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The equation P=W/t can also be written as P=ΔE/t, where ΔE is energy transferred.
Power is measured in joules (J).
A powerful electric motor will move the air in the room more slowly than a less powerful motor.
The work done by a crane in lifting a girder can be calculated using the formula W=F×d, where F is force and d is distance.
If a crane lifts a 20000 J girder in 40 seconds, the power of the crane\’s motor is 500 W.
The distance measured for the work done by a student running up stairs must be in the horizontal direction.
Energy is always conserved; the total amount of energy after the transfer is the same as before the transfer.
Efficiency can be calculated using the formula efficiency=total energy inputuseful energy output×100%.
An electric motor that takes in 10 kW of electricity and gives out 9.5 kW of kinetic energy has an efficiency of 85%.
The efficiency of a device can also be expressed as a decimal, such as 0.95 for 95%.
In a power station, if 70% of the energy transfers do not produce useful energy, the efficiency of the power station is 30%.
The efficiency of modern fossil-fueled power stations is closer to 70%.
A diesel engine giving out 200 kJ of kinetic energy from 500 kJ of stored chemical energy has an efficiency of 60%.
If power stations worldwide could use 25% less fuel, it would save millions of tonnes of coal or gas per year.
In some cities, homes are heated by hot water from a nearby power station, utilizing waste heat.