'Wasted' energy
Energy cannot be created or destroyed. It can only be converted from one form to another or moved. Energy that is 'wasted', like the heat energy from an electric lamp, does not disappear. Instead, it is transferred into the surroundings and spreads out so much that it becomes very difficult to do anything useful with it.
Electric
lamps
Ordinary electric lamps contain a thin metal filament that glows when electricity passes through it. However, most of the electrical energy is converted to heat energy rather than light energy. This (right) is the Sankey diagram for a typical filament lamp.
Modern
energy-saving lamps work in a different way. They convert
a greater proportion of electrical energy to light energy. This (right)
is the Sankey diagram for a typical energy-saving lamp.
From the diagram, you can see that much less electrical energy is "wasted"
i.e. converted to and then transferred to the surroundings as heat energy.
Calculating efficiency
The efficiency of a device such as a lamp can be calculated using this equation:
efficiency = useful energy transferred/energy supplied × 100
- The efficiency of the filament lamp is 10 ÷ 100 ×100
= 10%. This means that 10% of the electrical energy
supplied is converted to light energy (90% is converted to heat energy).
- The efficiency of the energy-saving lamp is 75 ÷ 100 × 100 = 75%. This means that 75% of the electrical energy supplied is converted to light energy (25% is converted to heat energy).
Note that the efficiency of a device will always be less than 100%.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science
]