Study Notes
Overview
The principle of conservation of energy is a cornerstone of physics, stating that energy cannot be created or destroyed, only transferred from one store to another. For your OCR GCSE Physics exam, a deep understanding of this topic (specification point 1.4) is crucial. It is frequently tested through both calculation and explanation questions, often carrying a high mark weighting. This guide will equip you with the knowledge to confidently tackle questions on energy stores, transfers, and efficiency, ensuring you can clearly articulate the concepts as an examiner would expect.
Key Concepts
Concept 1: The Principle of Conservation of Energy
This fundamental law is simple but profound. In any process, the total amount of energy in a closed system remains constant. It is vital to use precise language: energy is not 'lost' or 'used up', but rather 'transferred' or 'dissipated'. Using the term 'lost' will result in a loss of marks. The key is to identify where the energy came from (the initial store) and where it went (the final store).
Concept 2: Energy Stores and Transfers
OCR uses a model of energy stores and transfers. You must be able to identify the main energy stores and the pathways by which energy is transferred between them.
The 8 Energy Stores:
- Chemical: Energy stored in bonds between atoms (e.g., in food, fuel, batteries).
- Kinetic: Energy of a moving object.
- Gravitational Potential (GPE): Energy an object has due to its position in a gravitational field.
- Elastic Potential: Energy stored when an object is stretched or compressed.
- Thermal: The total kinetic and potential energy of the particles in an object.
- Magnetic: Energy stored when repelling poles have been pushed closer together or attracting poles have been pulled further apart.
- Electrostatic: Energy stored when repelling charges have been moved closer together or attracting charges have been pulled further apart.
- Nuclear: Energy stored in the nucleus of an atom.
The 4 Energy Transfer Pathways:
- Mechanically: An object moving due to a force acting on it (e.g., pushing, pulling, stretching).
- Electrically: A charge moving through a potential difference (e.g., current in a circuit).
- By heating: Due to a temperature difference.
- By radiation: Energy transferred as a wave (e.g., light, sound).
Concept 3: Efficiency
In real-world systems, not all input energy is transferred to a useful output store. Some energy is always dissipated, usually to the thermal store of the surroundings. Efficiency is a measure of how much of the input energy is usefully transferred.
Efficiency can be calculated as a ratio or a percentage. An efficiency greater than 1 or 100% is impossible as it would violate the principle of conservation of energy.
Mathematical/Scientific Relationships
- Kinetic Energy (KE): KE = ½mv² (Must memorise)
- KE = Kinetic Energy (Joules, J)
- m = mass (kilograms, kg)
- v = velocity (metres per second, m/s)
- Gravitational Potential Energy (GPE): GPE = mgh (Must memorise)
- GPE = Gravitational Potential Energy (Joules, J)
- m = mass (kilograms, kg)
- g = gravitational field strength (Newtons per kilogram, N/kg - on Earth, g ≈ 9.8 N/kg)
- h = height (metres, m)
- Efficiency: Efficiency = Useful Energy Output / Total Energy Input (Given on formula sheet)
Practical Applications
This topic is directly linked to the required practical investigating the specific heat capacity of materials. In this experiment, you heat a block of material using an electric heater and measure the temperature change. By measuring the energy input from the heater (Power x time) and the temperature change of the block, you can calculate the specific heat capacity. However, a key source of error is the dissipation of thermal energy to the surroundings, which makes the calculated value for specific heat capacity higher than the true value. This demonstrates that not all the electrical energy supplied is transferred to the thermal store of the block.