RESISTANCE
Mechanical Systems
Friction force resists motion.
Friction results from irregularities on
the surface of objects.
static friction – friction that keeps an
object stationary. Molecules of
each object electrically bond
together.
kinetic friction – friction that’s caused by an object in motion.
It’s less than static friction because molecules can’t bond.
Thermal Systems
Thermal resistance is the opposition to the flow of heat.
Heat flows from high temperature area to low temperature area.
As the heat moves through, it losses energy, and the temperature drops.
Thermal resistance depends on:
1) thickness of the wall - thicker increases resistance
2) cross-sectional area - smaller area increases resistance
3) thermal conductivity - less conductive increases resistance
The thermal conductivity of a material is determined by its omposition.
Friction force depends on how hard the surfaces are
pressed together, called the normal force.
It always pushes perpendicular to the direction of
motion.
The units for thermal resistance:
coefficient of friction – how much a material resists another object sliding past it.
Lubricants reduce friction.
It keeps the sliding
surfaces apart with a layer
of fluid.
which can be simplified to:
If there are multiple layers,
the total resistance = the sum of the resistances of each layer.
Rolling friction causes a circular object to
roll, not slide, across another object.
There is less movement between
surfaces.
Friction is not all BAD!
Shoes, wheels, brakes, clutch plates,
conveyer belts, tires.
Fluid Systems
Drag force resists fluid motion.
Drag results when an object moves through a fluid, or when a fluid moves around an
object.
Drag increases when:
- speed of the object or fluid increases
- size and shape of the object are less streamlined
- density and viscosity of the fluid increase
Two types of fluid flow:
Poiseuille’s Law – Volume flow rate in a tube increases
- with more pressure applied,
- and to the fourth power of the radius.
Also, a fluid’s viscosity causes the pressure to drop along the direction of flow.
Laminar (streamlined) slow, smooth flow over a
surface in layers called
steamlines that do not
mix.
frictional drag - force
produced by friction
between layers of
streamlines.
Turbulent - irregular flow
with eddies and whorls,
caused by high speeds or
an unstreamlined shape.
pressure drag - force
produced by the drop in
pressure in the wake.
Increases by the square
of the velocity. (v2)
Factors Affecting Flow Through a Pipe
1) Radius: Fluid resistance decreases as pipe radius and cross-sectional area increase.
(If the pipe’s bigger, the fluid flows more.)
2) Length: Fluid resistance increases as pipe length increases.
(If the pipe’s longer, the fluid flows less.)
3) Viscosity: Fluid resistance increases as viscosity increases.
(If the fluid’s thicker, it flows less.)
Viscosity – a fluid’s resistance to movement.
Determined by the strength of the electrical forces between its atoms and molecules.
Units of viscosity: Pa∙s , (Pascal∙second)
Liquid viscosities decrease with heat – molecule bonds become weaker.
Gas viscosities increase with heat – molecules collide with each other more often.
Terminal speed – the constant speed that occurs when
the drag force of a falling object equals the gravitational force.
4) Design: Abrupt changes in the direction of the flow, or obstructions/restrictions,
cause turbulence and increase resistance.
Electrical Systems
All substances classified based on how easily electrons can move through them.
conductors - electrons move easily
insulators - electrons can’t move easily
semiconductors - electrons move easier than in insulators, but not as well as in
conductors.
Series Circuits
Total Resistance = the sum of the individual resistances:
The same current has to go through each resistance, one at a time.
The total voltage = the sum of the voltage drops across the resistors.
Electrical resistance is the opposition to charge flow in electric circuits.
The resistance causes a loss of voltage (voltage drop) as current flows through.
With fluids, we have:
With electricity, it’s similar:
Parallel Circuits
The reciprocal of the total resistance = the sum of the reciprocals of each separate resistance.
Since it’s the reciprocal, the total resistance will always be less than the smallest
resistance.
The same voltage is across every branch.
The total current = the sum of all the currents of the branches.
Ohm’s Law – if resistance is constant,
the current is proportional to the voltage.
So, rearranging the equation,
ΔV = R · I
To find the current in each branch:
Unit of resistance: ohm (Ω)
Resistivity depends on:
1) length: Longer wire increases resistance.
2) radius/cross-sectional area: Smaller wire increases resistance.
3) material: Higher resistivity (less conductivity) increases resistance.