work, power and efficiency

• What is work in the context of physics?

  Work is the measure of energy transfer when a force moves an object through a distance.
• How is energy transferred when work is done?

  Energy has been transferred from one energy store to another.
• In what unit are energy transferred and work done measured?

  Both are measured in joules (J).
• What two factors determine the amount of work done on an object?

  The size of the force acting on the object and the distance through which the force moves the object.
• What is the equation used to calculate work done?

  Work done = force × distance
• What are the units for work done, force, and distance?

  Work done is in joules (J), force is in newtons (N), and distance is in metres (m).
• If a man pushes a box with a force of 10 newtons to move it a distance of 2 metres, how much work is done?

  Work done = 10 N × 2 m = 20 J
• How much work is done when a force of one newton causes a body to move through a distance of one metre?

  One joule of work is done.
• What is power in the context of energy transfer?

  Power is the rate at which energy is transferred when work is done on an object.
• What is the equation used to calculate power?

  Power = work done / time
• What are the units for power, work done, and time?

  Power is measured in watts (W), work done in joules (J), and time in seconds (s).
• How does an increase in power relate to energy transfer?

  The more powerful a device is, the more energy it will transfer each second.
• If motor one lifts a 2 N weight through a height of 10 m in 5 seconds, what is its power?

  Power = work done / time = (2 N × 10 m) / 5 s = 4 W
• If motor two lifts the same weight in 10 seconds, what is its power?

  Power = work done / time = (2 N × 10 m) / 10 s = 2 W
• How does the power of motor one compare to motor two?

  Motor one is twice as powerful as motor two.
• What does a very efficient device do with input energy?

  A very efficient device wastes very little of its input energy.
• What is efficiency in the context of energy transfer?

  Efficiency is how well a device transfers energy input to useful energy output.
• How can efficiency be calculated?

  Efficiency can be calculated as a decimal or a percentage using the proportion of useful energy transferred to total energy supplied.
• If a light bulb receives 200 J of energy and transfers 28 J usefully, what is its efficiency?

  Efficiency = useful energy transferred / total energy supplied = 28 J / 200 J = 0.14 or 14%
• What happens to most of the energy supplied to a light bulb?

  Most of the energy is dissipated as infrared radiation.
• What is the relationship between power and efficiency?

  Power is equal to useful energy transferred per second.
• What are some reasons devices waste energy?

  Friction, electrical resistance, and unwanted sound energy.
• How can devices be made more efficient?

  By reducing the energy wasted or dissipated to the surroundings.
• What are the useful and wasted energies for common electrical appliances?

  • Electric kettle:
  • Useful: Heats the water
  • Wasted: Internal energy heating the kettle, infrared radiation lost
  • Hair dryer:
  • Useful: Heats the air, kinetic energy of the fan
  • Wasted: Sound radiation, internal energy heating the dryer, infrared radiation lost
  • Light bulb:
  • Useful: Light radiation from the filament
  • Wasted: Infrared radiation lost
  • TV:
  • Useful: Light and sound radiation
  • Wasted: Internal energy heating the TV, infrared radiation lost
• How does the efficiency of an appliance relate to its energy transfer?

  The greater the amount of supplied electrical energy transferred to useful energy stores, the more efficient the device.
• How does a kettle transfer supplied electrical energy?

  A kettle transfers supplied electrical energy to both useful and wasted energy stores.
• What is the equation for calculating efficiency?

  Efficiency = useful energy transferred / total energy supplied
• What is the significance of an energy transfer diagram?

  It shows how supplied electrical energy is transferred to useful and wasted energy stores.
• What is the equation for calculating efficiency using power?

  Efficiency = useful power transferred / total power supplied