Until now, we've been talking about mechanical and electrical energy as two separate subjects. Now we'll discuss how we use mechanical energy to generate electrical energy, and electrical energy to generate mechanical energy.
Much of this interaction involves a unit of force at a distance called torque. Torque is the force created by a rotating shaft or wheel, such as a waterwheel, windmill or electric motor. Any value for torque must have two elements: the unit of force applied to (or received from) a wheel, and how far from the center of the wheel the force is applied (or received).
Both of these elements are necessary because any wheel creates a mechanical advantage that increases as you move farther and farther from its center of rotation. For instance, it's much more difficult to turn a bicycle wheel by the axle than it is to turn it by pushing along the tire. It takes more force near the wheel's center than it does at the wheel's perimeter.
In the U.S., we measure torque in pound-ft. of torque. One pound-ft. of torque is equivalent to one pound of force applied to a wheel one foot from the center of rotation. Do not confuse pound-ft. of torque with the linear ft-pounds of work we discussed earlier. A pound-ft. of torque performs 6.29 linear ft-pounds of work for every rotation of an axle.
To determine horsepower, we multiply the pound-ft. of torque by the revolutions per minute (rpm) of the axle and divide by the number 5,250. For example, if we were to apply 75 lb-ft. of torque to the crank in the illustration here, and sustain a rate of 70 revolutions per minute, that would generate one horsepower.
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