potential dividers

• • When two resistors are connected in series, through Kirchhoff’s Second Law, the potential difference across the power source is divided between them
  • Potential dividers are circuits that produce an output voltage as a fraction of its input voltage
  • This is done by using two resistors in series to split or divide the voltage of the supply in a chosen ratio
• • Potential dividers have two main purposes:
  • To provide a variable potential difference
  • To enable a specific potential difference to be chosen
  • To split the potential difference of a power source between two or more components
• • A potentiometer is similar to a variable resistor connected as a potential divider to give a continuously variable output voltage
  • It can be used as a means of comparing potential differences in different parts of the circuit
• • Variable and sensory resistors are used in potential dividers to vary the output voltage
  • This could cause an external component to switch on or off e.g. a heater switching off automatically when its surroundings are at room temperature
  • Sensory resistors used are light dependent resistors (LDRs) and thermistors
• • The voltmeter in both circuits is measuring Vout
  • Recall that the resistance of an LDR varies with light intensity
  • The higher the light intensity, the lower the resistance and vice versa
  • An LDR circuit is often used for street and security lights
  • The resistance of a thermistor varies with temperature
  • The hotter the thermistor, the lower the resistance and vice versa
• • From Ohm’s law V = IR, the potential difference Vout from a sensory resistor in a potential divider circuit is proportional to its resistance
  • If an LDR or thermistor's resistance decreases, the potential difference through it also decreases
  • If an LDR or thermistor's resistance increases, the potential difference through it also increases
• • Since the total p.d of the components must be equal to Vin, if the p.d of the sensory resistor decreases then the p.d of the other resistor in the circuit must increase and vice versa