1) **Heat transfer** has direction as well as magnitude, and thus it is a **vector quantity**

a) Must specify **both direction and magnitude** in order to describe heat transfer completely at a point.

b) Question: In the figure below tell us whether heat conduction is toward the inside (indicating heat gain) or toward the outside (indicating heat loss) or vice versa?

*Figure** Indicating direction for heat transfer (positive in the positive direction; negative in the negative direction).*

c) To avoid such questions, a coordinate system and indicate direction with plus or minus signs can be developed

2) Different Kinds of Coordinate systems

3) The notation T(x, y, z, t) implies that the temperature varies with the space variables x, y, and z as well as time.

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