Ohm's Law

Ohm's law states that the current (I) flowing through a conductor is directly proportional to the potential difference (V) across it, provided the temperature stays constant. Written as an equation it is V = IR, where R is the resistance. 'Directly proportional' means that if you double the voltage, the current also doubles; if you halve the voltage, the current halves. The graph of current against voltage for such a conductor is therefore a straight line through the origin. Georg Ohm published this relationship in 1827 after careful experiments with wires.

These three quantities describe any electrical circuit. Potential difference (voltage) is the energy given to each unit of charge by the cell, measured in volts. Current is how much charge flows past a point each second, measured in amperes. Resistance is how strongly the component opposes that flow, measured in ohms. A useful way to picture it: voltage is the push, current is the flow that results, and resistance is whatever gets in the way. Ohm's law ties all three together so that knowing any two lets you calculate the third.

A component that obeys Ohm's law is called ohmic: its resistance stays constant, so its current-voltage (I-V) graph is a straight line through the origin. A fixed resistor at constant temperature and a metal wire are good examples. Many components are non-ohmic. A filament lamp gets hotter as more current flows, its resistance rises, and its I-V graph curves into an S-shape that flattens off. A diode only allows current in one direction, so its graph is flat (almost no current) one way and rises steeply the other. Recognising the shape of an I-V graph tells you immediately whether a component is ohmic.

Ohm's law only holds at constant temperature because resistance depends on temperature. In a metal, the positive ions vibrate more when hot, so they get in the way of the moving electrons more often, raising the resistance. That is why a filament lamp does not obey Ohm's law: as current heats the filament, its resistance climbs and the current no longer rises in step with the voltage. In experiments we keep the current low or use short pulses so the wire does not heat up and the straight-line relationship is preserved.

The resistance of a particular wire is not random; it is set by three things. It increases with length (a longer wire opposes current more), decreases with cross-sectional area (a thicker wire lets current through more easily, like a wider pipe), and depends on the material through a property called resistivity. These combine into R = ρL/A. Copper has a very low resistivity, which is why it is used for connecting wires; nichrome has a high resistivity, which is why it is used for heating elements.

To test Ohm's law you build a simple circuit: a cell, the component under test, an ammeter in series to measure current, and a voltmeter in parallel across the component to measure voltage. A variable resistor (rheostat) lets you change the current in steps. You record several pairs of voltage and current readings, plot I against V, and look at the shape. For an ohmic conductor the points fall on a straight line, and its gradient equals 1/R. This is the standard required practical in both GCSE and Indian board syllabuses.