Write down m, c, L and Δθ separately before substituting, so each value is clearly identified.
- 💡For change-of-state questions, check whether the temperature is constant; if it is, use E = m × L rather than specific heat capacity.
- 💡Read heating graphs carefully: label the flat sections as changes of state and the sloping sections as temperature changes.
- 💡Give the energy unit as J or kJ and keep the same unit throughout a multi-step calculation.
- 💡Write the equation, substitute the values with units, then give the answer with the correct unit; this makes method clear even if the arithmetic slips.
- 💡Check whether the force and distance are in the same direction before using W = F s; if not, state that only the component along the motion does work.
- 💡Link the numerical answer back to energy transfer, for example '60 J of energy is transferred mechanically to the box'.
- 💡Clearly state the energy stores and the pathways separately, e.g., 'electrical work transfers energy to the thermal store'.
- 💡Write the equation E = Q × V, substitute values with units, then calculate and give the unit in joules.
- 💡Write the store names and their calculated values in a table before drawing any chart, so every store is accounted for.
- 💡State the common scale explicitly, for example 'each grid square represents 2 J', so the comparison is clear to the examiner.
- 💡Check conservation by adding the final store values and comparing with the initial total; show this check in your answer.
Common Mistakes
- Thinking a system must contain more than one object; correction: a single object, such as one ball, is a valid system.
- Confusing the system with its surroundings; correction: the surroundings are everything outside the chosen boundary.
- Assuming the system boundary is fixed by the question; correction: the boundary is a modelling choice and should be stated clearly.
- Saying energy is 'used up' or 'lost'; correction: energy is transferred to other stores, including thermal stores in the surroundings.
- Listing only one store change; correction: describe both the store that decreases and the store that increases.
- Confusing a closed system with an open system; correction: a closed system has no energy transfer to or from the surroundings, whereas an open system does.
- Saying energy is 'lost' or 'used up' when it is dissipated to the surroundings; correction: state that energy is transferred to the thermal store of the surroundings by heating.
- Naming only one store, for example saying a falling ball has kinetic energy; correction: describe both the gravitational potential store decreasing and the kinetic store increasing.
- Confusing stores with pathways, for example calling radiation a store; correction: work, heating and radiation are transfer pathways, while kinetic, thermal and chemical are stores.
- Saying the object 'has no energy' at the highest point; correction: the kinetic store is zero but the gravitational potential store is at its maximum.
- Treating gravitational potential energy as a property of the object alone; correction: it is a store of the object–Earth system because it depends on the separation between them.
- Ignoring air resistance entirely when the question mentions it; correction: state that work done against air resistance dissipates energy to the thermal store of the surroundings.
- Saying energy is 'used up' or 'lost'; correct this by stating energy is dissipated to thermal stores and remains conserved.
- Treating kinetic energy as destroyed on impact; correct by explaining it is transferred by forces doing work.
- Ignoring sound or thermal transfers and claiming all kinetic energy becomes one store; correct by listing all pathways and stores involved.
- Confusing force with energy and giving work done in newtons; correct by using joules for work and newtons for force.
- Assuming all work done by an applied force increases kinetic energy when friction is present; correct by subtracting energy dissipated to thermal stores.
- Conflating applied force with resultant force; correct by noting that work done by the resultant force equals the change in kinetic energy only.
- Using total distance rather than distance moved along the force direction; correct by resolving the force and displacement correctly.
- Saying energy is 'used up' or 'lost' when a vehicle slows down. Correction: energy is conserved and transferred to thermal energy stores of the brakes and surroundings.
- Ignoring air resistance and attributing all energy transfer to the brakes. Correction: both brake friction and air resistance transfer energy to thermal stores.
- Forgetting that gravitational potential energy may also change if the vehicle is on a slope. Correction: state whether the vehicle is on a level road or a slope and include any change in height.
- Stating that electrical work transfers energy directly to the water; correction: electrical work is done on the heating element, which then transfers energy to the water by heating.
- Saying the water's temperature continues to rise while it is boiling; correction: during a change of state, energy is transferred without a change in temperature.
- Assuming all electrical energy is transferred to the water's thermal energy store; correction: some energy is transferred to the surroundings by heating, so efficiency is less than 100%.
- Confusing the energy transfer with the temperature change; correction: energy transfer can cause a temperature change or a change of state, but not both at the same time.
- Using the mass in grams instead of kilograms: convert 250 g to 0.25 kg before substituting into any equation.
- Forgetting to square the speed in E = ½ × m × v²: square the velocity first, then multiply by mass and halve.
- Treating dissipated energy as destroyed: it is still transferred to the surroundings, usually by heating, so include it in the total.
- Mixing up the gravitational field strength value: on Earth use g = 9.8 N/kg unless the question states otherwise.
- Using the final temperature instead of the temperature change: Δθ is final temperature minus initial temperature.
- Assuming temperature rises during melting or boiling: during a change of state the energy breaks bonds, so temperature stays constant.
- Confusing specific heat capacity with specific latent heat: specific heat capacity applies to temperature change, specific latent heat to change of state.
- Forgetting that the unit of specific heat capacity is J/kg°C, so mass must be in kilograms.
- Using the total distance travelled rather than the distance moved along the line of the force; correct by resolving the force or using only the component in the direction of motion.
- Forgetting to convert centimetres to metres before multiplying; correct by dividing by 100 so the distance is in metres.
- Writing the unit of work as newtons or newton metres per second; correct by using joules (J), since 1 J = 1 N m.
- Conflating electrical work and heating: electrical work is the pathway to the resistor, heating is the pathway from the resistor to the surroundings.
- Using time instead of charge in the E = Q × V equation; ensure charge flow in coulombs is used.
- Assuming all the energy transferred by electrical work is useful; in a resistor, the energy increases the thermal store, which may be dissipated as wasted energy.
- Drawing before-and-after bars on different scales, which hides the redistribution; correction: use one axis or one stated scale for every bar.
- Forgetting to include a thermal store when friction or air resistance acts; correction: add the missing energy as a thermal transfer so the totals match.
- Mixing units, such as entering grams in E = mgh instead of kilograms; correction: convert all masses to kilograms and heights to metres before calculating.