Conservation of energy — AQA A-Level Physics

The conservation of energy is a fundamental principle in physics that states energy cannot be created or destroyed, only transferred between stores. In AQA A-Level Physics, you will apply this principle to mechanical systems, including objects moving under gravity, pendulums, and roller coasters, as well as to electrical and thermal systems. Understanding energy conservation allows you to solve problems involving work, power, and efficiency without needing to analyse forces in detail.

This topic is crucial because it underpins many other areas of physics, from mechanics to thermodynamics and electricity. It provides a powerful problem-solving tool: by accounting for all energy transfers, you can predict motion, calculate speeds, and determine energy losses. Mastery of energy conservation is essential for tackling more complex topics like circular motion, oscillations, and fields, where energy methods often simplify calculations.

• →Energy can be transferred between different stores (kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, electrostatic, magnetic) but the total energy in a closed system remains constant.
• →Work done by a force is the energy transferred when a force moves an object: W = Fs cos(theta), where theta is the angle between force and displacement.
• →The principle of conservation of energy: in a closed system, the sum of all energy stores before an event equals the sum after, accounting for any energy dissipated to the surroundings.
• →Dissipation of energy: energy transferred to less useful stores, often thermal energy of the surroundings, due to resistive forces like friction or air resistance.
• →Efficiency: the ratio of useful energy output to total energy input, often expressed as a percentage: efficiency = (useful output / total input) * 100%.

• States that energy cannot be created or destroyed, only transferred between stores or transformed.
• Defines a closed system as one in which no energy enters or leaves, so the total energy remains constant.
• Applies conservation of energy to a mechanical process, for example equating gravitational potential energy lost to kinetic energy gained when friction is negligible.
• Explains that when friction or drag acts, some energy is transferred to the internal energy store of the surroundings, often called dissipated or wasted energy.
• Uses conservation of energy to calculate an unknown quantity such as speed, height or energy transferred, showing that the total before equals the total after.

• 💡Write a clear 'before = after' energy equation, listing each store with its correct expression, before substituting numbers.
• 💡State explicitly that you are assuming the system is closed and that friction or air resistance is negligible when using conservation of mechanical energy.
• 💡If energy is dissipated, calculate the energy transferred to internal energy as the difference between initial and final mechanical energy.
• 💡Use standard symbols and units consistently, for example mgh for gravitational potential energy and ½mv² for kinetic energy.
• 💡Always start by identifying the energy stores involved and the transfers that occur. Write down the conservation of energy equation: total initial energy = total final energy + work done against resistive forces.
• 💡Use standard symbols and units correctly: J for energy, N for force, m for distance, m/s for speed. Show all steps in calculations, including rearranging equations before substituting numbers.
• 💡When explaining energy transfers in words, use precise terminology: 'transferred to', 'dissipated to', 'stored as'. Avoid vague terms like 'lost' or 'used up'.

• Claiming that energy is used up or lost; energy is conserved but may be transferred to less useful stores such as internal energy. Correct by saying energy is dissipated or transferred to the surroundings.
• Forgetting to include all energy stores in the system, such as elastic potential energy or internal energy, when writing an energy balance equation.
• Assuming that mechanical energy is always conserved; in the presence of friction or drag, mechanical energy decreases as internal energy increases, although total energy is constant.
• Mixing up the terms energy store and energy transfer; for example, saying kinetic energy is transferred to gravitational potential energy store rather than saying energy is transferred from the kinetic store to the gravitational potential store.
• Energy is 'used up' or 'lost' in processes like friction. Correction: Energy is conserved; it is transferred to thermal energy of the surroundings, which is less useful but not lost.
• If an object moves at constant speed, no energy is transferred. Correction: Energy may be transferred to overcome resistive forces, such as when a car travels at constant speed, chemical energy is transferred to thermal energy.
• Gravitational potential energy depends only on height, not on the path taken. Correction: It depends on the vertical height difference, but work done against friction depends on the path length, so the total energy transfer can vary.

1. 1Day 1-2: Review the different energy stores and the principle of conservation of energy. Learn the equations for kinetic energy, gravitational potential energy, and work done.
2. 2Day 3-4: Practice applying conservation of energy to simple systems like falling objects and pendulums, including calculations of speed and height.
3. 3Day 5-6: Study energy dissipation and efficiency. Solve problems involving resistive forces and calculate work done against friction.
4. 4Day 7-8: Attempt exam-style questions, focusing on multi-step calculations and explanations of energy transfers. Review mark schemes to understand examiner expectations.
5. 5Day 9-10: Revise common misconceptions and practice command words like 'describe', 'explain', and 'calculate'. Create a summary sheet of key equations and definitions.

• 📋Calculation questions: Often involve finding speed, height, or resistive force using conservation of energy. Advice: Write down the energy balance equation, cancel mass if possible, and show all working.
• 📋Explanation questions: Describe energy transfers in a given scenario, such as a bouncing ball or a roller coaster. Advice: Use precise language, mention specific stores, and state that total energy is conserved.
• 📋Practical-based questions: Analyse data from experiments like a pendulum or a ball rolling down a ramp to verify conservation of energy. Advice: Consider sources of error and how they affect results, such as air resistance or friction.
• 📋Graph questions: Interpret or sketch graphs of energy against time or position, such as kinetic energy vs height. Advice: Remember that total energy is constant, so the sum of kinetic and potential energy remains constant if no dissipation.