Ohm's Law, Resistivity and Drift Velocity
V = IR for an ohmic conductor, with R = ρ L/A, arising microscopically from drift velocity vd = I/(nAe).
What is Ohm's Law, Resistivity and Drift Velocity?
V = IR for an ohmic conductor, with R = ρ L/A, arising microscopically from drift velocity vd = I/(nAe).
Key formula / rule: R = ρ L/A; ρ depends on material and temperature.
Key points
- Compute resistance from resistivity and geometry.
- Explain conduction using the drift velocity model.
Common exam trap
Assuming electrons travel from one end to the other at the speed of the signal.
Definitions
- Term
Ohm's Law, Resistivity and Drift Velocity
- Meaning
V = IR for an ohmic conductor, with R = ρ L/A, arising microscopically from drift velocity vd = I/(nAe).
- Term
Ohm's Law, Resistivity and Drift Velocity — explanation
- Meaning
Resistivity is a material property while resistance also depends on geometry. Drift velocity is surprisingly small, yet the current starts almost immediately because the field propagates at near light speed.
Learning objectives
Compute resistance from resistivity and geometry.
Explain conduction using the drift velocity model.
Formulae
- Key point
R = ρ L/A; ρ depends on material and temperature.
- Key point
I = nAe vd; drift velocity is of the order of mm/s.
- Key point
Metals have positive temperature coefficients, semiconductors negative.
Prerequisites
PHY-U5-ELS-T2-S1-C1
Common mistakes
Assuming electrons travel from one end to the other at the speed of the signal.
Treating resistivity as geometry dependent.
Keywords
Resistivity
Drift
Velocity
Practice preview
Which of the following statements correctly defines Ohm's Law?…
easy
How does the drift velocity of electrons in a metallic conductor change if the current flowing through it is doubled, assuming the cross-sectional area and number density of free electrons remain constant?…
medium
A current of 1.6 A flows through a conductor of cross-sectional area 1 x 10^-6 m^2. If the number density of free electrons in the conductor is 1 x 10^29 m^-3, what is the drift velocity of the electrons? (Charge of elec…
medium