Cambridge IGCSE Physics 0625 – 4.2.2 Electric Current
4.2.2 Electric Current
Objective
Know that electric current is related to the flow of charge.
Key Concepts
Electric current is the rate at which electric charge passes a point in a circuit.
It is a scalar quantity; only magnitude is considered, not direction.
Conventional current direction is defined as the direction positive charge would move.
In metallic conductors the actual charge carriers are electrons, which move opposite to the conventional current direction.
Definition and Formula
The electric current \$I\$ is defined as the amount of charge \$\Delta Q\$ that passes a given point in a time interval \$\Delta t\$:
\$ I = \frac{\Delta Q}{\Delta t} \$
where
\$I\$ is the current (ampere, A)
\$\Delta Q\$ is the charge transferred (coulomb, C)
\$\Delta t\$ is the time taken (second, s)
Units and Symbols
Quantity
Symbol
Unit
Unit Symbol
Electric current
\$I\$
ampere
A
Electric charge
\$Q\$
coulomb
C
Time
\$t\$
second
s
Relation to Charge Carriers
In a conductor, the current can also be expressed in terms of the number of charge carriers, their charge, and their drift speed:
\$ I = n A v_d q \$
\$n\$ – number of charge carriers per unit volume (m\$^{-3}\$)
\$A\$ – cross‑sectional area of the conductor (m\$^{2}\$)
\$v_d\$ – drift velocity of the carriers (m s\$^{-1}\$)
\$q\$ – charge of each carrier (C). For electrons \$q = -e = -1.6 \times 10^{-19}\,\text{C}\$.
Conventional vs. Electron Flow
Conventional current: Assumes positive charges move from the positive terminal to the negative terminal of a source.
Electron flow: In metals, electrons (negative charge) move from the negative terminal to the positive terminal, opposite to conventional current.
Practical Implications
Current is measured with an ammeter placed in series with the component.
High currents can cause heating (\$P = I^{2}R\$) and may damage components.
Safety: The human body can be harmed by currents as low as 0.01 A (10 mA) passing through the heart.
Suggested Diagram
Suggested diagram: A simple circuit showing a battery, a resistor, an ammeter in series, and the direction of conventional current versus electron flow.
Summary Checklist
Current \$I\$ is the rate of charge flow: \$I = \Delta Q / \Delta t\$.
Unit of current is the ampere (A).
Conventional current direction is opposite to the actual electron movement in metals.
Current can be related to microscopic properties: \$I = n A v_d q\$.
Measuring current requires placing the measuring device in series with the circuit.