Edexcel AS Physics

Revision Notes

3.14 Potential Dividers & Variable Resistance

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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

Potentiometer diagram, downloadable AS & A Level Physics revision notes

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

Potentiometer symbol, downloadable AS & A Level Physics revision notes

  • The sliding contact has the effect of separating the potentiometer into two parts
    • Each part will have different resistances
    • Therefore output voltage will change

Potentiometer circuit diagram, downloadable AS & A Level Physics revision notes

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)

Sensory potential divider circuits, downloadable AS & A Level Physics revision notes

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

  • The voltmeter in both circuits is measuring Vout
  • 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 potential difference of the components must be equal to Vin:
    • 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, Vout across the LDR will decrease because resistance has decreased
    • If light intensity decreases, Vout across the LDR will increase because resistance has increased
  • An LDR circuit is often used for street and security lights
    • When light intensity falls, Vout 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, Vout across the thermistor will decrease because resistance has decreased
    • If temperature decreases, Vout across the thermistor will increase because resistance has increased
  • A thermistor circuit is used in fire alarms, ovens and digital thermometers
    • When temperature falls, Vout increases and so this can provide the voltage required to turn on a heater

Worked example

A potential divider consists of a fixed resistor R and a thermistor connected in series.WE - Thermistor potential divider question image, downloadable AS & A Level Physics revision notesWhich row of the table describes what happens to the potential difference across resistor R and the thermistor when the temperature of the thermistor decreases?WE - Thermistor potential divider question table image, downloadable AS & A Level Physics revision notes

ANSWER:  DWorked example - Thermistor potential divider (2), downloadable AS & A Level Physics revision notes

Step 1: Consider Ohm's Law

    • Due to Ohm’s Law (V = IR), both the resistor and thermistor are connected in series and have the same current I
      • If resistance R increases, the potential difference across the thermistor also increases

Step 2: Consider the electrical voltages rule

    • In series, the input potential difference is shared equally amongst the components due to the electrical voltages rule
      • Therefore, since the potential difference across the thermistor increases, the potential difference across the resistance R must decrease

   Step 3: State the answer

    • This is row D

Exam Tip

A potentiometer can also be used as a variable resistor if only two of the three possible terminals are used.

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