Advice 1: How to find efficiency

Coefficient of useful action shows the ratio of the useful work that is performed by a mechanism or device to expended. Often spent work take the amount of energy that the device consumes in order to perform the work.
You will need
  • - the car;
  • thermometer;
  • calculator.
In order to calculate the coefficient of useful action (CPA) divide the useful work Up to the work expended AZ and multiply the result by 100% (efficiency=AP/AZ∙100%). Get the result in percent.
When calculating the efficiency of the heat engine, useful work, consider the mechanical work done by a mechanism. For the spent work take the amount of heat generated by the burned fuel which is a source of energy for the engine.
Example. The average tractive power of the engine of the vehicle is 882 N. At 100 km it consumes 7 kg of gasoline. Determine the efficiency of its engine. First, find a good job. She is the product of force F for a distance S traveled by a body under the influence of AP=F∙S. Determine the quantity of heat allocated at burning 7 kg of gasoline, it will be spent working AZ=Q=q∙m, where q – calorific value of fuel, for gasoline it is equal to 42∙10^6 j/kg and m is the mass of fuel. Engine efficiency is equal to efficiency=(F∙S)/(q∙m)∙100%= (882∙100000)/( 42∙10^6∙7)∙100%=30%.
In the General case, to find the efficiency of any heat engine (internal combustion engine, steam engine, turbine, etc.) where the work is done by the gas has a coefficient of useful action is equal to the difference of heat given to the heater, Q1 and Q2 received by the fridge, find the difference between the warmth of the heater and of the refrigerator, and divide by the warmth of the heater efficiency= (Q1-Q2)/Q1. Here efficiency is measured in units of decimal fraction from 0 to 1 to convert the result into a percentage, multiply it by 100.
To get the efficiency of an ideal heat engine (Carnot), find the ratio of the temperature difference of the heater T1 and cooler T2 temperature the heater efficiency=(T1-T2)/T1. This is the maximum possible efficiency for a particular type of heat engine with given temperatures of the heater and of the refrigerator.
To find the spent motor work as the product of the power at the time of its execution. For example, if the motor of the crane capacity of 3.2 kW lifts a load weighing 800 kg to a height of 3.6 m for 10 s, then its efficiency is equal to the ratio of useful work AP=m∙g∙h, where m is the load mass, g≈10 m/S2 the acceleration of gravity, h is the height to which lifted the load, and spent working AZ=R∙t, where P – motor power t – the time it works. Will receive the formula for definition of efficiency=AP/AZ∙100%=(m∙g∙h)/(R∙t) ∙100%=%=(800∙10∙3,6)/(3200∙10) ∙100%=90%.

Advice 2: How to calculate efficiency

Coefficient of performance (COP) is a measure of the effectiveness of any system, be it a car engine, car or other mechanism. It shows how efficiently the system uses the received energy. To calculate efficiency very easily.
Most often efficiency is calculated from the ratio of the useful energy system to all total received energy in a certain period of time. It is worth noting that efficiency has no specific units of measurement. However, in the school curriculum this value is measured in percents. This ratio, based on the above data, calculated by the formula:
η = (A/Q)*100%, where η ("ETA") is the required efficiency, A useful operation of the system, Q is the total energy, A and Q are measured in Joules.
The above method of calculating efficiency is not the only one, as the useful work of the system (A) is calculated according to the formula:
A = Po-Pi where Po - power system energy from the outside, Pi - loss of energy in the system. Expanding the numerator of the above formula, it can be written in the following form:
η = ((Po-Pi)/Po)*100%.
To the calculation of the efficiency was more clear and evident, it is possible to consider the examples.
Example 1: Useful work equal to 75 j, the sum expended for its operation energy is 100 j, it is required to find the efficiency of the system. To resolve this problem, apply the first formula:
η = 75/100 = 0.75 or 75%
Answer: the efficiency of the proposed system is 75%.
Example 2: Input to operate the engine energy is 100 j, the energy loss during operation of this engine 25 j is necessary to calculate efficiency. For a solution use the second formula of calculating the required ratio:
η = (100-25)/100 = 0.75 or 75%.
The responses in both examples are the same, because in the second case was more parsed data to the numerator.
Many types of modern engines (e.g., rocket engine or turbowebguy) have several stages of its work, and each stage has its own efficiency, which is measured in any of these formulas. But to find the common ratio, you will need to multiply all known efficiency at all stages of operation of this engine:
η = η1*η2*η3*...*η.
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Efficiency cannot be more than one, because during the operation of any system inevitably loss of energy.
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