2.1 - Circuits Introduction

Current, potential difference and resistance

Three quantities describe what's happening in an electric circuit:

1. Current - The rate of flow of electric charge. In a metal, the charge is carried by electrons. Symbol $I$.
2. Potential difference (voltage) - The energy transferred per unit of charge, which pushes charge around the circuit. Symbol $V$.
3. Resistance - How much a component opposes the flow of charge. Symbol $R$.

Conditions for current to flow

Charge only flows as a current when both conditions are met:

1. The circuit is complete (closed) with no gaps.
2. There is a source of potential difference, such as a cell or battery.

In a single closed loop, current has the same value everywhere.

How the three are linked

Current through a component depends on both the potential difference across it and its resistance:

• Increasing the potential difference increases the current (if resistance is fixed).
• Increasing the resistance decreases the current (if potential difference is fixed).

Circuit diagrams and standard symbols

Circuits are drawn using standard symbols connected by straight lines:

• Cell and battery (two or more cells joined together)
• Switch (open and closed)
• Filament lamp
• Fixed resistor and variable resistor
• Fuse
• Diode and LED (light-emitting diode)
• Thermistor and LDR (light-dependent resistor)
• Ammeter and voltmeter
• Power supply
• Motor

In a cell symbol, the long line is the positive terminal and the short line is the negative terminal.