Work done — OCR GCSE Physics
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Work done explained
The law of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another.
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In a closed system, no energy enters or leaves from outside, so although energy moves between stores, the total energy stays the same. For example, a falling ball transfers energy from its gravitational potential store to its kinetic store, and some to thermal energy by heating the surroundings through air resistance, but the total within the closed system does not change. A multiple-choice question may ask you to pick the statement that correctly describes this. Choose the option that says energy is transferred between stores with no net change to the total in a closed system.
describe all the changes involved in the way energy is stored when a system changes for common situations (to include: an object projected upwards or up a slope, a moving object hitting an obstacle, an object being accelerated by a constant force, a vehicle slowing down, bringing water to a boil in an electric kettle)
Energy is conserved, but it moves between stores. For each named situation, track the store that empties and the store that fills. An object projected upwards or up a slope gains gravitational potential energy as it rises and loses kinetic energy as it slows; at the top the kinetic store is momentarily empty. A moving object hitting an obstacle transfers kinetic energy to thermal energy in the object and obstacle, plus sound. An object accelerated by a constant force has work done on it, so its kinetic store grows while the force's source store (for example chemical in a fuel) empties. A vehicle slowing down transfers kinetic energy to thermal energy in the brakes and surroundings. In an electric kettle, the chemical store of the supply empties, energy is transferred electrically to the heating element, and the water's thermal store rises until it boils.
describe the changes in energy involved when a system is changed by heating (in terms of temperature change and specific heat capacity), by work done by forces, and by work done when a current flows
A system can be changed in three ways, and each moves energy into or out of a store. Heating a substance raises its temperature; the energy needed depends on the mass, the temperature change and the specific heat capacity, using E = m × c × Δθ. A larger specific heat capacity means more energy is needed for the same temperature rise. Work done by a force transfers energy mechanically, for example when a force moves an object through a distance, using W = F × s. Work done when a current flows transfers energy electrically; the energy transferred depends on current, potential difference and time, using E = V × I × t. In each case the total energy is conserved and the change shows as a rise or fall in a store such as thermal, kinetic or chemical.
make calculations of the energy changes associated with changes in a system, recalling or selecting the relevant equations for mechanical, electrical, and thermal processes; thereby express in quantitative form and on a common scale the overall redistribution of energy in the system (to include: work done by forces, current flow, through heating and the use of kW h to measure energy use in electrical appliances in the home)
This criterion asks you to pick the right energy equation for a process and then calculate. For a mechanical process, work done by a force is W = F × s, where F is the force in newtons and s the distance moved along the force in metres, giving joules. For an electrical process, energy transferred by current flow is E = P × t or E = Q × V, with power in watts and time in seconds giving joules. For heating, use E = m × c × Δθ, with mass in kilograms, specific heat capacity in J/(kg °C) and temperature change in °C. Because all these give joules, you can put the results on one common scale and state how energy is redistributed. For home appliances, kilowatt-hours are used: 1 kW h = 1000 W × 3600 s = 3 600 000 J, so a 2 kW heater running 3 h uses 6 kW h, which is 6 × 3 600 000 = 21 600 000 J.
calculate the amounts of energy associated with a moving body, a stretched spring and an object raised above ground level
Three stored-energy equations are needed here. A moving body has kinetic energy E_k = ½ × m × v², with mass in kilograms and speed in metres per second, giving joules; note that doubling the speed quadruples the energy because speed is squared. A stretched spring has elastic potential energy E_e = ½ × k × x², where k is the spring constant in N/m and x the extension in metres. An object raised above ground level has gravitational potential energy E_p = m × g × h, with g taken as 9.8 N/kg and h the height in metres. For example, a 2 kg mass moving at 3 m/s has E_k = ½ × 2 × 3² = 9 J; a spring with k = 200 N/m stretched 0.1 m has E_e = ½ × 200 × 0.1² = 1 J; and a 2 kg mass raised 5 m has E_p = 2 × 9.8 × 5 = 98 J. Square the speed or extension before multiplying by the other factors.
Your focus
- State the law of conservation of energy.
- Describe energy transfers in a closed system with no net change to the total.
- Select the correct qualitative statement about energy conservation in a multiple-choice question.
Show all 16 objectives
- Describe the energy store changes for an object projected upwards or up a slope
- Describe the energy store changes for a collision, a constant-force acceleration and a vehicle slowing down
- Describe the energy store changes when a kettle brings water to a boil
- Describe energy changes when a system is heated in terms of temperature change and specific heat capacity
- Describe energy changes when work is done by forces
- Describe energy changes when work is done by a flowing current
- Select and apply the correct equation for a mechanical, electrical or thermal energy change
- Calculate energy in joules for work done, current flow and heating
- Convert between kilowatt-hours and joules for home appliances
- Describe quantitatively how energy is redistributed within a system
- Calculate the kinetic energy of a moving body using E_k = ½ × m × v²
- Calculate the elastic potential energy stored in a stretched spring using E_e = ½ × k × x²
- Calculate the gravitational potential energy of an object raised above ground level using E_p = m × g × h
Work done exam tips
Marking Points
- States that energy cannot be created or destroyed, only transferred between stores.
- Recognises that in a closed system no energy enters or leaves from outside.
- Concludes that the total energy of a closed system shows no net change.
- Describes energy moving between stores, such as gravitational potential to kinetic, while the total stays constant.
- Applies the law of conservation of energy to a described situation qualitatively.
- Identifies the store that empties and the store that fills for each named situation
- States that a rising object transfers kinetic energy to gravitational potential energy
- States that a collision transfers kinetic energy to thermal energy and sound
- States that a constant force does work that increases an object's kinetic energy
- States that a slowing vehicle transfers kinetic energy to thermal energy in the brakes
- States that a kettle transfers energy electrically from the supply to the water's thermal store
- States that heating raises temperature and the energy needed depends on mass, temperature change and specific heat capacity
- Uses E = m × c × Δθ to relate energy transferred by heating to temperature change
- Explains that a larger specific heat capacity means more energy for the same temperature rise
- States that work done by a force transfers energy mechanically, related by W = F × s
- States that work done when a current flows transfers energy electrically, related by E = V × I × t
- States that energy is conserved and appears as a change in a store
- Selects the equation matching the named process, such as W = F × s for work done by a force, before substituting values
- Converts time to seconds and power to watts when calculating electrical energy in joules
- Uses E = m × c × Δθ with mass in kilograms and temperature change in °C for a heating process
- Converts between kilowatt-hours and joules using 1 kW h = 3 600 000 J to place home appliance energy use on a common scale
- States the overall redistribution of energy in the system in quantitative terms, for example how much moves by work, by current flow and by heating
- Applies E_k = ½ × m × v² with mass in kilograms and speed in metres per second to find the energy of a moving body
- Squares the speed before multiplying by ½ and the mass when calculating kinetic energy
- Applies E_e = ½ × k × x² with the spring constant in N/m and the extension in metres for a stretched spring
- Applies E_p = m × g × h with g = 9.8 N/kg and height in metres for an object raised above ground level
- Gives each calculated energy in joules and checks that the magnitude is reasonable
Examiner Tips
- 💡Look for the option that mentions both transfer between stores and no net change to the total.
- 💡Check whether the system is described as closed before accepting a statement about constant total energy.
- 💡Reject any option that says energy is created, destroyed or used up.
- 💡Name both the store that empties and the store that fills for each situation
- 💡Use the words kinetic, gravitational potential, thermal and chemical precisely
- 💡Check whether the question asks for one situation or all five before answering
- 💡Write the equation you use before substituting values
- 💡Keep units consistent: mass in kg, temperature change in °C, energy in J
- 💡Check whether the question asks about heating, work by a force or work by a current before choosing an equation
- 💡Write down the equation and the quantity each symbol stands for before substituting numbers
- 💡Convert every value to SI units first, then convert the final answer to kW h only if the question asks for it
- 💡Check that your answer is in joules and is a sensible size for the appliance or process described
- 💡Write the equation, then square the speed or extension before doing any other multiplication
- 💡Convert centimetres to metres and grams to kilograms before substituting
- 💡Check the units of the spring constant; it must be in N/m for the answer to come out in joules
Common Mistakes
- Claims that energy is used up or destroyed during transfers; correct this by stating that energy is transferred, not destroyed.
- Assumes the total energy changes because one store empties; correct this by noting that energy moves to another store so the total is unchanged.
- Treats an open system as closed; correct this by checking whether energy enters or leaves from outside.
- Gives a numerical calculation when the question asks for a qualitative description; correct this by describing the transfers in words.
- Says energy is used up or destroyed; state that energy is conserved and only transferred between stores
- Claims a rising object gains kinetic energy; state that it loses kinetic energy and gains gravitational potential energy
- Treats the kettle as creating energy; state that energy is transferred electrically from the supply to the water
- Uses temperature change alone and ignores mass; include mass and specific heat capacity in the calculation
- Confuses specific heat capacity with the total energy; state that it is the energy per kilogram per degree Celsius
- Adds the three routes as if energy were created; state that each route transfers energy between stores
- Uses minutes or hours directly in E = P × t; convert the time to seconds so the answer is in joules
- Forgets to convert kilowatts to watts before multiplying by time; multiply the power in kW by 1000 first
- Mixes units so the answer is not in joules; keep force in newtons, distance in metres, mass in kilograms and time in seconds
- Treats a kW h as a unit of power; recognise it as a unit of energy equal to 3 600 000 J
- Multiplies mass by speed without squaring the speed; square the speed first in E_k = ½ × m × v²
- Uses the total length of the spring instead of its extension; use only the change in length in E_e = ½ × k × x²
- Omits the factor of ½ in the kinetic or elastic equation; include ½ in both
- Uses height in centimetres in E_p = m × g × h; convert the height to metres first