Current Electricity Basics

Current Electricity Overview

• Involves the flow of electrons or charges through a conductor.
• Electric current ( I ) measures the rate of electric charge flow in a circuit, represented by the formula: ( I = \frac{Q}{t} ) where ( Q ) is electric charge and ( t ) is time.
• Unit for current is the ampere (A), equivalent to ( \text{C s}^{-1} ).

Potential Difference and Electromotive Force

• Terminal potential difference (voltage) ( V ) represents the work done in moving a unit charge across two points in a circuit.
• Electromotive force (emf) ( E ) is the total work done per unit charge in a complete circuit: ( E = \frac{W}{Q} ) with ( W ) as work done.
• Voltage in a circuit not delivering current, or open circuit, is also described by emf.
• The unit of potential difference is the volt (V).

Factors Affecting Emf of a Cell

• Depends on the quality of chemicals or materials in the cell; stronger electrolytes result in lower internal resistance and greater emf.
• Influenced by the number of cells, time, and temperature; emf decreases with time and increasing temperature, such as in primary cells.

Internal Resistance

• Internal resistance ( r ) is the opposition to current flow caused by the cell producing the current.
• Determinants include:
  • Nature of chemicals or electrodes.
  • Amount and size of chemicals or electrodes.
  • Area of electrodes; internal resistance is inversely proportional to area (( r \propto \frac{1}{A} )).
  • Distance between electrodes; internal resistance is proportional to distance (( r \propto d )).
• Higher internal resistance limits the maximum current supplied by the cell, as described by the equation: ( I_{\text{max}} = \frac{E}{r} ).

Lost Volts and External Resistance

• Lost volt ( v ) refers to the potential difference across the internal resistance of a cell.
• External resistance ( R ) is the opposition encountered by current in a material, affecting the overall circuit performance.