0.22 Phy1200: energy -- power  (Page 2/3)

Power

To be correct from a physics viewpoint, we would need to say that Joe is delivering more power than Bill. In other words, power is a measure of the rate at which work is done. They both do the same amount ofwork, but Joe does it more quickly than Bill. Hence Joe delivers more power than Bill.

Power in equation form

Power is the ratio of work to time. In equation form,

Power = Work/time

The SI unit for power

The SI unit for power is the watt . One watt of power is being delivered when onejoule of work or energy is being delivered each second.

An electric heater

In other words, if you have an electric heater that is properly rated at 60 watts, it will deliver 60 joules of energy per second when it is turned on.

This means that somewhere in the world, someone or something must be doing work at a rate of 60joules per second in order to insert the energy into the electrical grid that your heater will be taking out of the grid and turning into heat energy.

Horsepower

For historical reasons, particularly in the U.S., we also use the term horsepower to describe the power delivered by a machine. This is particularly true in the automotive industry, but it applies to other kinds ofmachines as well. We might speak of a car with a 300 horsepower engine, or a clothes washing machine with a quarter-horsepower motor.

One horsepower is equal to approximately 750 watts.

Most machines do work

Most machines are designed to consume electrical or chemical energy and do work on an object. Some machines, such as the treadmill at the health center, are designed to consumehuman energy in order to do work.

In order for a machine to do work, it must consume energy in some form.

Power ratings for machines

Machines are often described by a power rating. The power rating indicates the rate at which that machine can do work on objects.

When I was in the military many years ago, there were potato peeling machines in kitchens in the messhalls. Their purpose was to do work on potatoes by removing the peel. Presumably a machine with a high power rating could peel more potatoes per hour than one with a lowerpower rating.

Automobile engines and horsepower

Automobile engines are often rated in terms of power using horsepower as the units. At the drag-race track, contests are held to determine which vehicle can movefrom point A to point B in the shortest amount of time.

Since work is measured as force multiplied by distance, and power is measured as the work done per unit of time, everything else being equal, one would expectthat the vehicle with the highest power rating would be the winner in moving a given distance in the shortest amount of time.

What about the units?

What are the units of power? We know that

Power = force * distance/seconds

We know that the units of force are

f = m*a = kg*m/s^2

We know that the units of time are seconds, and the units of distance are meters. Therefore,

Power = f*d/time = (kg*m/s^2)*(m/s) = (kg*m^2)/(s^3), or

Power = kg*(m^2)*(s^(-3))

Plug the right-hand expression into the Google search box and you will learn that