- #1
Brajesh kedia
Flux is the no. Of electric field lines per unit area..how can it have a unit
Drakkith said:Area always has a unit. Cm2, M2, Ft2, etc.
Brajesh kedia said:Frankly speaking i have not got ur answers
No, the flux is not defined in terms of number of lines. There is no way to count these lines, they don't have a physical existence. Not more than the longitude lines, for example. The reference to the number of lines (if it is used at all) is just to offer some sort of intuitive understanding of the flux. In this case it should be said that the flux is proportional to the number of field lines.Brajesh kedia said:Flux is the no. Of electric field lines per unit area..how can it have a unit
Are you saying that engineers don't regard the stress tensor as having units of force per unit area?stedwards said:Sure. Tell this to the mechanical engineers, students, professors, and text writers alike, attempting to explain the stress tensor and the pressure vector. Area, for these indocrinated, is unitless.
nasu said:No, the flux is not defined in terms of number of lines. There is no way to count these lines, they don't have a physical existence. Not more than the longitude lines, for example. The reference to the number of lines (if it is used at all) is just to offer some sort of intuitive understanding of the flux. In this case it should be said that the flux is proportional to the number of field lines.
The real definition is for example here (for magnetic field):
http://en.wikipedia.org/wiki/Magnetic_flux
Chestermiller said:Are you saying that engineers don't regard the stress tensor as having units of force per unit area? Chet
As an engineer with over 50 years of experience in solid mechanics and fluid mechanics, I can assure you that your recollection about this is not correct. The Cauchy stress relationship specifies dotting the stress tensor with a unit normal vector to an area to spit-out the traction vector (or stress vector) acting on the area. The pressure tensor is the isotropic part of the stress tensor. If the stress tensor happens to be isotropic (as in the case of a fluid at hydrostatic equilibrium, for example), the dot product of the stress tensor with the unit normal gives you the hydrostatic pressure p times the unit normal. So, since the stress tensor is being dotted with a unit normal, the units of the pressure vector are still force per unit area. I think what you are referring to is that if you dot the stress tensor with a unit normal times a differential element of area dA, you obtain the differential force acting on that area. In this case, the units of the differential force are those of force (since the area units cancel out).stedwards said:Oh, no. It's worse than that. The pressure tensor [itex]\sigma[/itex], or pressure matrix is a linear map. It takes a vector, normal to an area [itex]a[/itex] and spit-out a pressure [itex]P[/itex].
[itex]P= \sigma \cdot a[/itex]
Pressure has units of pressure. That's easy enough. Area has units area. Properly speaking, the stress tensor has units of Force per Length^4.
Maybe you don't like the idea of the area normal having units of area, and want it to have units of length. Now the stress tensor has different units, but these units are not pressure.
From my text : The units of the Poynting Vector, S = 1/μ0 E x B are in watts/meter2Brajesh kedia said:Flux is the no. Of electric field lines per unit area..how can it have a unit
The correct equation is not ##\vec{P}=\vec{σ}\centerdot \vec{a}##. The correct equation is ##\vec{F}=\vec{σ}\centerdot \vec{a}##, where vector ##\vec{F}## is the force acting on the area, not the pressure. Now, if you continue issuing this string of misinformation, you will be receiving infraction points, which can lead to a ban from Physics Forums.stedwards said:As an engineer with a wee bit less than 50 years of experience, I invite you balance the units as well as the variables.
For the equation [itex]P[F/A] = A[D^2] \sigma[F/A][/itex], the units do not balance. Units are in square brackets.
F =Force
P=Pressure
A=Distance^2
Everything you learned in school is wrong.
Chestermiller said:The correct equation is not ##\vec{P}=\vec{σ}\centerdot \vec{a}##. The correct equation is ##\vec{F}=\vec{σ}\centerdot \vec{a}##, where vector ##\vec{F}## is the force acting on the area, not the pressure. Now, if you continue issuing this string of misinformation, you will be receiving infraction points, which can lead to a ban from Physics Forums.
Chet
Yes, flux can have a unit. It is defined as the flow of a physical quantity through a given surface, and the unit of flux is determined by the unit of the physical quantity.
The common units of flux depend on the type of flux being measured. For example, electric flux is measured in volts per meter squared (V/m2), while magnetic flux is measured in webers (Wb).
Flux can be either a vector or a scalar quantity, depending on the type of flux being measured. For example, electric flux is a vector quantity, while magnetic flux is a scalar quantity.
Flux and flow rate are closely related concepts, as both involve the flow of a physical quantity through a given surface. Flux is the amount of the quantity that flows through the surface per unit time, while flow rate is the volume or mass of the quantity that flows through the surface per unit time.
Yes, flux can have negative values. This typically occurs when the direction of the flow is reversed, resulting in a negative value for the flux. For example, if the electric flux is entering a closed surface, it will have a negative value.