Electric current describes the flow of electric charge through a material, and it is one of the central ideas behind circuits, electronics, and power systems. In a metal wire, mobile electrons move through a fixed lattice of positive metal ions when an electric field is applied. Resistance tells how strongly a wire or device opposes that flow of charge.
Understanding current, resistance, and resistivity helps explain why wires heat up, why circuits need specific materials, and how electrical devices are designed.
Key Facts
- Current is charge flow per time: I = ΔQ/Δt
- Ohm's law for an ohmic conductor is V = IR
- Resistance depends on material and shape: R = ρL/A
- Resistivity ρ is a material property measured in ohm meters, Ω m
- Electron drift speed is related to current by I = nqAvd
- Conventional current points in the direction positive charge would move, opposite the drift direction of electrons in a metal
Vocabulary
- Electric current
- Electric current is the rate at which electric charge passes through a cross section of a conductor.
- Resistance
- Resistance is the opposition a component or material gives to the flow of electric current.
- Resistivity
- Resistivity is an intrinsic property of a material that describes how strongly it resists electric current.
- Drift velocity
- Drift velocity is the average slow velocity of charge carriers through a conductor due to an electric field.
- Ohmic conductor
- An ohmic conductor is a material or device that follows V = IR with constant resistance over a given range of conditions.
Common Mistakes to Avoid
- Confusing current direction with electron motion is wrong because conventional current points opposite the motion of electrons in a metal.
- Treating resistance and resistivity as the same quantity is wrong because resistance depends on length and area, while resistivity depends mainly on the material and temperature.
- Forgetting the wire's cross sectional area in R = ρL/A is wrong because a thicker wire gives charges more paths and therefore has lower resistance.
- Assuming electrons move through a circuit at nearly the speed of light is wrong because the electric signal spreads quickly, but the electron drift velocity is usually very slow.
Practice Questions
- 1 A wire carries a current of 2.5 A for 12 s. How much charge passes through a cross section of the wire?
- 2 A copper wire has resistivity 1.7 x 10^-8 Ω m, length 4.0 m, and cross sectional area 2.0 x 10^-6 m^2. What is its resistance?
- 3 Two wires are made of the same material and have the same length, but wire B has twice the diameter of wire A. Explain which wire has less resistance and why.