Rearranging I = Anev gives
Charge & Current - OCR A-Level Physics
- Rearranging I = Anev gives: $v = I / (Ane)$.
- For a given currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A)., mean 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 chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow, typically very slow (~1 mm/s).The average velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. of charge carriers through a conductor in the direction of currentThe rate of flow of charge. Measured in amperes (A). flow. depends on A and n.
- Thinner wire (smaller A) means faster drift velocityThe average velocity of charge carriers through a conductor in the direction of current flow, typically very slow (~1 mm/s). for the same current.
- Material with lower n means faster drift velocityThe average velocity of charge carriers through a conductor in the direction of current flow, typically very slow (~1 mm/s). for the same current.
- Two wires in series carry the same current (Kirchhoff's first lawThe sum of currents entering a junction equals the sum of currents leaving. A consequence of conservation of charge.).
Examiner Tips and Tricks
- A classic OCR exam question: two wires of different cross-sectional areas are connected in series.
- Describe and explain how drift velocity changes as electrons move from one wire to the other.
- State which variables are constant (I, n, e) and which change (A).
- If A halves, v doubles (they are inversely proportional at constant I, n, e).
- If a wire narrows, electrons must speed up to maintain the same current.
- Think of water flowing through a pipe that narrows - the flow speed increases.