Electricity Study Guide

When an electric current flows through a wire, it creates a magnetic field around that wire. This phenomenon is known as the magnetic effect of a current. We can demonstrate this effect using a simple setup. If we place a compass near a current-carrying wire, the compass needle will deflect, indicating the presence of a magnetic field. When the current is switched off, the compass needle returns to its original position, aligning with the Earth's magnetic field. Reversing the direction of the current would cause the compass needle to deflect in the opposite direction. The magnetic field lines around a straight current-carrying wire are concentric circles centred on the wire. The direction of these magnetic field lines can be determined using the right-hand grip rule. If you imagine gripping the wire with your right hand with your thumb pointing in the direction of the conventional current (from positive to negative), your fingers will curl in the direction of the magnetic field lines. The strength of the magnetic field is strongest closest to the wire and decreases with increasing distance from the wire. This principle forms the basis for many electromagnetic devices, including electromagnets and motors.

In electrical circuits, the flow of charge is fundamental. Electric current is defined as the rate of flow of electrical charge. Imagine a river; the current describes how much water flows past a point per second. Similarly, in an electrical circuit, the current describes how much charge flows past a point in a given amount of time. The greater the current, the more charge passes by per second. The longer the time that current flows, the greater the total amount of charge that has moved.

This relationship can be expressed by a simple formula: Quantity of Charge = Current × Time. In symbols, this is written as Q = I × t.

It is crucial to use the correct units for these quantities. Quantity of charge (Q) is measured in coulombs (C). Current (I) is measured in amperes (A). Time (t) is measured in seconds (s). If any of the values are given in different units, such as milliamperes or minutes, they must be converted to amperes and seconds first to ensure the calculation is correct.

For example, if a current of 2 amperes flows for 10 seconds, the total charge that has passed is Q = 2 A × 10 s = 20 C. This relationship is a cornerstone of understanding electricity and is frequently used to analyse and design electrical circuits. Knowing any two of these values allows you to calculate the third by rearranging the formula: I = Q/t or t = Q/I.