CIE A Level Physics復習筆記5.1.3 Efficiency
Efficiency of a System
- The efficiency of a system is?the ratio of the useful energy output from the system to the total energy 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
- Multiplying this ratio by 100 gives the efficiency as a percentage
- The efficiency is calculated using the equation:
Efficiency equation in terms of energy
- Efficiency can also be written in terms of power (the energy per second):
Efficiency equation in terms of power
Worked Example
Exam Tip
Efficiency can be in a ratio or percentage format. If the question asks for an efficiency as a ratio, give your answer as a fraction or decimal. If the answer is required as a percentage, remember to multiply the ratio by 100 to convert it, e.g. Ratio = 0.25, Percentage = 0.25 × 100 = 25 %
Solving Problems Involving Efficiency
- Recall the two equations for calculating efficiency are:
- Which to use will depend on whether you’re given a system calculating energies or power as shown in the examples below
Worked Example
The diagram shows a pump called a hydraulic ram.
In one such pump, the long approach pipe holds 700 kg of water. A valve shuts when the speed of this water reaches 3.5 m s-1 and the kinetic energy of this water is used to lift a small quantity of water by a height of 12m.The efficiency of the pump is 20%.Which mass of water could be lifted 12 m?
A.?6.2 kg? ? ? ? ? ? ? ?B.?4.6 kg? ? ? ? ? ? ? ?C.?7.3 kg? ? ? ? ? ? ? ?D.?0.24 kg
- The pump is what converts the water’s?kinetic energy?into?gravitational potential energy
- Since its efficiency is 20%, you would multiply the kinetic energy by 0.2 since only 20% of the?kinetic energy?will be converted (not?20% of the gravitational potential energy)
Exam Tip
Equations for kinetic and potential energies are important for these types of questions. Also, familiarise yourself with the different equations for power depending on what quantities are given.
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