Electricity Study Guide

Current-voltage (I-V) graphs are fundamental tools in electrical circuits, allowing us to visualise how the current flowing through a component changes as the voltage across it varies. For an ohmic resistor, the I-V graph is a straight line passing through the origin. This linearity demonstrates Ohm's Law (V = IR), implying that the resistance (R = V/I) remains constant regardless of the applied voltage or current. The gradient of this line is directly related to the reciprocal of the resistance (1/R). If the line is steeper, the resistance is lower; if it is less steep, the resistance is higher.

In contrast, non-ohmic components exhibit non-linear I-V graphs, indicating that their resistance changes with voltage or current. A filament lamp, for example, shows a curve that becomes less steep at higher currents and voltages. This is because as the current increases, the filament heats up significantly, causing its resistance to increase. The hotter the metal, the more the ions in the lattice vibrate, making it harder for electrons to pass through, thus increasing resistance. Similarly, a diode's I-V graph is highly non-linear. It has very high resistance in the reverse direction (blocking current), a very low resistance when forward-biased beyond a certain threshold voltage (typically around 0.6-0.7V for silicon diodes), and effectively no current flowing below this threshold. This characteristic allows diodes to act as rectifiers, permitting current flow in only one direction. These graphical representations are crucial for understanding the behaviour and applications of different circuit elements.

Electric circuits are fundamental to modern technology, and understanding how components are connected within them is crucial. There are two primary ways to connect components: in series or in parallel. In a <b>series circuit</b>, components are connected end-to-end, forming a single continuous loop for the current to flow through. This means that the current is the same at all points in a series circuit. If one component in a series circuit breaks or is removed, the entire circuit is broken, and current stops flowing to all other components. For example, older Christmas lights were often wired in series; if one bulb failed, the whole string went out. The total resistance in a series circuit is the sum of the individual resistances, which typically leads to a higher overall resistance and thus a smaller current for a given voltage. The total voltage supplied by the power source is divided among the components. In a <b>parallel circuit</b>, components are connected across the same two points, creating multiple pathways for the current. This means that the voltage across each component in a parallel circuit is the same. The total current from the power source splits among the different branches, and the sum of the currents in the individual branches equals the total current entering and leaving the junction. If one component in a parallel branch breaks, current can still flow through the other branches, allowing the remaining components to continue operating. For instance, household wiring is typically parallel, so turning off one light doesn't turn off all other appliances. The total resistance in a parallel circuit is less than the resistance of any single branch, leading to a larger total current for a given voltage. Current in a circuit is the rate of flow of charge.