Givenφ=x −y,sketchononegraphthecurvesφ=4,φ=1,φ=0, φ = −1, φ = −4. If φ is the electrostatic potential, the curves φ = const. are equipotentials, and the electric field is given by E = −∇φ. If φ is temperature, the curves φ =const. are isothermals and ∇φ is the temperature gradient; heat flows in the direction −∇φ.
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