work power and efficiency

• Energy

  A key principle in physics, as it allows work to be done
• Power


  The rate at which energy is transferred
• Efficiency

  The amount of energy that is usefully transferred
• Work

  The measure of energy transfer when a force (F) moves an object through a distance (d)
• Energy transferred = work done
• Energy transferred and work done are both measured in joules (J)
• Calculating work done

  1. Depends on the size of the force acting on the object

  2. Depends on the distance through which the force causes the body to move in the direction of the force
• Work done (W)

  Measured in joules (J)
• Force (F)


  Measured in newtons (N)
• Distance (d)

  Measured in metres (m)
• One joule of work is done (or one joule of energy is transferred) when a force of one newton causes a body to move through a distance of one metre
• Calculating work done

  • Force of 10 N causes a box to move a horizontal distance of 2 m, so work done = 10 x 2 = 20 J
• Calculating work done

  • Horizontal force of 50 N causes a trolley to move a horizontal distance of 30 m, so work done = 50 x 30 = 1,500 J
• Calculating force

  • 12,000 J of energy is supplied to move a small truck a distance of 80 m, so force = 12,000 / 80 = 150 N
• Work

  When energy is transferred to an object
• Power


  The rate at which energy is transferred
• Power is measured in watts (W)
• One watt is equal to one joule per second (J/s)
• For every extra joule that is transferred per second, the power increases by one watt
• Calculating power

  Power = work done ÷ time taken
• Calculating power

  • Power = W/t
• Efficiency

  • The amount of energy that is usefully transferred
• When work is done on an object, energy is transferred
• The more powerful a device is, the more energy it will transfer each second
• Calculating power

  • Motor one: P = 20J/5s = 4W

  Motor two: P = 20J/10s = 2W
• Motor one is twice as powerful as motor two since it transfers twice as much energy per second
• Calculating power

  • Hairdryer: P = 48,000J/60s = 800W
• Efficiency


  The fraction of the energy supplied to a device which is transferred in a useful form
• A very efficient device will waste very little of its input energy. A very inefficient device will waste most of its input energy
• Dissipated


  The spreading out and transfer of energy stores into less useful forms, such as thermal energy causing the surroundings to heat up
• It is not possible to have an efficiency of greater than 1 or efficiency percentage of 100%. This would mean that more energy is being transferred than is being supplied, which would break the law of conservation of energy
• Devices waste energy for various reasons including friction between their moving parts, electrical resistance, and unwanted sound energy
• Devices can be made more efficient by reducing the energy that they waste or dissipate to the surroundings
• One example is lubrication being used to reduce the friction between moving parts of a machine
• Work

  The transfer of energy
• Power


  The rate at which energy is transferred
• Efficiency

  The amount of energy that is usefully transferred
• Electrical appliances used in the home


  • Electric kettle
  • Hair dryer
  • Light bulb
  • TV
• Electric kettle

  • Useful energy: Energy that heats the water
  • Wasted energy: Internal (thermal) energy heating the kettle, Infrared radiation lost to the surroundings
• Hair dryer

  • Useful energy: Internal (thermal) energy heating the air, Kinetic energy of the fan that blows the air
  • Wasted energy: Sound radiation, Internal (thermal) energy heating the hairdryer, Infrared radiation lost to the surroundings