Resistance is the opposition to motion or flow. In some situations, resistance
must be overcome—for example to create or maintain motion of objects, or
flow of fluids, charge, or heat. In other situations, resistance is useful—for
example, to slow or stop motion of objects and to control flow of fluids,
charge, and heat.
Mechanical resistance opposes motion
between solid surfaces. This opposition
is called friction. A force of friction
exists whenever one solid object slides
against another solid object. The
direction of the force is opposite the
direction of the object’s velocity. For
example, when you move a chair by
sliding it across the floor, you must
exert a force greater than the opposing
force of friction. When you apply the
brakes on a car, friction between the
brake pad and a rotating wheel opposes
the wheel’s motion and slows its rate of
rotation.
Fluid resistance opposes the flow of fluids. The resistance is caused by
friction between molecules in the fluid. This opposition is called drag when a
solid object moves through a fluid (or when a fluid flows past a solid
object). To paddle a kayak through water, you must exert a force greater than
the opposing force of drag. A sky diver has a terminal speed because the
drag force balances the gravitational force. A fluid flowing through a pipe
has a pressure drop along the pipe because of drag.
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RESISTANCE
Electrical resistance opposes the flow of electric charge, or current. This
resistance is caused by electrons colliding with atoms in the conducting path.
Some materials have low resistance, and some have high resistance. For
example, a toaster has two wires—a power cord and a heating element. The
power cord is made of copper. Copper has a low resistance—it is a good
conductor of electricity because it has many electrons that can easily move
in the wire. But the toaster’s heating
element has a high resistance. In this
material, electrons are more tightly bound
to the atoms and do not move as easily
from place to place. When electrons
collide with atoms in the heating element,
they transfer energy to the atoms. This is
the source of heat.
Thermal resistance opposes the flow of heat. Some materials have low
thermal resistance and some have high resistance. Metals have low thermal
resistance—they are good conductors of heat. The metal handle on a pot of
boiling water is hot because the pot conducts heat to the handle. You can
safely handle the pot if you use a potholder between your hand and the
metal. A potholder has a high thermal resistance—it is not a good conductor
of heat.
C HAPTER 4
RESISTANCE
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