Ch 5 · Energy, Work, Power and Energy Resources · IGCSE Physics 0625
Topic 5 of 5 in Motion, Forces and Energy — IGCSE Physics 0625 — 19 lessons.
Introduction
Energy is the idea that links every topic in physics. A falling ball, a boiling kettle, a wind turbine and a nuclear power station can all be described in the same way: energy moves from one store to another by a transfer, and the total amount never changes. In this lesson you will name stores and transfers, calculate work, power and (Supplement) kinetic and gravitational potential energy and efficiency, and compare the resources we use to generate electricity.
Key ideas
1. Energy stores. Energy is measured in joules (J). The stores you need are:
🧑🏫 Teacher's explanation
Think of energy as a currency that can be stored in different "bank accounts" (stores) like kinetic, chemical, or gravitational potential. You can't create money out of thin air, and you can't make it disappear; you can only move it from one account to another or spend it on something else (transfers). For example, when you drop a ball, you are "spending" its gravitational potential energy, which immediately appears in its kinetic energy account as it speeds up.\n\nWhen we talk about "work done," we are simply measuring how much energy was transferred by a force. If you push a box, you are transferring energy from your chemical store into the box's kinetic store. Power is just the speed at which you spend that energy. If you spend \$100 in one hour, your power is low; if you spend that same \$100 in one minute, your power is very high!\n\nIn the real world, no machine is perfect. Some energy always "leaks" out as wasted heat (internal energy) to the surroundings. We use Sankey diagrams to visualize this: the wider the arrow, the more energy is flowing. The total width of the input arrow must always equal the sum of all output arrows.
📖 Definitions
🔍 Worked example — Easy
A car of mass 1200 kg is traveling at 20 m/s. Calculate its kinetic energy.
🔍 Worked example — Standard
A crane lifts a 500 kg crate to a height of 15 m in 25 s. Calculate the power output of the crane.
🔍 Worked example — Exam-style
An electric motor is used to lift a weight. The motor uses 2000 J of electrical energy. 1500 J is transferred to gravitational potential energy, and the rest is lost as heat. Calculate the efficiency of the motor.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Example 1 — falling object (Supplement). A 0.50 kg ball is dropped from a height of 3.2 m. Ignoring air resistance, find the loss in gravitational potential energy and the speed just before it hits the ground.
🔍 Worked example — Easy
A crate is pushed across a floor with a constant horizontal force of 50 N over a distance of 12 m. Calculate the work done on the crate.
🔍 Worked example — Standard
A 0.50 kg ball is dropped from a height of 4.5 m. Calculate its speed just before it hits the ground, assuming no air resistance. (Use g = 9.81 m/s^2)
🔍 Worked example — Exam-style
An electric motor is used to lift a 25 kg mass through a height of 8.0 m in 5.0 s. The motor is 75\% efficient. Calculate the total electrical energy supplied to the motor.
⚠️ Common errors
Common mistakes and examiner tips
Practice
Answers: (1) Chemical store of the battery → (electrical work) → kinetic store of the blades, plus internal (thermal) store of the motor and surroundings (by heating) and some sound. (2) W = 4900 × 12 = 58 800\ J ≈ 5.9 × 10^4\ J; P = 58 800/20 = 2940\ W ≈ 2.9\ kW. (3) Δ E = Pt = 2000 × 90 = 1.8 × 10^5\ J. (4) E_k = tfrac12 × 70 × 8.0^2 = 2240\ J ≈ 2200\ J. (5) 900/2500 × 100\% = 36\%; wasted power = 2500 - 900 = 1600\ MW. (6) v = √(2gΔ h) = √(2 × 9.8 × 20) = √(392) = 20\ m/s (19.8 m/s). (7) Geothermal, nuclear, tidal. (8) Advantage: renewable, no CO₂ emitted while generating. Disadvantage: output depends on the wind, so not available on demand (also noise, visual impact, large area).