Electricity Study Guide

Static electricity is a common phenomenon that occurs when there is an imbalance of electric charges on the surface of an object. This imbalance is primarily due to the transfer of electrons, which are negatively charged subatomic particles. When two insulating materials are rubbed together, electrons can be transferred from one object to the other. The specific material that gains electrons becomes negatively charged, as it now has an excess of negative charges. Conversely, the material that loses electrons becomes positively charged, as it now has a deficit of negative charges (meaning a relative excess of positive charges from the atomic nuclei). The amount of charge transferred depends on the materials involved and how vigorously they are rubbed. Different materials have different affinities for electrons; some tend to give them up easily, while others tend to gain them. Once charged, these objects can then exert forces on other charged or uncharged objects, leading to phenomena like attraction (e.g., a charged comb picking up small pieces of paper) or repulsion (e.g., two identically charged balloons pushing each other away). The charges tend to remain static, or in one place, on insulating materials because electrons cannot easily move through them, unlike conductors where charges can flow freely. This charge imbalance persists until a path is provided for the electrons to move, such as through a discharge or by touching an earthed object, which neutralises the charge.

Resistance is a property of a material that opposes the flow of electric current. It converts electrical energy into other forms like heat. The relationship between voltage (V), current (I), and resistance (R) in an electrical circuit is described by Ohm's Law: V = I × R. This formula allows us to calculate any of these three quantities if the other two are known. Voltage, measured in volts (V), is the electrical potential difference across a component, driving the current. Current, measured in amperes (A), is the rate of flow of charge. Resistance is measured in ohms (Ω).

For some resistors, known as 'ohmic' resistors (like a standard metal wire at constant temperature), the resistance value remains constant regardless of the current flowing through them, assuming temperature also stays constant. This means that if you double the voltage across such a resistor, the current through it will also double. When plotted on a voltage-current graph, an ohmic resistor shows a straight line passing through the origin.

However, for other components, such as filament lamps or thermistors, the resistance changes as the current changes. In a filament lamp, for example, as current increases, the filament heats up significantly. The increased thermal vibrations of atoms make it harder for electrons to pass through, thus increasing its resistance. This means that if you double the voltage across a filament lamp, the current will not necessarily double; it will increase, but by a smaller factor than expected if resistance were constant. Similarly, the resistance of a thermistor changes with temperature, which can be indirectly affected by current. Understanding these differences is crucial for analysing and designing electrical circuits.