Efficiency of Electricity

Not all uses and production methods of electricity are created equally. Some consume a lot of energy and produce a low power output, others are almost 100% efficient. When you plug your phone in to charge, you will typically notice after some time that both the charging brick and the battery area of your phone will get HOT! This is considered wasted energy, because not all of it has gone into charging the device and instead heated the room.

Another example is the display you are using to read this information, it transforms the electricity into light which is constantly being generated by the device and consumes electricity. As a by-product of the light being made and colliding with the glass before it leaves the device it will also get hot over time.

Measuring Efficiency

Efficiency is measured in percentage, by dividing the “Useful power output” by the “Total power input” (equation below)

{\mathrm  {Efficiency}}={\frac  {{\mathrm  {Useful\ power\ output}}}{{\mathrm  {Total\ power\ input}}}}
  • An Electric kettle is more than 90% (comparatively little heat energy is lost during the 2 to 3 minutes a kettle takes to boil water).
  • A premium efficiency electric motor: more than 90%.
  • A large power transformer (used in the electrical grid) may have efficiency of more than 99% (essentially it’s a really good wire). Early 19th century transformers were much less efficient, wasting up to a third of the energy passing through them.
  • A steam power plant used to generate electricity may have 30-40% efficiency.
  • Solar panels are up to 20% efficient with technology today (2022)
  • Mobile phone towers and devices are extremely inefficient at about 30%, due to transmission loss via air

Practice Questions

  1. What is the efficiency when 120J is supplied to a toaster and 50J of energy is transferred into heat?

  2. What is the efficiency when 5J is supplied to a buzzer and 2J is transferred into sound energy?

  3. The efficiency of a LED lamp is 80% if 60J is supplied to the lamp, what is the useful energy?

  4. The efficiency of the kettle is 75%. If 80J of energy is transferred into heat energy, what is the total energy supplied to the kettle?
  5. A TV is supplied with 380J of energy. 160J is transferred into light energy, 130J is transferred into heat energy and 90J is transferred into sound energy. What is the efficiency?

  6. An electric motor transfers 400J into kinetic (movement) energy, 280J into heat energy and 120J into sound energy. What is the efficiency of the motor?

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