IG Phy – Work, Power and Efficiency (II)

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.