Contact area
Encyclopedia
When two objects
touch, a certain portion of their surface areas will be in contact with each other. Contact area is the fraction of this area that consists of the atom
s of one object in contact with the atoms of the other object. Because objects are never perfectly flat due to asperities, the actual contact area (on a microscopic
scale) is usually much less than the contact area apparent on a macroscopic
scale. Contact area may depend on the normal force
between the two objects due to deformation.
The contact area depends on the geometry of the contacting bodies, the load, and the material properties. The contact area between two parallel cylinders is a narrow rectangle. Two, non-parallel cylinders have an elliptical contact area, unless the cylinders are crossed at 90 degrees, in which case they have a circular contact area. Two spheres also have a circular contact area.
fact for many materials that F = μN, where F is the frictional force
for sliding friction, μ is the coefficient of friction, and N is the normal force
. There isn't a simple derivation for sliding friction's independence from area.
of a wire is dependent on the cross-sectional area, so one may find the contact area of a metal by measuring the current
that flows through that area (through the surface of an electrode
to another electrode, for example.)
Physical body
In physics, a physical body or physical object is a collection of masses, taken to be one...
touch, a certain portion of their surface areas will be in contact with each other. Contact area is the fraction of this area that consists of the atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s of one object in contact with the atoms of the other object. Because objects are never perfectly flat due to asperities, the actual contact area (on a microscopic
Microscopic
The microscopic scale is the scale of size or length used to describe objects smaller than those that can easily be seen by the naked eye and which require a lens or microscope to see them clearly.-History:...
scale) is usually much less than the contact area apparent on a macroscopic
Macroscopic
The macroscopic scale is the length scale on which objects or processes are of a size which is measurable and observable by the naked eye.When applied to phenomena and abstract objects, the macroscopic scale describes existence in the world as we perceive it, often in contrast to experiences or...
scale. Contact area may depend on the normal force
Normal force
In mechanics, the normal force F_n\ is the component, perpendicular to the surface of contact, of the contact force exerted on an object by, for example, the surface of a floor or wall, preventing the object from penetrating the surface.The normal force is one of the components of the ground...
between the two objects due to deformation.
The contact area depends on the geometry of the contacting bodies, the load, and the material properties. The contact area between two parallel cylinders is a narrow rectangle. Two, non-parallel cylinders have an elliptical contact area, unless the cylinders are crossed at 90 degrees, in which case they have a circular contact area. Two spheres also have a circular contact area.
Friction and contact area
It is an empiricalEmpirical
The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....
fact for many materials that F = μN, where F is the frictional force
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
for sliding friction, μ is the coefficient of friction, and N is the normal force
Normal force
In mechanics, the normal force F_n\ is the component, perpendicular to the surface of contact, of the contact force exerted on an object by, for example, the surface of a floor or wall, preventing the object from penetrating the surface.The normal force is one of the components of the ground...
. There isn't a simple derivation for sliding friction's independence from area.
Methods for determining contact area
One way of determining the actual contact area is to determine it indirectly through a physical process that depends on contact area. For example, the resistanceElectrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
of a wire is dependent on the cross-sectional area, so one may find the contact area of a metal by measuring the current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
that flows through that area (through the surface of an electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
to another electrode, for example.)