energy

• What is the main reason useful devices work?

  They transfer energy from one store to another.
• What happens to some of the energy during energy transfers?

  Some energy is usually wasted by being transferred to a useless energy store.
• How is the efficiency of a device determined?

  The less energy wasted, the more efficient the device is said to be.
• How can you improve the efficiency of energy transfers?

  • Insulating objects
  • Lubricating them
  • Making them more streamlined
• What is the formula for calculating efficiency?

  Efficiency = Useful output energy transfer / Total input energy transfer
• How can you calculate efficiency if you know the power input and output?

  Efficiency = Useful power output / Total power input
• If a blender is 70% efficient with a total input power of 600 W, what is the useful power output?

  420 W
• Why is useful energy output not usually equal to total energy input?

  No device is 100% efficient, and wasted energy is usually transferred to thermal energy stores.
• What is the exception to the rule that no device is 100% efficient?

  Electric heaters are usually 100% efficient.
• What happens to energy when using an electric drill?

  Its energy transfers to different energy stores but ultimately ends up in thermal energy stores.
• What is the conservation of energy principle?

  Energy can be transferred, used, stored, or dissipated, but it can never be created or destroyed.
• What happens to energy during an energy transfer in a closed system?

  Some energy is always dissipated, usually as thermal energy.
• Describe the energy transfer in a closed system with a cold spoon and hot soup.

  • Cold spoon heats up
  • Hot soup cools down slightly
  • Energy transfers from the soup to the spoon
  • No energy leaves the system
• How is power defined in physics?

  Power is the rate of energy transfer or the rate of doing work.
• What is the unit of power?

  Watts
• How can power be calculated?

  Power can be calculated using the equations: $P =$$\frac{E}{t}$ and $P =$$\frac{W}{t}$.
• What does it mean for a machine to be powerful?

  A powerful machine can exert a strong force in a short space of time.
• What happens when two cars with the same engine run the same distance?

  • Both cars transfer the same amount of energy
  • The car that takes less time is considered more powerful
  • Power is not just about energy transfer but also time
• What happens to energy when it is transferred to an object?

  The energy is stored in one of the object's energy stores.
• What is the relationship between energy and energy stores?

  • Energy is never used up
  • It is transferred between different energy stores
  • Different objects can hold energy in various forms