Chapter 1.10 Resistance and resistivity - Summary Edexcel A Level Physics Student Book 1 - Physics (9PH0)

1.10 - Resistance and resistivity
V2
● ∵ V =IR , P=IV , ∴ P=I 2 R , P=
R
● Power is dissipated in resistors (particularly if that resistor is in the form of a filament
of a lamp)
● The electrical energy transferred in the resistor increases the potential energy +
random KE of atoms of material of resistor (internal energy of atoms)


Resistivity
● ***Definition: specific property of a material which measures its electrical
resistance/how strongly a material resists electric current
ρ(resistivity [Ωm ])× l (length of electrical component [m])
● *** R(resistance [Ω])= 2 ,
A (cross sectional areaof electrical component [ m ])
RA
ρ=
l
1
○ R∝
A
● ∵ (for filament lamps/diodes/thermistors) R does NOT remain constant, ∴
resistivity is affected by temperature changes
○ Ohmic metallic conductor - n does NOT depend on temperature (so it is
constant)
○ Filament lamps (*PTC) - temperature ↑, R ↑, ∴ ρ ↑
○ Diodes - when V ≳ 0.7 V, R ↓↓, ∴ ρ ↓↓
○ Thermistors (NTC) - temperature ↑, R ↓, ∴ ρ ↓
● Resistivity (of a metal) increases with its temperature; as I = nqvA, if drift velocity
decreases then current decreases so resistance increases {vice versa}




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