Potential Dividers & Variable Resistance (Edexcel International A Level Physics)

Potential Dividers & Variable Resistance

The Potentiometer

• A potentiometer is a single component which can act as a potential divider.

  • It consists of a coil of wire with a sliding contact

  • A variable output voltage can be varied by moving a slider along the component

A potentiometer is a type of variable resistor

• The circuit symbol is drawn as an arrow next to the resistor, to represent the sliding contact

• The sliding contact has the effect of separating the potentiometer into two parts

  • Each part will have different resistances

  • Therefore output voltage will change

Moving the slider (the arrow in the diagram) changes the resistance (and hence potential difference) of the two parts of the potentiometer

• If the slider in the above diagram is moved upwards, the resistance of the lower part will increase and so the potential difference across it will also increase

• Therefore, the variable resistor obtains a maximum or minimum value for the output voltage

• If the resistance is 3 Ω:

  • Maximum voltage is when the resistance is 3 Ω

  • Minimum voltage is when the resistance is 0 Ω

Thermistors & Light Dependent Resistors (LDRs)

• Sensory resistors are used in potential dividers to vary the output voltage

  • This could cause an external component to switch on or off

  • For example, a heater switches off automatically when its surroundings are at room temperature

• Examples of the variable sensory resistors used are thermistors and light-dependent resistors (LDRs)

LDR and thermistor in a potential divider circuit with a fixed resistor R

• The voltmeter in both circuits is measuring V_out

• From Ohm’s law V = IR, the potential difference V_out 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 potential difference of the components must be equal to V_in:

  • If the potential difference of the sensory resistor decreases then the potential difference across the other resistor in the circuit must increase 

  • If the potential difference of the sensory resistor increases then the potential difference across the other resistor in the circuit must decrease 

The resistance of an LDR...

• Varies with light intensity

  • The higher the light intensity, the lower the resistance

  • The lower the light intensity, the higher the resistance

• Therefore:

  • If light intensity increases, V_out across the LDR will decrease because resistance has decreased

  • If light intensity decreases, V_out across the LDR will increase because resistance has increased

• An LDR circuit is often used for street and security lights

  • When light intensity falls, V_out increases and so this can provide the voltage required to turn on a lamp

The resistance of a thermistor...

• Varies with temperature

  • The hotter the thermistor, the lower the resistance 

  • The cooler the thermistor, the higher the resistance

• Therefore:

  • If temperature increases, V_out across the thermistor will decrease because resistance has decreased

  • If temperature decreases, V_out across the thermistor will increase because resistance has increased

• A thermistor circuit is used in fire alarms, ovens and digital thermometers

  • When temperature falls, V_out increases and so this can provide the voltage required to turn on a heater