A metal plate is located in an xy-plane such that the temperature T at (x,y) is inversely proportional to the distance from the origin, and the temperature at point P(3,4) is 100 (i.e. the temperature at any point (x,y) is described by the function
T(x,y) = 500/(x^2 + y^2)^1/2
a) in what direction does the T increase most rapidly at P? Write the vector representing that direction explicitly.
b) Find the rate of change of T at P in the direction i + j.
c) In what direction does T decrease most rapidly at P?
The gradient of T at (3,4) will give the maximum rate of change of T at (3,4)
grad(T(x,y)) = (del(T)/del(x)).i + (del(T)/del(y)).j
where, del(T)/del(x) is the partial derivative of T with respect to x.
T = 500/(x^2 + y^2)^1/2
del(T)/del(x) = 500x/(x^2 + ...
This shows how to determine direction of increase (and the vector), rate of change, and direction of decrease.