Mean drift velocity
Charge & Current - OCR A-Level Physics
- With no p.d. applied, free electronsElectrons not bound to any particular atom, free to move through a conductor. Also called delocalised or conduction electrons. zig-zag randomly at high speeds.
- Random motion cancels out: the average flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). is zero.
- When a p.d. is applied, electrons still zig-zag but are also pushed along the wire.
- This creates a slow net flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). along the conductor.
Key Definition
Mean drift velocity
The average speed of charge carriers along a conductor when a potential difference is applied. Measured in m s^-1.
The average speed of charge carriers along a conductor when a potential difference is applied. Measured in m s^-1.
$$I = Anev$$
Examiner Tips and Tricks
- $I = Anev$ is a key OCR equation.
- You must know how to rearrange it for any variable.
- A common exam question asks you to calculate v or n.
- Always check your units: A must be in \(m^{2}\), not mm^2.
- Drift velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.The average velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). carriers through a conductor in the direction of currentThe rate of flow of charge. Measured in amperes (A). flow, typically very slow (~1 mm/s). in copper wire is typically ~10^-4 m s^-1 (very slow).
- The signal (electric field) travels at close to the speed of light.
- This is why a lamp turns on instantly even though electrons crawl.
Common Mistake
MEDIUM
Wrong: Using cross-sectional area in mm^2 or cm^2 in $I = Anev$.
Right: Always convert area to \(m^{2}\). For a wire of diameter d: $A = \pi(d/2)^2. If d = 0.22 mm$, then $d = 0.22 x 10^-3 m$.
Right: Always convert area to \(m^{2}\). For a wire of diameter d: $A = \pi(d/2)^2. If d = 0.22 mm$, then $d = 0.22 x 10^-3 m$.
Two diagrams side by side: (1) electrons in a wire with no p.d. - random zig-zag arrows in all directions, net movement zero; (2) electrons with p.d. applied - zig-zag arrows but with a net drift direction from negative to positive terminal.