Electricity Study Guide

Resistance is a measure of how much an electrical component opposes the flow of electric current. It converts electrical energy into other forms of energy, such as heat. The unit of resistance is the Ohm (Ω). According to Ohm's Law, for a given voltage, a higher resistance means a lower current, and a lower resistance means a higher current. However, not all components behave in the same way. Some components, known as ohmic resistors, have a constant resistance provided the temperature remains constant. This means that if you double the voltage across them, the current through them will also double, maintaining a constant ratio of voltage to current, which is the resistance. Examples of ohmic resistors include fixed resistors and many metallic wires at a constant temperature. For other components, such as filament lamps and thermistors, the resistance changes as the current and thus the temperature changes. In a filament lamp, as the current increases, the filament heats up, causing its resistance to increase. This means that doubling the voltage across a filament lamp will not double the current, as its resistance will have increased. This non-ohmic behaviour is crucial in understanding how different electrical circuits operate and how energy is dissipated within them. Diodes are another example of non-ohmic components where the resistance is very high in one direction and low in the other, and it also changes significantly with voltage and current.

Static electricity is the imbalance of electric charges within or on the surface of a material. Unlike current electricity, which involves the flow of electrons, static electricity refers to a stationary electric charge. One common way to produce static electricity is through triboelectric charging, or rubbing certain materials together. When two insulating materials are rubbed against each other, electrons can be transferred from one material to the other. The material that gains electrons becomes negatively charged, and the material that loses electrons becomes positively charged. The amount and nature of the charge depend on the materials involved.

Once an object acquires a static charge, it can exert forces on other charged or uncharged objects. There are fundamental rules governing these forces: like charges repel each other (e.g., two positively charged objects will push each other away), and opposite charges attract each other (e.g., a positively charged object and a negatively charged object will pull towards each other). A charged object can also attract an uncharged object through a process called electrostatic induction, temporarily rearranging charges within the uncharged object.

A 'spark' is a visible discharge of static electricity. It occurs when the electric field between two oppositely charged objects or a charged object and the ground becomes strong enough to ionize the air molecules between them. This ionization creates a conductive path, allowing the accumulated electric charge to rapidly flow, resulting in a sudden flash of light, heat, and often a crackling sound. Sparks can range from small, innocuous discharges encountered in daily life (like touching a doorknob after walking across a carpet) to large, dangerous lightning strikes caused by charges built up in clouds. Understanding these phenomena is crucial for safety and for many technological applications.