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Heat flux can be directly measured using a single heat flux sensor located on either surface or embedded within the material. Using this method, knowing the values of k and x of the material are not required. The multi-dimensional case is similar, the heat flux goes "down" and hence the temperature gradient has the negative sign:
The magnetic flux is the net number of field lines passing through that surface; that is, the number passing through in one direction minus the number passing through in the other direction (see below for deciding in which direction the field lines carry a positive sign and in which they carry a negative sign). [2]
Also, the surface has to be actually oriented, i.e. we use a convention as to flowing which way is counted positive; flowing backward is then counted negative. The surface normal is usually directed by the right-hand rule. Conversely, one can consider the flux the more fundamental quantity and call the vector field the flux density.
Illustration of the "reference directions" of the current (), voltage (), and power () variables used in the passive sign convention.If positive current is defined as flowing into the device terminal which is defined to be positive voltage, then positive power (big arrow) given by the equation = represents electric power flowing into the device, and negative power represents power flowing out.
A Left Hand Rule for Faraday's Law. The sign of ΔΦ B, the change in flux, is found based on the relationship between the magnetic field B, the area of the loop A, and the normal n to that area, as represented by the fingers of the left hand. If ΔΦ B is positive, the direction of the emf is the same as that of the curved fingers (yellow ...
B 0 is the flux density very close to each pole, in T, A is the area of each pole, in m 2, L is the length of each magnet, in m, R is the radius of each magnet, in m, and; x is the separation between the two magnets, in m = relates the flux density at the pole to the magnetization of the magnet.
Radiative forcing is defined in the IPCC Sixth Assessment Report as follows: "The change in the net, downward minus upward, radiative flux (expressed in W/m 2) due to a change in an external driver of climate change, such as a change in the concentration of carbon dioxide (CO 2), the concentration of volcanic aerosols or the output of the Sun." [3]: 2245
The force on a negatively charged particle is in the opposite direction. If both the speed and the charge are reversed then the direction of the force remains the same. For that reason a magnetic field measurement (by itself) cannot distinguish whether there is a positive charge moving to the right or a negative charge moving to the left.