Electric current is the continuous rate of transport of electric charge through a conductor. Defining current quantitatively via $I = Q/t$ and distinguishing between conventional current and physical electron drift is fundamental in Cambridge IGCSE Physics.

Electric Charge ($Q$) is a fundamental property of matter, measured in Coulombs (C). The elementary charge of a single electron is $e = -1.6 \times 10^{-19}\text{ C}$.

Electric Current ($I$) is defined as the rate of flow of electric charge past a point in a circuit per unit time:

Where:

• $I$ is electric current in Amperes (A)
• $Q$ is electric charge in Coulombs (C)
• $t$ is time elapsed in seconds (s)

Rearranging gives the total charge equation:

One Ampere ($1\text{ A}$) is defined as one Coulomb of charge passing a point per second ($1\text{ A} = 1\text{ C/s}$).

1. Conventional Current: Historical convention established before the electron's discovery. Defined as the flow of positive charge from the positive (+) terminal of a supply to the negative (-) terminal.
2. Physical Electron Flow: In metallic conductors, current is carried by free delocalized valence electrons. Because electrons carry negative charge, they are repelled by the negative terminal and attracted toward the positive (+) terminal.

Advanced Physical Analysis

Potential dividers split voltage in proportion to component resistances ($V_{\text{out}} = V_{\text{in}} \times R_2/(R_1+R_2)$). Thermistors and light-dependent resistors (LDRs) serve as sensing components in control circuits.

Extended Experimental Procedures and Quantitative Analysis

Experimental investigations require meticulous measurement and systematic error reduction. Digital timers, micrometers, and vernier calipers reduce uncertainty. Data analysis involves plotting linear graphs, calculating gradients to represent rate of change or physical constants, and comparing experimental values against theoretical expectations to quantify percentage error ($|\text{Exp} - \text{Theo}| / \text{Theo} \times 100\%$).

Quantitative Problem-Solving & Error Mitigation

Solving complex physical problems requires systematic unit conversions, identifying given variables, isolating unknown terms, and verifying dimensional consistency. Experimental uncertainties are reduced through repeated measurements, averaging, zeroing digital instruments, and minimizing parallax error during scale readings.

Electric current $I = Q/t$ measures charge flow rate. Conventional current flows from positive to negative terminals, while negative electrons drift from negative to positive terminals.