17.3 - 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).

Conductors, insulators and current direction

• Conductors let charge pass through easily (e.g. metals like copper and aluminium, and graphite).
• Insulators do not let charge pass through easily (e.g. plastic, rubber, wood, glass).

Free electrons

• In a metal, the outermost electrons are free to move through the structure (free electrons or delocalised electrons).
• An electric current in a metal is the flow of these free electrons.
• Insulators have no (or almost no) free electrons, so charge cannot flow through them.

The direction of current

• Conventional current flows from the positive terminal to the negative terminal.
• Free electrons are negatively charged, so they flow the opposite way, from negative to positive.

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.

Current is a flow of charge

• An electric current is the rate of flow of charge.
• In metals, current is carried by free electrons moving through the material.

Conditions for current to flow

Charge will only flow if both conditions are met:

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

Electromotive force and potential difference

• The electromotive force (e.m.f.) of a source is the electrical work done by the source in moving a unit charge around a complete circuit (the energy the source gives to each unit charge).
• The potential difference (p.d.) across a component is the work done by a unit charge as it passes through the component.
• Both e.m.f. and potential difference are measured in volts (V).

Formula for potential difference and e.m.f.

Where:

• $V$ (or $E$) = Potential difference (or e.m.f.), in volts (V)
• $W$ = Work done, in joules (J)
• $Q$ = Charge, in coulombs (C)

Worked example - E.m.f. of a battery

A battery supplies 4,800 J of energy to 400 C of charge as the charge passes through it. Calculate the e.m.f. of the battery.