A.3
Work, Energy & Power
Work done, kinetic and potential energy, conservation of energy, efficiency and power.
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Key Concepts, Work, Energy & Power
Conservation of Energy
The principle of conservation of energy states that energy cannot be created or destroyed, only transferred from one form to another. The total energy of a closed system remains constant. In IB Physics, this means the total mechanical energy (kinetic + gravitational potential + elastic potential) is conserved in the absence of friction or other dissipative forces. When friction is present, mechanical energy is not conserved, but total energy still is (the missing energy has been transferred to thermal energy).
Work Done
Work is done when a force causes a displacement. The equation is W = Fs cosθ, where F is the applied force in Newtons, s is the displacement in metres, and θ is the angle between the force and the direction of displacement. If the force and displacement are parallel, cosθ = 1 and W = Fs. If the force is perpendicular to displacement (like a normal force on a horizontally moving object), cosθ = 0 and no work is done. Work done equals the energy transferred to or from the system and is measured in Joules.
Kinetic and Potential Energy
Kinetic energy is the energy an object possesses due to its motion: Ek = ½mv². Gravitational potential energy is the energy stored due to an object's height in a gravitational field: Ep = mgh, where g = 9.81 m/s² near Earth's surface. Elastic potential energy is stored when a spring or elastic material is deformed: Ee = ½kx², where k is the spring constant and x is the extension. In the absence of friction, total mechanical energy (Ek + Ep + Ee) is constant, so any decrease in one form results in a corresponding increase in the others.
Sankey Diagrams
A Sankey diagram is a visual representation of energy transfers in a system. The width of each arrow is proportional to the amount of energy it represents. The main arrow entering on the left shows total input energy. It then splits into useful output energy (continuing straight ahead) and wasted energy (typically shown branching downward as heat or sound). Sankey diagrams make it easy to visualise efficiency at a glance: the wider the useful output arrow relative to the input, the more efficient the process.
Power
Power is defined as the rate of doing work, or the rate of energy transfer: P = W/t, measured in Watts (W), where 1 W = 1 J/s. For a force causing motion, power can also be written as P = Fv, where F is the driving force and v is the velocity. This version is useful for problems involving moving vehicles or objects at constant speed, where the driving force equals the resistive force.
Efficiency
Efficiency is the ratio of useful output energy (or power) to total input energy (or power), expressed as a decimal or percentage. Efficiency = useful output energy / total input energy, or equivalently, efficiency = useful output power / total input power. No real system is 100% efficient because some energy is always lost to the surroundings, usually as heat or sound. A system with an efficiency of 0.75 (or 75%) transfers 75% of its input energy usefully and wastes 25%.
Energy Density
Energy density is the amount of energy stored per unit mass (or per unit volume) of a fuel source, measured in J/kg or J/m³. A fuel with a high energy density releases a large amount of energy per kilogram burned. For example, hydrogen has a very high energy density by mass, making it attractive as a fuel. Energy density is used to compare different fuel sources and to solve problems involving the energy available from a given mass of fuel.
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Frequently Asked Questions, IB Physics Work, Energy & Power
What is Work, Energy & Power in IB Physics? ↓
Work done, kinetic and potential energy, conservation of energy, efficiency and power. This topic is part of Theme A (Space, Time & Motion) in the current IB Physics syllabus.
Is Work, Energy & Power SL or HL in IB Physics? ↓
Work, Energy & Power is covered by both SL and HL students in the current IB Physics syllabus. HL students study additional depth and extension content beyond the SL core.
What equations do I need for IB Physics Work, Energy & Power? ↓
The key equations for Work, Energy & Power are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Work, Energy & Power? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Work, Energy & Power for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.