Page 123 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 5 Mechanical and Electrical Power, Work, and Energy 103

A load (light bulb, motor, and so on) resists the flow of electricity by turning the
electrical energy into some other form—light in the case of the light bulb and motion
in the case of the motor. In an ideal circuit, all the electrical energy is converted into
other forms of energy like this. In reality, some energy is always lost, most of the time
as heat.
Technically, you can’t lose energy. It just gets turned into forms that are not useful. For
example, heat is a form of energy, but when our circuit has extra power that causes
our motor to heat up, that heat is not useful to us, so we say that the energy is lost.
Some appliances, like a toaster, take advantage of this. A toaster is like a big resistor
that just takes the current from the wall and turns it into heat to toast our bread.
Voltage, current, and resistance are related by Ohm’s law:

Voltage (V) = Current (I) × Resistance (R)

Batteries actually have internal resistance, which just means that they’re not 100%
efficient, and this internal resistance makes the batteries heat up. Cheaper batteries
tend to have higher internal resistance, which is a direct power loss. 2

Electrical power (P), measured in watts (W), is the combination of current and voltage:

Power (P) = Current (I) × Voltage (V)

For example, a 60-watt light bulb needs 0.5A at 120V. The more work you need
to do, the more power you need. Compact fluorescent light (CFL) bulbs are
environmentally friendly because they use much less power than standard
incandescent light bulbs to produce the same amount of light. A 13-watt CFL can
3
produce as much light as a standard 60-watt bulb. The CFL bulbs are more efficient,
which allows them to turn more of the input energy into light energy.
Now we can bring everything together. In the first section of this chapter, you learned
that mechanical power is measured in horsepower. So, 746 watts equals 1 horsepower,
and power equals torque times rotational velocity, and power also equals current
times voltage—what does all this mean? It means you can calculate some important
values. For example, if you have an electric motor rated in watts, you can figure out
the torque for a given velocity (or figure out the velocity at a given torque).
If your eyes are starting to cross and you’re thinking of throwing this book into the
closet, don’t worry. It’s not important to remember all these equations. However, it is