IB Physics--B.5-Qc#4-Paper 2-Electrical resistors made by forming a thin film of carbon

Electrical resistors can be made by forming a thin film of carbon on a layer of an insulating material.
A carbon film resistor is made from a film of width 8.0 mm and of thickness 2.0 μm. The diagram shows the direction of charge flow through the resistor.
a. The resistance of the carbon film is 82 Ω. The resistivity of carbon is 4.1 x
10 Ω m. Calculate the length l of the film.
b. . The film must dissipate a power less than 1500 W from each square
metre of its surface to avoid damage. Calculate the maximum allowable current for the resistor.
c. The current direction is now changed so that charge flows vertically
through the film. Deduce, without calculation, the change in the resistance.
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