A copper wire has cross-sectional area A = 0
Charge & Current - OCR A-Level Physics
- A copper wire has cross-sectional area $A = 0.5 \times 10^{-6}$ m$^2$.
- Number densityMass per unit volume of a material. Measured in kg m⁻³.The number of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). carriers per unit volume in a material. Measured in m⁻³. for copper: $n = 8.50 \times 10^{28}$ m$^{-3}$.
- 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). through the wire: $I = 2.90$ A.
- Find the 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 charge. 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., v.
$$\begin{aligned}
v &= \frac{I}{Ane} \\
&= \frac{2.90}{0.5 \times 10^{-6} \times 8.50 \times 10^{28} \times 1.6 \times 10^{-19}}
\end{aligned}$$
$$v = 4.3 \times 10^{-4} \text{ m s}^{-1}$$
Examiner Tips and Tricks
- Use standard form throughout.
- Writing e as 0.00000000000000000016 C is asking for decimal point errors.
- Always use $1.6 \times 10^{-19}$ C.