As the Ebola virus outbreak continues to run amok in West Africa, scientists are looking ahead to the possibly pivotal use of experimental drugs and vaccines against the disease. It will take months to test, produce and deploy the therapies. But researchers hold out hope that these products — even incompletely vetted — might help to turn the tide against an illness that has defied public health efforts to bring it under control.

Thursday, 13 October 2011

Power

In physics, power is the rate at which work is performed or energy is converted. Power can also be defined as the product of a force times the velocity of its point of application, in which case work is the integral of power along the trajectory of the point of application of the force.
As a simple example, burning a kilogram of coal releases much more energy than does detonating a kilogram of TNT, but because the TNT reaction releases energy much more quickly, it delivers far more power than the coal. If ΔW is the amount of work performed during a period of time of duration Δt, the average power Pavg over that period is given by the formula
P_\mathrm{avg} = \frac{\Delta W}{\Delta t}\,.
It is the average amount of work done or energy converted per unit of time. The average power is often simply called "power" when the context makes it clear.
The instantaneous power is then the limiting value of the average power as the time interval Δt approaches zero.
P = \lim _{\Delta t\rightarrow 0} P_\mathrm{avg} = \lim _{\Delta t\rightarrow 0} \frac{\Delta W}{\Delta t} = \frac{dW}{dt}\,.
In the case of constant power P, the amount of work performed during a period of duration T is given by:
W = PT\,.
In the context of energy conversion it is more customary to use the symbol E rather than W.
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