OCR Specification focus:
‘Recognise different energy forms and transfers; track energy changes between objects and stores.’
Energy is the foundation of all physical processes. Understanding how energy exists in different forms and moves through various transfer pathways is essential to describing and predicting the behaviour of physical systems.
Forms of Energy
Energy can exist in several distinct forms, each associated with a particular type of physical process or system. Although energy itself is conserved, it can be stored or transferred between different energy stores. In any system, energy is never created or destroyed, only changed in form.
Mechanical Energy
Mechanical energy refers to the energy associated with the motion and position of an object. It has two main components:
Kinetic energy – the energy of motion, possessed by any object that is moving.
Potential energy – stored energy due to an object’s position within a force field (for example, gravitational potential energy in Earth’s gravitational field or elastic potential energy in a stretched spring).
Thermal Energy
Thermal energy is associated with the random motion of particles within a material. As the temperature of an object increases, its particles vibrate or move more vigorously, leading to an increase in thermal energy. When two objects are in contact and have different temperatures, thermal energy transfers from the hotter to the colder object until equilibrium is reached.
Chemical Energy
Chemical energy is stored within the bonds between atoms and molecules. During chemical reactions, these bonds are broken and reformed, resulting in energy being released or absorbed. Fuels such as petrol and food store chemical energy that can be converted into kinetic or thermal energy when oxidised.
Electrical Energy
Electrical energy is associated with the movement of electric charges through a conductor. It is commonly generated by converting other energy forms, such as mechanical energy in turbines, and can be transported efficiently over long distances. Electrical energy can be converted into light, sound, or mechanical energy in electrical devices.
Nuclear Energy
Nuclear energy arises from changes within the nucleus of an atom. It can be released through nuclear fission (splitting heavy nuclei) or nuclear fusion (combining light nuclei). Both processes involve the conversion of a small amount of mass into a large amount of energy, as described by Einstein’s equation E = mc².
Radiant or Electromagnetic Energy
This is energy carried by electromagnetic waves, including visible light, infrared, ultraviolet, and radio waves. It travels through space without the need for a medium and can be absorbed, transmitted, or reflected by different materials.
Sound Energy
Sound energy is associated with the vibration of particles in a medium, such as air, water, or solids. It transfers through the medium as a longitudinal wave, with regions of compression and rarefaction carrying energy away from the source.
Energy Stores and Pathways
The concept of energy stores and transfer pathways helps to visualise how energy moves between objects and systems. Energy can be thought of as being held in various stores and transferred by specific mechanisms known as pathways.
Common Energy Stores
Kinetic store – associated with the motion of an object.
Gravitational potential store – linked to an object’s position in a gravitational field.
Elastic potential store – present when a material is stretched or compressed.
Thermal store – related to the internal energy due to particle motion.
Chemical store – present within chemical bonds.
Nuclear store – found within atomic nuclei.
Energy Transfer Pathways
Energy transfers occur through well-defined pathways.
The three mechanisms of heating: conduction through a solid or stationary fluid, convection between a surface and moving fluid, and thermal radiation exchanged between surfaces. Each panel indicates temperature labels and heat flux arrows, adding mechanism-level detail to the “by heating” pathway. Source.
These include:
Mechanically, by a force moving through a distance.
Electrically, when electric current flows due to a potential difference.
A battery–switch–lamp circuit demonstrating the electrical transfer pathway: charge moves through the circuit and energy is delivered to the lamp. This figure focuses solely on the pathway mechanism without extra component detail. Source.
By heating, resulting from a temperature difference between two systems.
By radiation, such as light or infrared radiation travelling through space.
Each pathway involves a mechanism for transferring energy between stores, without creating or destroying energy.
Tracking Energy Transfers
In analysing physical systems, physicists track energy as it flows from one store to another. The key is to identify the start and end stores and the pathway used for the transfer. For example:
When a pendulum swings, energy transfers between the gravitational potential store and the kinetic store.
When a motor lifts a weight, electrical energy from the power supply transfers mechanically to increase the weight’s gravitational potential energy.
When a hot object cools, thermal energy transfers by heating to the surroundings’ thermal store.
By tracking these exchanges, we can ensure that the total energy remains constant, even though the form of energy changes.
Energy Dissipation and Useful Transfers
Not all energy transfers are useful. Some energy is inevitably dissipated, typically as unwanted thermal energy to the surroundings. This is often described as energy loss, though technically the energy is not lost but spread out and becomes less available for useful work.
Common examples include:
Friction converting kinetic energy to thermal energy in moving parts.
Electrical resistance converting electrical energy to thermal energy in wires.
Sound energy radiating away from vibrating objects, reducing useful energy remaining in the system.
Reducing dissipation is an important aspect of improving efficiency in mechanical and electrical systems.
Conservation of Energy in Transfers
The principle of conservation of energy underlies all energy transfer analysis. In any closed system, the total energy remains constant — it is simply redistributed among different stores or transferred between objects. Tracking energy changes helps to verify that this principle is upheld in all processes.
Energy Transfer Pathway: The process by which energy moves from one store to another, such as mechanically, electrically, by heating, or by radiation.
Understanding these pathways allows physicists to analyse and model complex systems, from everyday machines to large-scale natural processes, ensuring accurate predictions of behaviour and energy flow.
FAQ
An energy store refers to the location or form in which energy resides — for example, kinetic, thermal, or gravitational potential. It represents where energy is held.
An energy transfer pathway describes how energy moves between stores. Pathways include mechanical, electrical, by heating, and by radiation.
In essence, stores are “where” the energy is, and pathways are “how” the energy gets from one store to another.
Energy is a scalar quantity because it has magnitude but no direction. It represents the amount of energy in a system, not the direction of transfer.
Although energy can be transferred between objects, the pathway (such as heating or electrical transfer) has no vector direction in space like force or velocity does.
This distinction means energy can be added or subtracted through arithmetic, not vector addition.
Every energy transfer pathway involves useful and wasted energy outputs.
Useful energy is the part that achieves the desired effect (e.g., kinetic energy when a motor turns).
Wasted energy is dissipated, often as thermal energy or sound.
Pathways like mechanical or electrical transfer can never be perfectly efficient because of friction, resistance, or heating effects that spread energy into the surroundings.
Yes. Most systems contain multiple energy stores that interact simultaneously.
For example, a heated spring-loaded toy has:
Elastic potential energy in the compressed spring.
Thermal energy in the materials due to internal friction.
Possibly chemical energy remaining in its power source.
When released, these stores can transfer energy along different pathways — mechanical motion, sound, and thermal losses.
Radiation transfers energy through electromagnetic waves and does not require a medium. Energy can move through a vacuum, as in sunlight reaching Earth.
By contrast, conduction and convection involve particle interactions:
Conduction transfers energy via direct particle collisions in solids.
Convection involves movement of fluid particles carrying energy.
Radiation is the only non-contact energy transfer pathway, making it unique among thermal transfer mechanisms.
Practice Questions
Question 1 (2 marks)
State two different energy transfer pathways and give one example for each.
Mark Scheme:
1 mark for identifying each valid energy transfer pathway.
1 mark for providing a suitable example for each.
Acceptable answers include:
Mechanically – e.g., a force moving a car along a road.
Electrically – e.g., a current lighting a lamp.
By heating – e.g., a hot pan transferring energy to food.
By radiation – e.g., the Sun transferring energy to Earth by electromagnetic waves.
Question 2 (5 marks)
A student describes a system in which an electric motor lifts a weight vertically. Explain how the energy changes and transfers in this system, naming the energy stores and transfer pathways involved. Include in your answer what eventually happens to the energy once the motor has stopped.
Mark Scheme:
1 mark: Identifies electrical energy supplied to the motor.
1 mark: States that energy is transferred mechanically from the motor to the weight.
1 mark: Identifies increase in gravitational potential energy of the weight.
1 mark: Mentions that some energy is dissipated as thermal energy due to friction or resistance.
1 mark: Explains that energy is conserved overall, eventually spreading out to the surroundings as thermal energy.
