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Tuesday, July 05, 2016

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Resistance, Capillarity and Viscosity

Oyetoke Tobi - Tuesday, July 05, 2016

What is Resistance in Physics

Resistance can be defined as the opposition to the flow of charge or current. Resistances are compound designed to give resistance to the current.

Resistors are made of wire, carbon or graphite wound round an insulator with wire ends at terminal for connections.

Types of Resistors 

Wire Resistors: it is made of wire, the change in resistance with temperature.
Fixed Resistors: This contains coil of wire of sufficient length to provide required resistance.
Variable Resistors (Rheostat):it is a wire round with sliding contact used in attaining the length of wire used for resistance.
Resistance Box: This raises plugs which retained indicate the resistance value in the circuit.

Factors that affect the resistance of a wire

Length Of Wire (L): As the length of wire increase the resistance increases I.e length.
Conductivity is a measure of the extent to which a material will allow current to flow easily through it. It is reciprocal of the Resistivity.

What is Capillarity

If three tubes with fine Biro with different diameter are dip into clean water. It is observed that water rises in the tubes but the narrower the Biro, the higher the height to which the water rises. This action is known as CAPILLARITY. It is defined as the tendency of a liquid to rise or fall in a narrow tube.

Two terms will be required to explain capillary, they are cohesion and adhesion.

Cohesion is the force of attraction between molecules of the same kind.

Adhesion is the force of attraction between molecules of different kind.

These forces explain the different action of water and mercury when spilled on a clean glass surface.

Adhesion of water to glass molecules is stronger than the cohesive force between water molecules; hence water spreads out on a clean glass surface and wets the glass.

Cohesive of mercury molecules is greater than the adhesive of mercury to glass molecules. Hence mercury does not wet glass.
Cohesion and adhesion as well as surface tension forces ate responsible for the capillarity of liquids.

Viscosity - Friction in Fluids 

This is the international friction between the layers of a liquid example gas in motion. Liquids which pour slowly are said to be more Viscous than those which pour faster. 

When a stone is thrown down a cylinder of a viscous liquid the downward motion of the body is opposed by the viscosity of the liquid. The more the viscosity of the liquid, the more the opposition to the movement of the stones and hence the slower its motion.

Terminal velocity (Speed)

When a stone falls through a viscous liquid it is subjected to three forces its weight (w) acting downwards upthrust (u) acting upwards Viscous force  (v) opposing its motion that is upwards.

Application Of Viscosity 

Oils, greases and air are used as lubricants because of their viscosity. The viscosity of oils and greases decrease with increase in temperature, hence lubricating effects are lowered at high temperature.

Comparison Of Viscosity And Friction 

* They both oppose motion 
* They can both be reduced by the use of grease and oil.

Differences between Viscosity and Friction 

Viscosity is the internal friction which exists between layers of a liquid or gas in motion while Friction is a force which acts at the surface of separation of two bodies in contact.

Submitted by Floxy


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