3. Resistance variation

      At the end of this chapter you should be able to:
• appreciate that electrical resistance depends on four factors
• appreciate that resistance R=ρl/a, where ρ is the resistivity
• recognize typical values of resistivity and its unit
• perform calculations using R=ρl/a
• define the temperature coefficient of resistance,α
• recognize typical values for α
• perform calculations using Rθ=R0(1+αθ)
• determine the resistance and tolerance of a  fixed resistor from its colour code
• determine the resistance and tolerance of a fixed resistor from its letter and digit code