A cup of hot coffee slowly cooling down on a desk is a perfect example of energy leaving a system. In physics, a system is defined simply as an object or a group of objects. A closed system is one where energy can be exchanged with the surroundings, but matter cannot enter or leave. According to the principle of the conservation of energy, the total energy within a closed system always remains constant, even as it moves between different stores.
The internal energy store is the total energy stored by all the atoms and molecules that make up a system. It is calculated as the sum of the kinetic energy (due to the random motion of particles) and the potential energy (due to the positions of particles and the chemical bonds between them).
When energy is transferred to a system's internal energy store by heating, it increases the kinetic energy of the particles, resulting in a temperature change. However, during a change of state (like melting or boiling), the temperature remains constant because the energy is instead used to change the potential energy store by breaking chemical bonds.
The energy transferred to heat a substance depends on its mass, the temperature change, and its specific heat capacity (the energy required to raise of a substance by ).
Where:
Calculate the energy required to heat a concrete block by . (Specific heat capacity of concrete = )
Step 1: Identify the known variables.
Step 2: Substitute the values into the formula.
Step 3: Calculate the final answer with units.
Why does a metal spoon feel cold when you first pick it up, but quickly warms in your hand? Energy naturally transfers from a hotter object (the source) to a cooler object (the sink). This transfer continues until both objects reach the exact same temperature, a state known as thermal equilibrium.
The OCR specification identifies two primary heating pathways. Heating by particles includes conduction (energy transfer via direct contact and particle vibration) and convection (where hot regions in fluids rise and cold regions sink). Heating by radiation involves energy transfer via infrared electromagnetic waves; unlike conduction or convection, radiation does not require any physical medium or particles to travel through.
Any energy transferred to non-useful stores—typically the thermal store of the surroundings—is described as dissipated or wasted energy. Materials with low thermal conductivity transfer energy slowly and are used as insulation to reduce this dissipation.
Pushing a broken-down car along a flat road requires a huge amount of effort, transferring energy from your muscles into the car's motion. Work done occurs whenever a force makes an object move over a distance. This process transfers energy from one store to another via the mechanical pathway.
The amount of energy transferred is exactly equal to the work done. One Joule () of energy is transferred when a force of one Newton () moves an object by one metre (). If a force is applied but the object remains perfectly stationary, the work done is .
Where:
A crane lifts a steel beam with a weight of vertically upwards by . Calculate the work done.
Step 1: Identify the known variables.
Step 2: Substitute the values into the formula.
Step 3: Calculate the final answer with units.
When describing energy changes, you must identify the full sequence. For example, when a car brakes, energy is transferred from the kinetic store of the car, via the mechanical pathway, to the thermal store of the brakes and surroundings (due to friction).
Every time you switch on your smartphone, a tiny battery forces electrons through microscopic circuits to power the screen. Electrical work is done when a charge moves through a potential difference in a closed circuit. This transfers energy from a power supply to components via the electrical pathway.
As an electric current flows, moving electrons continuously collide with vibrating positive ions in the metal lattice of the wire. These collisions transfer kinetic energy from the electrons to the ions, increasing the thermal store of the wire (a heating effect). The total energy supplied by the power source is always equal to the useful energy transferred plus the energy dissipated to the surroundings.
Where:
A motor draws a current of for . Calculate the total energy transferred.
Step 1: Identify the known variables.
Step 2: Substitute the values into the formula.
Step 3: Calculate the final answer with units.
When describing energy transfers in an electrical circuit, follow the step-by-step path. In a battery-powered lamp, energy moves from the chemical store of the battery along the electrical pathway to the thermal store of the filament before being transferred to the surroundings via radiation (light) and heating by particles.
Students often list 'work' as an energy store. In the OCR specification, work is a process or pathway (mechanical or electrical), never a store.
When asked to define internal energy, you must explicitly state that it is the sum of BOTH the kinetic and potential energy stores of the particles to secure the full mark.
In 6-mark 'Describe' questions about energy transfers, examiners expect a step-by-step structural analysis: state the initial energy store, name the specific pathway, and state the final energy store.
Always convert time into seconds (s) and power into Watts (W) before calculating electrical work done, or your final answer will be orders of magnitude off.
Remember that work is only done if the object moves in the same direction as the applied force; holding a heavy box stationary transfers via the mechanical pathway.
System
An object or a group of objects being studied or observed.
Closed system
A system where energy can be exchanged with the surroundings, but matter cannot enter or leave, meaning total energy remains constant.
Conservation of energy
The fundamental principle that energy cannot be created or destroyed, only transferred, stored, or dissipated.
Internal energy store
The total energy stored by all the particles in a system, calculated as the sum of their kinetic and potential energies.
Kinetic energy
The energy an object or particle has due to its motion.
Potential energy
The energy stored by particles due to their positions relative to each other and the chemical bonds between them.
Temperature change
An increase or decrease in the average kinetic energy of the particles in a substance.
Change of state
A physical change in matter (e.g., melting or boiling) where energy changes the potential energy store of particles without altering the temperature.
Specific heat capacity
The amount of energy required to raise the temperature of 1 kg of a substance by 1 °C.
Source
The hotter object in a thermal energy transfer.
Sink
The cooler object that receives energy in a thermal energy transfer.
Thermal equilibrium
The point at which two objects in contact reach the exact same temperature and net energy transfer stops.
Heating by particles
The OCR-specific term for energy transfer pathways that require a physical medium, including conduction and convection.
Conduction
Energy transfer through a material via direct contact and the vibration of adjacent particles.
Convection
Energy transfer in fluids where hotter, less dense regions rise and cooler, denser regions sink.
Heating by radiation
Energy transfer via infrared electromagnetic waves, which does not require a physical medium.
Dissipated
Energy that is transferred to non-useful stores, usually spreading out into the thermal store of the surroundings.
Thermal conductivity
A measure of how quickly energy is transferred through a material by conduction.
Work done
The amount of energy transferred when a force makes an object move a certain distance.
Mechanical pathway
The method of transferring energy from one store to another via the action of a force moving an object.
Electrical work
The energy transferred by a power supply or to a component when an electric current flows.
Potential difference
The work done (or energy transferred) per unit of charge passing between two points in a circuit.
Electrical pathway
The method of transferring energy when a charge moves through a potential difference.
Electric current
The continuous rate of flow of electric charge through a conductor.
Put your knowledge into practice — try past paper questions for Physics A
System
An object or a group of objects being studied or observed.
Closed system
A system where energy can be exchanged with the surroundings, but matter cannot enter or leave, meaning total energy remains constant.
Conservation of energy
The fundamental principle that energy cannot be created or destroyed, only transferred, stored, or dissipated.
Internal energy store
The total energy stored by all the particles in a system, calculated as the sum of their kinetic and potential energies.
Kinetic energy
The energy an object or particle has due to its motion.
Potential energy
The energy stored by particles due to their positions relative to each other and the chemical bonds between them.
Temperature change
An increase or decrease in the average kinetic energy of the particles in a substance.
Change of state
A physical change in matter (e.g., melting or boiling) where energy changes the potential energy store of particles without altering the temperature.
Specific heat capacity
The amount of energy required to raise the temperature of 1 kg of a substance by 1 °C.
Source
The hotter object in a thermal energy transfer.
Sink
The cooler object that receives energy in a thermal energy transfer.
Thermal equilibrium
The point at which two objects in contact reach the exact same temperature and net energy transfer stops.
Heating by particles
The OCR-specific term for energy transfer pathways that require a physical medium, including conduction and convection.
Conduction
Energy transfer through a material via direct contact and the vibration of adjacent particles.
Convection
Energy transfer in fluids where hotter, less dense regions rise and cooler, denser regions sink.
Heating by radiation
Energy transfer via infrared electromagnetic waves, which does not require a physical medium.
Dissipated
Energy that is transferred to non-useful stores, usually spreading out into the thermal store of the surroundings.
Thermal conductivity
A measure of how quickly energy is transferred through a material by conduction.
Work done
The amount of energy transferred when a force makes an object move a certain distance.
Mechanical pathway
The method of transferring energy from one store to another via the action of a force moving an object.
Electrical work
The energy transferred by a power supply or to a component when an electric current flows.
Potential difference
The work done (or energy transferred) per unit of charge passing between two points in a circuit.
Electrical pathway
The method of transferring energy when a charge moves through a potential difference.
Electric current
The continuous rate of flow of electric charge through a conductor.