Work - Energy Principle

1. Work - Energy Principle If some work is done on an object then its energy will change. It may gain or lose g.p.e , e.p.e or k.e. The work - energy principle simply states that the work done is equal to the gain in energy. This principle leads to an important conservation law: “For an isolated system (i.e. no external force acting), the total energy possessed by the system is constant.”

2. This basically says that no work done means no change in total energy. “Energy is neither created nor destroyed but may be transformed from one type to another.” We often do not observe this experimentally since in practice an isolated system is difficult to achieve. Usually friction etc acts.

3. Power and Efficiency The word “power” in Physics has a distinct meaning. Power is defined as the rate of doing work or the rate of change of energy. E t P = The unit of power is the watt 1 W = 1 J s-1

4. Since work done = force x distance, P = F s t But distance over time is velocity => P = F x v e.g a car travelling at constant speed of 30 m s-1 with a driving force of 2000 N uses a power of 2000 x 30 = 60000 W

5. An alternative unit of energy is derived from the kilowatt. The kilowatt-hour is the amount of energy used by a 1kW appliance in 1 hour. It is equal to 1000x3600J i.e. 3600000 or 3.6 x 106 J Household electricity costs about 6p a kWh.

6. Efficiency The second law of thermodynamics says no energy transfer is perfect: some is always lost as heat. The efficiency of a machine is a measure of how good the machine is at transferring energy into a useful form. Efficiency = useful energy out x 100 % total energy in e.g a motor converting 100 J of electrical energy to 75 J of k.e. has an efficiency of 75%