The potential divider is a circuit that splits voltage among electrical components, such that each of them receives the exact voltage they require. They usually contain two resistors in series with the supply: one of them is fixed, while the other is a variable resistor.

The use of a variable resistor helps to produce voltage of variable outputs, which can be used to operate the devices of different voltage requirements.

Some of the examples of a variable resistor are rheostat, potentiometer, thermistor and LDRs.

A typical potentiometer consists of a uniform resistance wire a metre long, stretched above a wooden plank or metre rule. It is based upon the principle that the voltage drop across the length of a uniform cross-section wire is directly proportional to the length of the wire when a constant current is passed through it.

Here, V_AC ∝ L_AC

During null deflection or balanced condition, no current flows between the cell and the potentiometer wire. At this condition, the voltage across the wire AY is equal to the emf or potential difference between the terminals of the cell. However, they act in opposite directions and as a result, no current flows between them.

At balanced condition, $\frac{\text{E}_{X}}{\text{E}_{O}} = \frac{\text{AY}}{\text{AB}}$

The resistance of a thermistor changes according to the change in temperature. In a negative temperature coefficient thermistor, the resistance decreases when there is an increase in temperature. They are used in refrigerators, digital thermometers, air conditioners and fire alarms.
Light-dependent resistors or LDRs are triggered by the intensity of light. The LDR shows a high resistance when the light intensity is low and vice versa. They are used in security alarms, automatic street lights and automatic brightness control in televisions and computers.

10.04 - Potential Dividers

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