Electricity Study Guide

The rotation in electric motors is a direct consequence of the motor effect, which states that a force is exerted on a current-carrying conductor when it is placed in a magnetic field. For a motor to operate continuously, this force must cause a rotational movement, and its direction must periodically reverse to maintain rotation in a single direction. A basic DC electric motor consists of a coil of wire (armature) placed within a magnetic field, typically provided by permanent magnets or electromagnets. When a current flows through the coil, each side of the coil experiences a force due to the magnetic field. According to Fleming's Left-Hand Rule, the direction of this force is perpendicular to both the current direction and the magnetic field direction.

Consider a rectangular coil: on one side, the current flows in one direction, and on the opposite side, it flows in the reverse direction. If these sides are within the magnetic field, they will experience forces in opposite directions, creating a turning effect or torque. For example, if current flows into the page on one side and out of the page on the other, one side might experience an upward force and the other a downward force. This pair of forces causes the coil to rotate. To ensure continuous rotation in the same direction, the direction of the current in the coil must be reversed every half turn. This is achieved by a component called a commutator, which is a split-ring device that reverses the current direction in the coil every 180 degrees of rotation. Carbon brushes make contact with the commutator, allowing current to flow from the power supply to the rotating coil. Without the commutator, the coil would simply oscillate back and forth or stop after a half rotation. The strength of the motor effect force, and thus the speed and torque of the motor, can be increased by increasing the current, using a stronger magnetic field, or increasing the number of turns on the coil and its area.

In a direct current (DC) circuit, electrical charge flows in only one direction. This is typically created by sources like batteries, which have distinct positive (+) and negative (-) terminals. The positive terminal is where current conventionally leaves the source, and the negative terminal is where it conventionally returns. Electrons, which are negatively charged, actually flow from the negative terminal to the positive terminal. However, by convention, current direction is defined as the direction of positive charge flow.

Circuit diagrams are used to represent these electrical circuits visually. They use standardised symbols to show the different components present and how they are connected. Understanding these symbols is crucial for designing, analysing, and fault-finding circuits. For example, a cell is represented by two parallel lines of different lengths, with the longer line indicating the positive terminal and the shorter line the negative terminal. A battery is several cells joined together. Wires are shown as straight lines, and connections between wires are marked with a dot if they are meant to be joined, or crossing without a dot if they are not connected. Other common components include resistors (rectangles or zig-zag lines), lamps (circles with a cross inside), ammeters (circles with an 'A' inside) for measuring current, and voltmeters (circles with a 'V' inside) for measuring potential difference. Learning these symbols and conventions allows you to accurately interpret and draw circuit diagrams for a wide range of electrical systems.