Edexcel GCSE Physics (1PH0) — Energy – forces doing work
Every time you use a mobile phone, some of the battery's energy powers the screen, but some of it just makes the device feel warm. This happens because no device is perfectly effective at transferring energy. Efficiency is a measure of the proportion of the total energy supplied to a device that is successfully transferred into useful energy stores.
Because of the Law of Conservation of Energy, the total energy input must always equal the useful energy output plus the wasted energy. Wasted energy is typically dissipated (spread out) to the surroundings as thermal energy due to friction, air resistance, or electrical resistance. The only exception to this is an electric heater, which is sometimes considered efficient because thermal energy is its intended useful output.
How do we mathematically prove that an LED bulb is better than an old filament bulb? We calculate its efficiency using either energy values or power values, depending on the data provided.
Efficiency is a dimensionless quantity, meaning it has no units because it is a ratio of identical units (such as Joules divided by Joules) that cancel each other out. It can be expressed as a decimal ratio between and , or as a percentage between and . Efficiency can never exceed (or ) because it is physically impossible to transfer more energy out of a system than was put into it.
Calculating exact efficiency values helps engineers design better, less wasteful appliances. When answering exam questions, always identify your useful and total values first, and ensure you round your final answer to the same number of significant figures as the data provided.
An electric motor is supplied with 2500 J of electrical energy. It usefully transfers 500 J to a kinetic energy store. Calculate the efficiency of the motor as a decimal ratio.
Step 1: Identify the values.
Step 2: Substitute into the equation.
Step 3: Calculate.
A television has a total power input of 150 W. It dissipates 105 W of power as thermal energy to the surroundings. Calculate its percentage efficiency.
Step 1: Find the useful power.
Step 2: Substitute into the equation.
Step 3: Convert to a percentage.
You can calculate numbers on a page, but visually mapping an energy split makes it instantly obvious how much is being wasted. A Sankey diagram is a specific type of flow diagram used to represent these energy transfers. The defining rule of a Sankey diagram is that the width (thickness) of each arrow is directly proportional to the amount of energy or power it represents.
The total input energy is represented by the flat end of the arrow on the left. This then splits into useful energy (the straight horizontal arrow) and wasted energy (the arrow bending downwards). In Edexcel exams, Sankey diagrams are often drawn on a grid, meaning you must count the squares to determine the exact energy values. The total width of the input arrow will always perfectly equal the combined widths of the output arrows.
Candidates often invert the division by placing the larger 'total' number on top of the 'useful' number, resulting in an impossible efficiency greater than 1.
In Edexcel mark schemes, if you provide a percentage answer you must explicitly write the '%' sign, but if providing a decimal ratio you should not attach any units or symbols.
Always use the specific word 'dissipated' rather than 'lost' when describing wasted energy in your exam answers to show examiners you understand energy is conserved.
When analysing Sankey diagrams on a grid, if the total input is not labelled, simply count the squares for the useful and wasted arrows and add them together to find the total.
Efficiency
A measure of the proportion of total energy supplied to a device that is transferred into useful energy stores.
Total energy supplied
The total amount of energy put into a system or device from an external source.
Useful energy
Energy transferred to the specific store that a device is designed to provide.
Law of Conservation of Energy
The fundamental principle that energy cannot be created or destroyed, only transferred between stores.
Dissipated
A term describing energy spreading out into the surroundings and becoming less useful, usually as thermal energy.
Sankey diagram
A flow diagram where the width of the arrows is directly proportional to the amount of energy or power they represent.
Put your knowledge into practice — try past paper questions for Physics
Efficiency
A measure of the proportion of total energy supplied to a device that is transferred into useful energy stores.
Total energy supplied
The total amount of energy put into a system or device from an external source.
Useful energy
Energy transferred to the specific store that a device is designed to provide.
Law of Conservation of Energy
The fundamental principle that energy cannot be created or destroyed, only transferred between stores.
Dissipated
A term describing energy spreading out into the surroundings and becoming less useful, usually as thermal energy.
Sankey diagram
A flow diagram where the width of the arrows is directly proportional to the amount of energy or power they represent.
