Practical skills Study Guide

Understanding how current flows through different electrical components in response to a potential difference (voltage) is fundamental in physics. The relationship between current and potential difference is known as the component's 'characteristic'. This practical investigation (PAG P6) involves building simple circuits to measure current and potential difference across various components, such as a resistor, a filament lamp, and a diode. By varying the potential difference applied across the component and measuring the corresponding current, we can plot graphs to visually represent these characteristics.

For an ohmic resistor, the current is directly proportional to the potential difference, resulting in a straight line through the origin on a V-I graph, assuming constant temperature. This adherence to Ohm's Law (V=IR) is a key concept. However, components like a filament lamp do not obey Ohm's Law. As the current increases, the filament heats up, increasing its resistance, which means the V-I graph will be a curve. A diode is an even more extreme example, showing almost no current flow in one direction (reverse bias) and significant current flow only once a certain forward potential difference (threshold voltage) is reached.

The ability to construct these circuits accurately, collect reliable data, and interpret the resulting graphs is crucial for understanding the behaviour of electrical components in real-world applications. Identifying the independent variable (potential difference, often controlled by a variable power supply or changing the number of cells), the dependent variable (current, measured by an ammeter), and control variables (like temperature for an ohmic resistor) is essential for a fair test. Proper use of ammeters (in series) and voltmeters (in parallel) is also critical for accurate measurements.

When a force is applied to an object, such as a spring, it can cause the object to change shape. For elastic objects like springs, this change in shape is often an extension or compression. The relationship between the applied force and the resulting extension in a spring is a fundamental concept in physics. Up to a certain point, known as the limit of proportionality, the extension of a spring is directly proportional to the force applied to it. This means if you double the force, the extension doubles. This proportional relationship is described by Hooke's Law.

To investigate this, you can set up an experiment where a spring hangs vertically from a retort stand, and known masses (which exert a known gravitational force, F = mg) are added to it. The initial length of the spring is measured, and then the length of the spring is measured again after each mass is added. The extension is calculated by subtracting the initial length from the new length. By plotting a graph of force (on the y-axis) against extension (on the x-axis), you can observe this direct proportionality as a straight line passing through the origin. The point where the graph starts to curve is the limit of proportionality. Beyond this limit, the spring no longer behaves elastically; it may undergo plastic deformation or even break, meaning it will not return to its original length once the force is removed. This experiment allows for safe observation of these principles, provided appropriate safety measures are taken to prevent weights from falling or springs from snapping back unexpectedly.