# Power and yield

Part of the power that enters through the electrical network, in a blender, is transformed into mechanical energy and the other part is dissipated in heat.

In many practical situations it is important to know the speed with which a job can be done. For example, when overtaking, the driver must be sure that his car will be able to perform that task without endangering the lives of passengers.

Another obvious example can clearly illustrate the importance that the time interval has in carrying out the work: which is faster, filling a water tank with a dropper or with a bucket? Going from São Paulo to Piauí by car or by plane?

We can define average power ( *P _{m}* ) as the quotient between the work done and the time interval spent. Mathematically, we have:

The SI unit of power is the **watt** (W), named after James Watt, the inventor who perfected the steam engine. So we have:

Other power units such as horse-power (HP) and horsepower (hp) are also used:

*– horse-power (HP): 1 HP = 746 W
– horse-power (hp): 1 hp = 735 W*

The HP unit was introduced by James Watt in a very simplified way, as he was concerned with making himself understood by the lay public. So, he used the figure of the horse to present his idea about power: a 30 HP machine corresponds to the power of 30 British horsepower.

To perform a task, every machine, every organism, needs a certain power, that is, a certain amount of energy per unit of time. However, not all the required power is used to perform the task. For example, in the blender, part of the power that enters through the electrical network is transformed into mechanical energy and the other part is dissipated in heat.

Thus, every machine has an **efficiency** (η), which is the quotient between the useful power (that is, used in performing the task) and the total power. By definition we have: