Power

• What is the definition of 'Power'?

  The rate at which energy is transferred (or rate at which work it done).
• State 2 equations for power. Give SI units for all quantities involved.


  Power = Energy transferred / Time

  Power = Work done / Time

  Energy (J), Work done (J), Time (S)
• What is the unit of power?

  Watt (W).
• What is one Watt equivalent to?

  1 joule per second (J/s).
• What does a power rating of 10W mean for an electrical device?

  It converts 10 joules of energy every second.
• Two motors lift the same mass through the same height. Motor A does this in half the time of Motor B. Which dissipates the most power?

  Motor A.

  The energy transferred is the same but the time taken is less (P=E/T).
• What is efficiency?

  The proportion of energy that is converted usefully, rather than wasted/dissipated.
• How do you calculate efficiency?

  Efficiency = Useful energy out / Total energy in
• What are the units of efficiency?

  Efficiency is a ratio so has no units, but it can be expressed as a percentage.