Physical Principals (Cross/ Plunkett) Flashcards
Laminar flow
Describes the situation in which any fluid (liquid or gas) passes smoothly and steadily along a given path, described by the Hagen Pousielle formula.
Flow = Pi. pressure. radius^4 / 8. viscosity. length of tube
Fluid flowing near the edge of the tube approaches zero, fluid centre of tube is twice the average speed of the fluid column as a whole.
Turbulent Flow
Describes the situation where fluid flows unpredictably along a path, with multiple eddy currents and not parallel to the sides of the tube. Due to its unpredictable flow, there is no equation to describe this. There is a number that can be calculated, Reynolds number, that identifies whether flow is likely to be laminar or turbulent.
Re = density. velocity. diameter / viscosity
Bernoulli Principle
Increase in the flow velocity of an ideal fluid will be accompanied by a simultaneous reduction in its pressure.
The Venturi Effect
Its is an extension of the Bernoulli principle - the effect by which the introduction of a CONSTRICTION to fluid flow within a tube causes the velocity of the fluid to increase, therefore, the pressure of the fluid to fall.
The Coanda Effect
Describes the tendency of a stream of fluid flowing in proximity to a convex surface to follow the line of the surface rather than its original course.
The moving column of fluid entrains molecules lying close to the curved surface, creating a relatively low pressure, contact point. The pressure away from the curved surface is relatively higher, preferentially pushing the fluid towards the surface.
Heat
It is the form of energy that is passed between two samples owing to a difference in their temperatures.
What is temperature?
It is the measure of the mean kinetic energy of the molecules in a substance and a property of a substance which will determine which direction energy will flow when an object is in contact with another of a different temperature.
Triple point
The temperature at which all three phases of water - solid, liquid and gas - are in equilibrium at 611.73 kPa. It occurs at 0.01 degrees C.
what is Kelvin?
One kelvin is equal to 1/273.16 of the thermodynamic triple point of water. A change in temperature of 1K is equal in magnitude to that of 1 degrees C.
Celsius
common measure of temperature in which a change of 1 degrees C is equal to magnitude to a change of 1K.
Principle of a resistance wire
The underlying principle of this method of measuring temperature is that the resistance of a thin piece of metal increases as the temperature increases. Fragile and slow response time.
Principle of a thermistor
Resistance of certain semiconductor materials falls as temperature increases. Fast response but suffer from calibration error and deteriorate over time.
The Seebeck Effect
At the junction of two dissimilar metals, a voltage will be produced, the magnitude of which will be in proportion to the temperature difference between two such junctions. This effect is used by a thermocouple - uses copper and constantan.
Absolute humidity
The total MASS of water vapour in the air per unit volume (kg.m-3)
Relative humidity
The RATIO of the amount fo water vapour in the air compared with the saturated vapour pressure (amount of water present if fully saturated) at that temperature.
