Revise Potential dividers for Cambridge 9702 (10.3)

At its heart, a potential divider is a simple series circuit. When resistors are connected in series across a voltage source, the total voltage supplied is distributed amongst them. This distribution isn't random; it's directly proportional to each resistor's individual resistance. The larger the resistance, the greater the share of the total voltage it "drops" across itself.

In a series circuit, the current ($I$) is the same through every component. According to Ohm's Law ($V=IR$), the voltage drop across a resistor is proportional to its resistance. For two resistors, $R_1$ and $R_2$, in series with an input voltage $V_{in}$, the output voltage ($V_{out}$) across $R_2$ is given by:

$V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}$

To create an output voltage that isn't fixed, we can incorporate a variable resistor. A potentiometer is a three-terminal variable resistor, often wired into a potential divider circuit. By sliding its contact (wiper), the resistance ratio changes continuously, allowing for a continuously adjustable output voltage, perfect for volume controls or brightness adjustments.

Sensors like Light Dependent Resistors (LDRs) and thermistors can replace fixed resistors in a potential divider. An LDR's resistance decreases with more light, while an NTC thermistor's resistance drops with increasing temperature. This makes the output voltage responsive to environmental changes, forming the basis of many automatic control systems like streetlights or temperature alarms.

Potentiometers are also critical in highly accurate null measurement techniques. The key advantage is that at the null point (zero current flow), no current is drawn from the component being measured. This prevents the measurement device from altering the circuit's conditions, ensuring a more accurate determination of its actual potential difference. A galvanometer detects this precise null point.