IB DP Physics: SL复习笔记2.3.7 Efficiency

• The efficiency of a system is a measure of how well energy is transferred in a system
• Efficiency is defined as:

The ratio of the useful power or energy transfer output from a system to its total power or energy transfer input

• If a system has high efficiency, this means most of the energy transferred is useful
• If a system has low efficiency, this means most of the energy transferred is wasted
• Determining which type of energy is useful or wasted depends on the system
  • When electrical energy is converted to light in a lightbulb, the light energy is useful and the heat energy produced is wasted
  • When electrical energy is converted to heat for a heater, the heat energy is useful and the sound energy produced is wasted

• Efficiency is represented as a percentage, and can be calculated using the equation:

• The energy can be of any form e.g. gravitational potential energy, kinetic energy
• The efficiency equation can also be written in terms of power:

• Where power is defined as the energy transferred per unit of time

Step 1: Write down the efficiency equation 

Step 2: Rearrange for the power input  

Step 3: Calculate the power output

• • The power output is equal to energy ÷ time
  • The electric motor transferred electric energy into gravitational potential energy to lift the load

Gravitational potential energy = mgh = 7.2 × 9.81 × 5 = 353.16 J

Power = 353.16 ÷ 3 = 117.72 W

Step 4: Substitute values into power input equation 

• The pump is what converts the water’s kinetic energy into gravitational potential energy
• Since its efficiency is 20%, the kinetic energy can be multiplied by 0.2 since only 20% of the kinetic energy will be converted (not 20% of the gravitational potential energy)